1.1.1 Migration in the Context of Human Adaptation to a Naturally Changing Climate

Apart from Antarctica and the hottest, driest parts of a small number of deserts, there are few terrestrial spots on this planet where people have not lived or attempted to live on a permanent basis. The history of our species since it emerged approximately 200,000 years ago is of people on the move, and for most of that history, climate played an important role. Radiating out from our origins in East Africa, Homo sapiens have over the millennia moved and adapted biologically, behaviorally, and technologically to a wide range of environments. Hot or cold, wet or dry, rugged or flat, continental or island, if people could get there, they did, and attempted to establish homes and livelihoods. Sometimes “home” was not a single location, but a wider territory over which people would travel as they hunted, fished, and gathered, often in synchronicity with the seasons. In other locations where resources and climate permitted, people established fixed settlements, some of which over time prospered and grew into towns and cities, while others dwindled and were abandoned (McLeman Reference McLeman2011). The act of moving around in search of favorable locations was an important component of larger processes of behavioral adaptation that made our species so successful, if we measure “success” in ecological terms such as the number of individuals in a species (eight billion and counting in our case) and the wide variety of habitats we occupy.

There are, however, limits to physical and behavioral adaptation. Humans are biologically incapable of withstanding exposure to very hot or very cold temperatures for extended periods of time, and we must have a supply of water to drink daily. In this sense, climate places an important check on the types of places to which people are able to move; it defines our geographical range, to import another term from ecology. Within areas that are biologically and climatically viable for us, the distribution and density of human settlements is neither uniform nor random; instead, settlement patterns historically reflected the availability, quality, and distribution of resources critical for our survival, with climate again playing a determining factor. Its influence varies across scales from the global to the local. Resources potentially available for human use vary considerably among Arctic, temperate, and tropical environments due to climate; they also vary between the north side and the south side of a hill, and windward and leeward sides of a coastal mountain. Large- and small-scale variations in climatic conditions interact with other ecological processes to render certain locations more desirable than others for human settlements.

The geographical expansion of the human population over the millennia has been further shaped by continuous and ongoing changes in the Earth’s climate over long and short periods of time due to natural processes. Human populations have learned how to adapt to the seasonality of the Earth’s climate and the inevitable and generally predictable year-to-year variations in temperatures, precipitation, growing season length, and other weather conditions that affect livelihoods (see Box 1.2). People have also had to adapt to changes in climate that unfold over longer periods of times, along with unexpected short-term fluctuations triggered by natural processes that occur within and beyond the Earth’s atmosphere. One of the more obvious long-term climatic processes that has shaped human population movements and patterns has been the slow climatic cycle of ice age to warm interglacial period and back again to ice age. These glacial/interglacial periods play out over thousands of years, driven principally by Milankovitch cycles, named after the physicist who first documented them (NASA 2020). These are small variations in the orbit of the Earth around the Sun, in the tilt of the Earth’s axis, and in the steadiness of the Earth’s spin on its axis. Milankovitch cycles change the distance of the Earth from the Sun (the closer we are, the more solar energy we receive, and average global temperatures warm up) and the orientation of the Earth’s surface toward the Sun (locations that are more perpendicular to the Sun receive more radiation and become warmer than others).

Global climate patterns can fluctuate irregularly due to natural processes such as volcanic activity and fluctuations in solar radiation. Recent centuries have seen only occasional large volcanic eruptions and a somewhat steady occurrence of small ones, but in periods when there has been a lot of volcanic activity fluctuations in global climate conditions have occurred. The specific effects of volcanic eruptions on climate are complicated to disentangle (Chim et al. Reference Chim, Aubry, Abraham, Marshall, Mulcahy, Walton and Schmidt2023). Ash, particulate matter, and sulfur gasses emitted into the air during eruptions exert a cooling effect on the climate, but carbon dioxide and water vapor that are also emitted into the air trap heat, and the combined effects may be to temporarily cool the climate and then subsequently warm it (but not always). Temporary variations in the amount of energy received from the Sun can also stimulate changes in the climate. On average, the amount of solar energy received from the Sun equals 341 watts per square meter, but there can be variations over relatively short periods of time. The amount of energy emitted by the Sun varies very slightly over the course of a 11-year cycle, and on occasions there are storms on the surface of the Sun that cause flares or bursts of additional energy to be emitted (often referred to as “sunspots,” for that is how they appear to us from the Earth) (NASA Reference Earth Data2024). Should a solar flare happen to point toward the Earth when it occurs, our planet can receive a slight increase in the amount of energy received, in turn leading to a slight temporary warming. In addition to variations in climate stimulated by orbital variations, volcanic activity, and solar flares, there are multiple naturally occurring, cyclical oscillations in the Earth’s climate driven by interactions between oceans and atmosphere, the best known of which is the El Niño Southern Oscillation (ENSO) (Box 1.3).

Throughout the longer course of human history, there have been many times and places when naturally occurring short- and long-term changes in climatic conditions have led to large-scale movements of people into and out of particular areas. In the past thousand years, two periods stand out as being particularly notable for the extent to which climatic changes affected the movements and distribution of people. The first is the Medieval Warm Period (MWP) that ran from the tenth to the early fourteenth centuries CE, a time when average temperatures across much of Europe, Central Asia, Africa, and the Americas were as warm or warmer than they are today (Mann et al. Reference Mann, Zhang, Hughes, Bradley, Miller, Rutherford and Ni2008). Excluding present-day climate, average temperatures during the twelfth-century peak of the MWP were likely the warmest experienced since the last ice age (i.e. the last ten thousand years). The impacts of the MWP were favorable for people in Europe, where the climate of the preceding thousand years had been highly variable, characterized by long periods of harsh winters, droughts, extreme storm events, and notable sea level change along the coasts (Lamb Reference Lamb1995, Grove Reference Grove, Wefer, Berger, Behre and Jansen2002). The MWP created a benign climate that led to increased European agricultural productivity and expansion northward of warm weather crop production, even leading to the establishment of vineyards in southern England (Grove Reference Grove, Wefer, Berger, Behre and Jansen2002). During the MWP, Scandinavian Norse (popularly known as the Vikings) began migrating to and establishing permanent settlements on the Faroes, Iceland, and Greenland (the coastal areas of which were indeed green during the summers of the MWP), and the building of smaller, ephemeral hunting and fishing settlements on Newfoundland (Dugmore et al. Reference Dugmore, McGovern, Vésteinsson, Arneborg, Streeter and Keller2012). Inuit settlements across the North American Arctic and Greenland also expanded during this period of relatively mild conditions (Friesen et al. Reference Friesen, Finkelstein and Medeiros2020). Elsewhere, MWP climate conditions were not so benign. A series of severe, extended droughts coincided with the collapse and depopulation of the great cities of the Mayan empire in Central America in the tenth-century and the twelfth-century abandonments of large pueblos (hilltop settlements) and Anasazi cliff dwellings in what is today the southwestern US (Haug et al. Reference Haug, Günther, Peterson, Sigman, Hughen and Aeschlimann2003, Wahl et al. Reference Wahl, Byrne, Schreiner and Hansen2007, Lekson and Cameron Reference Lekson and Cameron1995) (Figure 1.1). Inter-city conflict, deforestation, and an inflexible political hierarchy likely amplified the effects of MWP droughts on Mayan cities (Shaw Reference Shaw2003, Orlove Reference Orlove2005); it is less clear whether droughts alone were responsible for settlement abandonments in the Pueblo and Anasazi territories or if other factors also came into play. Across Asia, climatic conditions were highly variable during the MWP (this was actually a cold period in East Asia), and some scholars have suggested the expansion of the Mongol emperor of Genghis Khan and his successors into China and Eastern Europe may have been necessitated by persistent dry conditions in Central Asia in the twelfth century (Fagan Reference Fagan2004).

Figure 1.1 Remains of Anasazi cliff dwellings at Mesa Verde, present-day Colorado. These apartment-style settlements carved out of sandstone cliff walls are found at multiple locations in the US southwest and supported thousands of people until their sudden abandonment in the twelfth century CE.

Photo by R. McLeman.

The MWP was followed by a period of falling average temperatures and greater climate instability in the northern hemisphere that reached its nadir between the sixteenth and nineteenth centuries in what has been described as the “Little Ice Age” (Grove Reference Grove1988). During this period, European winters became longer and colder, ice skating was widely enjoyed in the Netherlands, and England’s River Thames froze over on several occasions (Huntley Reference Huntley1957, Robinson Reference Robinson2005). Agricultural settlements in marginal areas of Europe were abandoned, as were Norse settlements on Greenland (Lamb Reference Lamb1995). Tremendous storms struck the northern European coast on several occasions, causing erosion and the abandonment of many coastal settlements (Clarke et al. Reference Clarke, Rendell, Tastet, Clave and Masse2002). In the eastern North American Arctic, colder temperatures coincided with a southward movement of Inuit settlements (Friesen et al. Reference Friesen, Finkelstein and Medeiros2020). Indigenous settlements known as Greater Cahokia in mid-continental North America were abandoned during serious droughts in the years 1350–1450 (Pompeani et al. Reference Pompeani, Bird, Wilson, Gilhooly, Hillman, Finkenbinder and Abbott2021). In West Africa, the great empire of Mali collapsed in the late sixteenth century during a period of repeated catastrophic flooding in the Niger River valley followed by severe droughts upstream at Timbuktu (Makaske et al. Reference Makaske and Vries2007). In India, the recently built capital city of Fatehpur Sikri had to be abandoned due to drought and lack of water (Hillel Reference Hillel1991).

The examples given earlier are just some of many past instances when large-scale population displacements and migrations coincided with notable climatic events and conditions (see McLeman Reference McLeman2011 for further examples). The simultaneous occurrence of a major climate event and a significant event in human history does not prove that the former caused the latter. With migration events of long ago our access to details about human systems and local climate conditions is limited, so it is important to be cautious about assuming a migration event was caused by climate and to not ignore other concurrent events of a political, social, or economic nature that may have had a causal influence. Nonetheless, it is reasonable to view the MWP and the Little Ice Age as being noteworthy periods of human history when migration and displacement patterns were heavily influenced by climate change.

This raises the question: Have we entered a new period when climate change – now being driven by human activity as opposed to natural processes – is once again becoming a dominant influence on migration and displacement? It is too soon to say for certain, but the warning signs are clearly there. The climate is clearly changing rapidly – much more rapidly than at any time humans have been on the planet – and this will test our ability to adapt unlike ever before. Fortunately, because human activity is the cause of the changes now occurring to the climate, it is within our ability to prevent further changes. Whether we will collectively decide to do so is also too soon to say. As of the time of writing this book in 2024, GHG emissions are rising steadily with only modest signs of slowing despite many promises made by politicians from around the world. For at least the next several decades, the climate will continue to warm, generating more frequent and severe hazards that will almost certainly lead to higher rates of migration and displacement in the most highly exposed areas.

1.2.1 Basic Functioning of the Earth’s Climate and the Effect of Greenhouse Emissions

The Earth’s climate is regulated by interactions between solar radiation arriving on a continuous basis from the Sun and the gaseous composition of the Earth’s atmosphere. The amount of energy entering the Earth’s atmosphere via solar radiation is balanced over the long term by an equal amount of energy being radiated back into space. If this were not the case, there would be no long-term stability of the climate. For example, if the amount of incoming solar radiation exceeded the amount of energy leaving the atmosphere, temperatures on the Earth’s surface would rise rapidly. Conversely, if the amount of energy escaping the atmosphere exceeded the amount coming in, the Earth would cool rapidly. On average, the Earth receives just over 340 Watts per meter squared of energy in the form of solar radiation on an ongoing basis. Radiation is produced in different wavelengths depending on the temperature of the emitting source. These wavelengths range from exceedingly tiny gamma rays – roughly the length of the nucleus of a single atom – to radio waves that may be hundreds of kilometers long. The shorter the wavelength, the more concentrated the energy that is carried. Solar radiation enters the Earth’s atmosphere in a range of wavelengths that include the near-infrared part of the electromagnetic spectrum, visible light, and ultraviolet light (Figure 1.2). Most of the ultraviolet radiation is prevented from entering lower portions of the atmosphere and reaching the Earth’s surface by ozone molecules in the stratosphere – a function that makes life on the Earth possible, since ultraviolet radiation damages the cells of living organisms.

Figure 1.2 The electromagnetic spectrum. Solar radiation arriving at the Earth’s atmosphere is primarily in the near-infrared, visible light, and ultraviolet portions of the spectrum, with ultraviolet light being intercepted by ozone molecules in the stratosphere before it can reach the Earth’s surface. Radiation leaving the Earth’s atmosphere is in wavelengths in the far infrared part of the spectrum.

Adapted from MyNASAData.

To maintain the energy balance, the incoming radiation from the Sun must equal the amount of energy leaving the Earth’s atmosphere. Slightly less than 30% of the incoming solar radiation is immediately reflected back into space from shiny surfaces, such as the tops of clouds and snow cover on the ground, and therefore has no impact on the Earth’s climate (Figure 1.3). Just under half of the incoming solar radiation reaches the Earth’s surface and is absorbed, with the remaining 23% of the incoming radiation being directly absorbed by gasses in the atmosphere before it reaches the surface. Energy that is absorbed by the surface and in the atmosphere gets re-emitted back into the air; however, it is re-emitted at a longer wavelength than incoming radiation, in the far part of the infrared spectrum, because the emitting source is not as hot as the Sun. In the absence of an atmosphere (such as is the case on the Moon), this re-emitted radiation would escape directly to outer space. However, only about 12% immediately escapes to space, because our atmosphere contains small concentrations of gasses that absorb infrared radiation and re-radiate it in all directions, including back toward the Earth’s surface. This causes air temperatures at the Earth’s surface to warm up. These same gasses – carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) – do not interfere with the short-wave radiation arriving from the Sun, but only with the infrared radiation trying to escape the atmosphere. In this way, they perform a function similar to glass in a greenhouse, which allows sunlight to enter and warm the interior surfaces but slows heat from escaping, creating temperatures inside the greenhouse that are warmer than the outside air. For this reason, carbon dioxide, methane, and nitrous oxide are referred to as GHGs because of their heat trapping greenhouse effect. If there were no GHGs in the atmosphere, the Earth’s average surface temperature would be approximately –15°C, almost 30°C colder than it actually is.

Figure 1.3 Energy in balance. The amount of energy reaching the Earth from the Sun is balanced by the amount escaping the atmosphere to outer space. However, much of the outgoing energy gets intercepted by carbon dioxide and other trace gasses in the atmosphere and re-radiated multiple times before it escapes, causing heat to build up at the Earth’s surface. This phenomenon is referred to as the greenhouse effect.

For a detailed explanation, visit https://earthobservatory.nasa.gov/features/EnergyBalance).

GHGs are not naturally abundant; concentrations of carbon dioxide have historically fluctuated within a range of 180–300 molecules per million molecules of air, and methane and nitrous oxides are even less common, measured in parts per billion molecules of air. Water vapor is also a GHG; its concentration in the air varies significantly from one location to another, being high in moist tropical environments and near zero in desert environments. Globally, water vapor averages roughly 1% of the air in lower parts of the atmosphere. Starting in the mid-1800s, atmospheric concentrations of GHGs, particularly carbon dioxide, began rising rapidly (Figure 1.4). This increase is not caused by any natural processes described in Section 1.1.1, but by human activity: the consumption of fossil fuels; the production of cement and steel; and, rapid reduction of the Earth’s forest cover. Fossil fuels include coal, oil, and natural gas that are extracted from underground and burned, with the combustion process releasing carbon dioxide gas into the air. Burning 1 kg of coal (which is nearly pure carbon) releases over 3 kg of carbon dioxide gas into the atmosphere (the mass of the carbon atom plus the mass of two oxygen atoms joined to it). The industrial processes used to make cement and steel also emit carbon dioxide into the air and collectively comprise up to roughly 5% of total anthropogenic GHG emissions. Plants – and forests in particular – remove large amounts of carbon dioxide from the atmosphere and store it in their tissues via photosynthesis. When it is summer in the northern hemisphere – which has a much larger land area than the southern hemisphere and therefore the largest mass of terrestrial vegetation – the amount of photosynthesis occurring is so great it reduces the amount of carbon dioxide in the air temporarily. When the northern winter arrives much of the vegetation goes dormant, and carbon dioxide levels rise again. This seasonal effect of vegetation on atmospheric carbon dioxide is seen in the annual up-and-down fluctuation in measurements of carbon dioxide recorded since the 1950s at the weather observatory at Mauna Loa, Hawaii (Figure 1.5). Deforestation reduces the amount of photosynthesis that occurs, thereby weakening the effectiveness of a natural process that removes carbon from the atmosphere. This process is known as a carbon sink, two other key carbon sinks being photosynthesizing phytoplankton that float on ocean surfaces and direct absorption of carbon dioxide from the air by the ocean surface to form carbonic acid, making seawater gradually more acidic. The collective capacity of the Earth’s three main natural carbon sinks is unable to keep pace with human emissions of carbon dioxide, hence its rapid accumulation in the atmosphere.

Figure 1.4 Atmospheric concentrations of carbon dioxide over the past 800,000 years. Since 1958, concentrations have been measured directly on a continuous basis. Concentrations for previous millennia are measured from air bubbles trapped within cores of ice taken from Greenland and Antarctica. As snow and ice is deposited over time, air becomes trapped. The lower down the core, the older the air.

Source: NASA Reference Earth Data2024.

Figure 1.5 Daily measurements of atmospheric carbon dioxide made at Mauna Loa, Hawaii. The overall trend since the late 1950s shows a steady increase; the continuous up-and-down fluctuation reflects the seasonality of photosynthesis levels of terrestrial vegetation. This graph is popularly known as the “Keeling curve” after the scientist who first began taking such measurements.

Source: NASA Reference Earth Data2024.

Although carbon dioxide is the most abundant GHG (apart from water vapor), atmospheric concentrations of methane have also been rising rapidly over the past 150 years (Figure 1.6). Individual methane molecules trap far more heat in the atmosphere than individual carbon dioxide molecules, but methane molecules remain in the atmosphere for much shorter periods of time than carbon dioxide molecules (7–12 years vs hundreds of years). Methane forms whenever the decomposition of organic material occurs in oxygen-deprived environments, such as buried trash, the stomachs of animals, and wetlands. The fossil fuel known as “natural gas” is methane. Key anthropogenic sources of methane emissions are agriculture, landfills, and oil and gas production. Atmospheric concentrations of nitrous oxide, a GHG that is released as a by-product of combustion and the manufacture of synthetic fertilizers, have risen by approximately 18% over the past century (NASA Reference Earth Data2024).

Figure 1.6 Atmospheric concentrations of methane over the past thousand years.

Source: NASA Reference Earth Data2024.

A consequence of rising atmospheric concentrations of GHGs has been a rapid increase in average temperatures at the Earth’s surface. Widespread global coverage of thermometer-recorded daily temperatures dates back to the mid-1800s, and so scientists use average global temperatures for the period 1850–1900 as a baseline or reference period for comparison, referring to it as pre-industrial temperatures. Any significant deviations in average global temperatures from this reference period are described as temperature anomalies. In 2017, average global temperature for the first time rose to more than 1°C warmer than the pre-industrial reference period, with most of this increase occurring since the year 1950 (IPCC Reference Masson-Delmotte, Zhai, Pörtner, Roberts, Skea, Shukla, Pirani, Moufouma-Okia, Péan, Pidcock, Connors, Matthews, Chen, Zhou, Gomis, Lonnoy, Maycock, Tignor, Waterfield, Cambridge and New York2018). Scientists expect temperatures will continue to rise so long as anthropogenic GHG emissions continue to rise, with average global temperatures reaching 1.5°C warmer than pre-industrial by the late 2030s or mid-2040s; the significance of this will be explained shortly. Figure 1.7 charts average annual global temperatures from 1850 to 2024. Each year’s temperature is compared against the mean average for the entire period, represented by the horizontal line set at 0°. Years with temperatures colder than the long-term average have bars pointing downward and those with temperatures warmer than average pointing upward. As can be clearly seen, every year since the mid-1970s has been warmer than the average, and although there is year-to-year variability, the trend is toward consistently warmer global average temperatures.

Figure 1.7 Average global temperatures, 1850–2024, shown as deviations from the mean for the entire time series.

Source: National Centers for Environmental Information 2024.

This raises the question of how much warmer average global temperatures will get, and at what rate of change. The simple answer is it will depend upon future rates of GHG emissions. If GHG emissions continue to rise at current rates, average global temperatures by the year 2100 will be approximately 2.8°C warmer than pre-industrial levels (World Meteorological Organization 2024). If GHG emissions rise faster than current rates and no action is taken to attempt controlling them – what scientists refer to as a high emissions scenario – average global temperatures at the end of the century could be 4°C or warmer than pre-industrial temperatures (Arias et al. Reference Arias, Bellouin, Coppola, Jones, Krinner, Marotzke, Naik, Palmer, Plattner, Rogelj, Rojas, Sillmann, Storelvmo, Thorne, Trewin, Achutarao, Adhikary, Allan, Armour, Bala, Barimalala, Berger, Canadell, Cassou, Cherchi, Collins, Collins, Connors, Corti, Cruz, Dentener, Dereczynski, Di Luca, Diongue Niang, Doblas-Reyes, Dosio, Douville, Engelbrecht, Eyring, Fischer, Forster, Fox-Kemper, Fuglestvedt, Fyfe, Gillett, Goldfarb, Gorodetskaya, Gutierrez, Hamdi, Hawkins, Hewitt, Hope, Islam, Jones, Kaufmann, Kopp, Kosaka, Kossin, Krakovska, Li, Lee, Masson-Delmotte, Mauritsen, Maycock, Meinshausen, Min, Ngo Duc, Otto, Pinto, Pirani, Raghavan, Ranasighe, Ruane, Ruiz, Sallée, Samset, Sathyendranath, Monteiro, Seneviratne, Sörensson, Szopa, Takayabu, Treguier, van den Hurk, Vautard, Von Schuckmann, Zaehle, Zhang, Zickfeld, Masson-Delmotte, Zhai, Pirani, Conners, Péan, Berger, Caud, Chen, Goldfarb, Gomis, Huang, Leitzell, Lonnoy, Matthews, Maycock, Waterfield, Yelekçi, Yu and Zhou2021). Conversely, if the international community acts rapidly to reduce GHG emissions such that they plateau over the next decade or so and are reduced to near zero by the year 2050, average global temperature by the end of the century could be between 1.5°C and 2°C warmer than pre-industrial average temperatures. These differences in future warming trajectories are important, because the impacts of future warming – including floods, storms, droughts, wildfires, heat waves, crop failures, and the spread of infectious disease vectors – increase in frequency, severity, and/or geographical scope as temperatures rise (reviewed in detail in Chapters 2 and 3). Sea levels that are currently rising at a rate of approximately 3.4 mm per year will begin to rise faster, threatening ever larger numbers of people living in coastal areas (Chapter 4). Scientists have warned for decades that an increase of 1.5°C over pre-industrial temperatures would have dangerous consequences for human well-being and for ecological systems, and under the Paris Agreement of the UNFCCC, the international community has committed to not allowing temperatures to exceed that threshold (see Chapter 6). An assessment conducted by the IPCC found that even the seemingly small difference between a 1.5°C and 2°C increase in average global temperatures would increase the number of people exposed to severe climate risks by several hundred million, and cause sea levels to rise by an extra 0.1 meters, which would in turn threaten the homes of an additional 10 million people (IPCC Reference Masson-Delmotte, Zhai, Pörtner, Roberts, Skea, Shukla, Pirani, Moufouma-Okia, Péan, Pidcock, Connors, Matthews, Chen, Zhou, Gomis, Lonnoy, Maycock, Tignor, Waterfield, Cambridge and New York2018). As a result, it is in our collective best interest to avoid every last fraction of a degree of warming possible.

1.2.2 Climate Change Risks

When we discuss the potential future impacts of anthropogenic climate change – including the possibility that large numbers of people might be displaced from their homes – we are talking about things that might happen but that could potentially be avoided. There could also be unwelcome things that happen but that we are not able to foresee based on current evidence. The IPCC uses the term risk to describe any potentially adverse impacts of climatic variability or change. Risk is in turn a function of four key components:

1. 1. Hazard: The potential occurrence of a climate- or weather-related event or condition that may cause loss of life, injury, or other health impacts, as well as damage and loss to property, infrastructure, livelihoods, service provision, ecosystems, and environmental resources.

2. 2. Exposure: The presence of people; livelihoods; species or ecosystems; environmental functions, services, and resources; infrastructure; or economic, social, or cultural assets in places and settings that could be adversely affected by a hazard.

3. 3. Vulnerability: The propensity or predisposition to be adversely affected by a hazard, which may include a particular sensitivity or susceptibility to harm and/or lack of capacity to cope and adapt.

4. 4. Adaptation: Adjustments and responses to actual or expected climate and its effects, in order to moderate harm or exploit beneficial opportunities.

Figure 1.8 illustrates the interaction of hazard, exposure, vulnerability, and adaptation in the creation of risk in a generic sense. Hazard, exposure, and vulnerability are all positively related to risk; that is, as any of these factors increase, so too does risk (and conversely, if any shrink, so does risk). Adaptation – when it is done well – operates in the opposite way: as adaptation increases, risk decreases (and vice versa). The term “response” is also included in Figure 1.8 because sometimes peoples’ responses to climate hazards are simply reactive in nature, and may not address the underlying causes of risk – such as rebuilding a waterside house that has been damaged by a flood exactly as it was before, without making any adjustments to prevent it from being damaged by future floods (this happens more often than one might expect). In such a case, a response has occurred, but it is not a beneficial adaptation.

Figure 1.8 IPCC diagram showing how climate-related risk is formed through interactions of hazard, exposure, and vulnerability, with adaptation and other responses having the potential to moderate risk. Diagram and definition of terms in preceding paragraph adapted from IPCC AR6 WGII Report, Chapter 1 (Ara Begum et al. Reference Begum, Lempert, Ali, Benjaminsen, Bernauer, Cramer, Cui, Mach, Nagy, Stenseth, Sukumar, Wester, Pörtner, Roberts, Tignor, Poloczanska, Mintenbeck, Algeria, Craig, Langsdorf, Löschke, Möller, Okem and Rama2022a) and Figure 1.5 Panel (b) in Ara Begum, R., R. Lempert, E. Ali, T.A. Benjaminsen, T. Bernauer, W. Cramer, X. Cui, K. Mach, G. Nagy, N.C. Stenseth, R. Sukumar, and P. Wester, 2022: Point of Departure and Key Concepts. In: Climate Change 2022: Impacts, Adaptation, and Vulnerability. Contribution of Working Group II to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [H.-O. Pörtner, D.C. Roberts, M. Tignor, E.S. Poloczanska, K. Mintenbeck, A. Alegría, M. Craig, S. Langsdorf, S. Löschke, V. Möller, A. Okem, B. Rama (eds.)].

Cambridge University Press, Cambridge, UK and New York, NY, USA, pp. 121-196, doi:10.1017/9781009325844.003. Reprinted with permission.

A simple example that we will return to in later chapters explains how the concept of risk works in a practical sense: In the Pacific and Indian Oceans, there are hundreds of thousands of people living on low-lying coral islands known as atolls, where none of the land is more than a meter or two above sea level. Simply by virtue of living on an atoll, one’s home, livelihood, community, and infrastructure are exposed to a variety of coastal hazards, including storm surges that cause damaging floods, penetration of salt water into agricultural soils and underground fresh water supplies, and rising sea levels that may one day lead to complete inundation of the atoll. However, although all atolls are exposed to similar hazards, the vulnerability of atoll communities and nations varies from one to another, and as a result, the relative level of risk differs. For example, to reduce its vulnerability and risk, the government of the Maldives – a country situated on a chain of atolls south of India – is building artificial islands with apartment complexes well above the zone of potential flooding or inundation, and these are capable of housing tens of thousands of people (Jayasinghe Reference Jayasinghe2023). To finance this, Maldivians have established high-end tourist resorts that generate significant revenues from international visitors and entered into close political relationships with the government of China, with Chinese companies helping to construct the artificial islands. Other atoll nations, such as Kiribati, Tuvalu, and Vanuatu, are too distant and isolated to attract deep-pocketed international tourists or foreign benefactors. Their populations face a far more imminent risk of being displaced by climate hazards, a situation we explore in greater detail in Chapter 4.

The targeted use of tourist revenues and construction of artificial islands is an example of adaptation in action. Adaptation can be initiated at any societal scale, from the international community or nation-state level down to local communities, households, or individuals. Artificial islands, sea walls, and hazard early warning systems are examples of adaptations that typically require institutions and higher levels of government to coordinate, finance, and implement. Examples of individual- or household-led adaptations might include people living in flood-prone areas storing their valuables on the second story of their homes or farmers in drought-prone areas planting crops that require low amounts of soil moisture. These are just a few of the many types of in situ adaptations (i.e. adaptations that do not involve moving someplace else) for particular climate hazards described in chapters that follow. Migration is also a possible form of adaptation, which we explain in Section 1.6. It is also important to note that adaptation may be initiated proactively – that is, in anticipation of a potential risk – or after a hazard event has been experienced, so as to reduce the potential for loss or harm from future similar events. The ability (or inability) to adapt successfully is referred to as adaptive capacity and is determined by a wide range of non-environmental factors, including political processes, economic wealth, cultural norms, social relations, technological expertise, knowledge, skills, and acumen.

1.2.3 Hazards: Sudden-Onset, Slow-Onset, Compound, and Cascading

Hazards come in many different types. The ones assessed in this book fall within the larger category of what scientists call natural hazards. There are non-natural hazards as well, such as chemical spills, leaky pipelines, fires deliberately set by people, and so forth, but we do not consider them here. While all natural hazards have the potential to generate migration and displacement, not all have links to climate – volcanoes, earthquakes, and tsunamis being common exceptions – and these are also not assessed in this book. The climate hazards we assess in Chapters 2–4 differ in many ways, one important one being the speed at which they occur. Some, such as tropical cyclones, severe floods, and wildfires can materialize within a matter of days – and in the case of tornadoes, within a matter of hours or minutes – giving exposed populations little time to prepare for their onset. Climate scientists often refer to these as sudden-onset or rapid-onset hazards. By contrast, droughts, sea level rise, and rising average temperatures that may eventually make some areas of the world too hot to live in are examples of hazards that emerge over the course of months, years, or decades, allowing people time to prepare and adapt. We call these slow-onset hazards.

Climate hazards – both slow- and rapid-onset – are often perceived by the public and governments as being exceptions from “normal” weather patterns, but in reality, they are inherent features of the Earth’s climate system that simply occur less frequently than do benign weather conditions (Hewitt Reference Hewitt2019). In the broader scholarly field of natural hazard research, it has long been recognized that infrequent climate events and conditions become hazardous to human well-being due to social, economic, cultural, political, and other non-climatic processes that concentrate vulnerable people in dangerous locations without adequate protection and/or ability to cope (Burton et al. Reference Burton, Kates and White1978, Blaikie et al. Reference Blaikie, Cannon, Davies and Wisner1994, Hewitt Reference Hewitt2013). Consistent with this, a key focus in subsequent chapters of this book is how climate processes and human systems interact to create hazards, exposure, vulnerability, and risk, and the options available to us to reduce such risks.

For analytical purposes, this book focuses on summarizing generally observed migration/displacement patterns and outcomes in response to specific types of sudden- and slow-onset climate hazards. However, reality is often more complex. Climate hazards do not always occur as discrete, unique events; a drought may be followed by flooding (or vice versa), and heavy rains associated with tropical cyclones may generate floods or landslides far from the storm’s center. Climate hazards can also occur at the same time a community is dealing with a non-climatic hazard, such as an economic downturn or a health crisis. Scientists refer to these types of cases as cascading hazards (i.e. when successive hazards are experienced one after another) or compound hazards (when two or more hazards are experienced at the same time) (Zscheischler et al. Reference Zscheischler, Martius, Westra, Bevacqua, Raymond, Horton, van den Hurk, AghaKouchak, Jézéquel, Mahecha, Maraun, Ramos, Ridder, Thiery and Vignotto2020). As an example of a cascading hazard, the COVID-19 virus arrived in the Bahamas only six months after Hurricane Dorian caused widespread loss of life, injuries, and damage to homes and infrastructure, and recovery efforts were not completed when authorities had to implement responses to the public health risks of the pandemic (UNDP 2022). An example of a compound hazard occurred in the 1930s on the North American Great Plains, when widespread severe droughts struck in the midst of the Great Depression, leading to widespread farm abandonment and migration out of affected areas (McLeman and Smit Reference McLeman and Smit2006). Both compound and cascading hazards have the potential to overwhelm the adaptive capacity of exposed populations that might have been able to cope with a single hazard, leading to migration outcomes that would not have otherwise emerged.

1.2.4 The Relationship between Climate-Related Migration and Risk

Breathing is not a risk – it is something people do naturally – but breathing polluted air is a risk. Similarly, migration is not in and of itself a risk, it is simply something people do. When people move voluntarily to seek opportunities or to help their families adapt to climate hazards, such acts of migration do not cause harm and are therefore by definition not a risk (Gilmore et al. Reference Gilmore, Wrathall, Adams, Buhaug, Castellanos, Hilmi, McLeman, Singh and Adelekan2024). However, when migration occurs involuntarily or if people are undertaking migration that is inherently dangerous, such as paying organized criminals to transport them clandestinely across oceans or deserts, it is a risk. This is not simply an academic distinction. The words we use are important, and by associating migration with the term risk without any regard for the implications of doing so can feed into anti-immigrant sentiments and narratives in receiving countries. Migration out of communities exposed to climate hazards can, depending on circumstances, serve an important role in building adaptive capacity in the sending community, thereby reducing vulnerability and risk. An example of this would be out-migration of household members to seek wage labor opportunities with the goal of remitting money home to help the household build more robust housing capable of withstanding severe weather. Conversely, immobile individuals or groups that lack the ability to migrate may miss out on opportunities to gain remittances that facilitate adaptation and consequently see their risk increase over time.

Having completed our summary of climate science and climate risks, we now turn to the general question of why people move. At the conclusion of this chapter, we link climate science and climate risks with migration and adaptation processes, setting the stage for the more detailed analysis of migration and displacement in the context of specific climate hazards in the chapters that follow.

1.3.1 Migration and Displacement: Complex Causes, Diverse Outcomes

Every migrant’s experience is unique and personal to that individual. The reasons that lead up to the decision to move, the timing of the decision, the destination (or destinations), the reasons for selecting one destination over other possibilities, whether migration is temporary or becomes indefinite, the costs and benefits that result from migrating – these all differ from one person to another (see Box 1.6 for a discussion of temporal and spatial scales of migration). Yet in academia, in policymaking, and in popular discourse, we tend to describe migrants and migration not as unique individuals on the move, but as flows and stocks of anonymous people, and assign them labels according to generally understood categories of why people move: economic migrants, job seekers, guest workers, international students, nomads, asylum seekers, refugees, trafficking victims, and snowbirds, to name just some of the labels. These labels are easily recognizable, but the reality is that individuals may be on the move for many reasons. Consider someone who applies for a visa to study at a college or university in another country. They may simultaneously have multiple intentions and motivations. While studying, they might work part time and remit money home. They might be planning to remain in the destination country indefinitely and pursue a career there, or they may plan to return home after graduation and hope that having international qualifications improves their future career prospects. Or maybe they’re not sure what they will do next. Perhaps during their studies, they will fall in love with a fellow student, and this might alter their previously held plans and aspirations. Perhaps they will become unwell or do poorly at school and be forced to return home without the diploma or degree they hoped for. The key is to think of the act of migration not as simply a movement of person X from point A to point B, but as part of the larger trajectory of an individual as they move through the physical world and through the social world over the course of a lifetime. By doing so, we realize people may move multiple times to multiple places, sometimes returning back to places where they once lived, perhaps never going back. Every migrant’s story is unique and continually changing.

Now, having established that we ideally ought to think of every migrant as an individual on a unique journey through physical and social space, we authors contradict ourselves for the remainder of this book by generalizing the most common reasons why people move.

Scholars have many broad theories and explanations of why people migrate. Some theories relate to the decision to leave one place for another, other theories consider what draws people to particular destinations and how they become incorporated (or not) into the receiving communities, and still others consider the factors that mediate the migration process as a whole. What they all have in common is an understanding that migration outcomes are diverse, and are the product of complex interactions of cultural, economic, environmental, political, and social processes operating at multiple scales, from the local to the global. Some of the factors that influence a given individual’s decision to move (or not) are particular to that individual’s household or family, such as relationships with parents and siblings, the health of family members, the household’s livelihood(s), and financial savings, among many others. Other factors that influence migration decisions are beyond the influence or control of the household, driven by what we may refer to as macro-level processes, and include a wide range of possibilities such as interest rates, the costs of travel, cultural norms, and the presence or absence of political instability in a given country. These and other factors that shape migration decisions operate at multiple scales and are continuously interacting with one another, and so the conditions under which migration occurs are highly dynamic and always changing. Figure 1.9 provides a simplified illustration of how these complex interactions between macro-level processes and household characteristics affect the decision of an individual to migrate (or not). Each of the factors shown in the figure is now summarized in turn.

Figure 1.9 Simplified representation of how macro-level processes and household characteristics shape migration decision-making.

Adapted from Foresight 2011, McLeman 2014.

1.3.2 Economic Processes That Influence Migration

Economic conditions are an important influence on migration. Places that experience economic growth and a corresponding increase in employment opportunities often attract migrants from other places with slower growth and/or lower wages (Todaro Reference Todaro1969, Karras and Chiswick Reference Karras and Chiswick1999). The relatively large number and range of potential income opportunities in cities is an important driver of rural–urban migration, especially for young adults of working age (Todaro Reference Todaro1969, Sassen Reference Sassen and Scott2001). Migration can stimulate additional economic growth in the receiving area, as new arrivals bring skills that can enhance the productivity of businesses and increase demand for goods and services, assuming that newcomers are able to integrate and participate fully in the economy (Engler et al. Reference Engler, MacDonald, Piazza and Sher2020). Urban centers that develop large pools of economic and human capital may hold a stronger attraction to potential migrants than other places (Sassen Reference Sassen and Scott2001). In some economic sectors, such as agriculture, mining, forestry, and construction, large numbers of workers are required at specific locations and/or at certain times of the year, and temporary migrant workers may be recruited from elsewhere to fill such jobs as needed. In other economic sectors, such as light manufacturing and the garment industry, production facilities can be located anywhere that has reasonably good facilities for shipping goods to market and access to electrical power; in these industries, companies may deliberately seek out suppliers in countries where large numbers of low-wage workers are available. This became an important influence on migration patterns within many countries starting in the 1970s and 1980s, when global production of textiles, clothing, and footwear shifted dramatically from high-income countries to low-income countries in Asia and, to a lesser extent, Latin America (International Labour Organization 1996). Governments in low-income countries have often attempted to attract such facilities by establishing special zones where offshore companies can operate with low or no taxes, so long as production is for export. These concentrated areas of low-wage jobs draw laborers from across the host country and/or adjacent ones, generating new migration flows in directions that did not previously exist (Chang Reference Chang2015). Oil-rich, low-population countries in the Middle East region have in recent decades become a key destination for wage-seeking migrants, and now host over 24 million temporary migrant workers, 83% of them male, drawn from countries across Asia and Africa (International Labour Organization 2024).

Adverse economic conditions, as experienced through such phenomena as high rates of unemployment, commodity price swings, changing interest rates, price inflation, and changing currency valuations, can also have a strong influence on migration. For example, during the Great Depression of the 1930s, large numbers of unemployed North Americans migrated from city to city in search of work, some crisscrossing the continent on railway cars or on foot in search of work (Mitchell Reference Mitchell1996). During the “Great Recession” of 2007–2008, immigration to the US from other countries slowed temporarily because economic growth slowed and unemployment rose sharply (Singer and Wilson Reference Singer and Wilson2010).

The specific ways in which economic considerations influence migration decision-making are context specific. Economists have historically assumed that migrants (or people thinking about possibly migrating) are primarily opportunity-seekers that weigh the monetary costs and benefits of whether or not to migrate and assess potential destination options based on the information available to them (Todaro Reference Todaro1969). Labor markets and wage differentials combine to create potential migration pathways for people with particular types of job skills, training, aptitudes, and knowledge (Sjaastad Reference Sjaastad1962, Todaro Reference Todaro1969, Borjas Reference Borjas2001). The influence of wage differentials on migration is most evident in movements of people within the borders of countries (i.e. internal migration), where there are typically fewer restrictions on movement as compared with international migration, where high-wage/high-income countries often restrict entry of low-skilled labor-seeking migrants but deliberately favor and facilitate the entry of higher skilled workers from other countries. For example, under the North American Free Trade Agreement, Mexican citizens that hold university degrees in particular professional programs are eligible to be sponsored by prospective US employers for temporary work permits, but other Mexican workers are not, and may not have legal pathways to working in the US. The European Union takes a different approach to labor migration under the Schengen agreement, allowing citizens of member countries freedom to move about and seek employment. Because borders are less open to them, lower-skilled workers seeking to move internationally to higher-wage countries may get pushed into clandestine, riskier forms of migration that raise their potential to become victims of exploitation by untrustworthy employers and criminal organizations (Salt and Stein Reference Salt and Stein1997).

A different economic interpretation of migration behavior is known as the new economics of labor migration (or NELM), first described in the 1980s (so not especially “new” anymore). Instead of focusing on the cost-benefit analysis made by individuals, NELM considers the household to be the key decision-making unit, and suggests that migration is one of a variety of strategies that households may use to diversify income sources and reduce exposure to hardship and risk (Stark and Bloom Reference Stark and Bloom1985). An important point here is that the desire to avoid losses or harm can be as powerful a motivation for migration as is the desire to acquire wealth. So it is, for example, that in many cultures, young adults will leave the family home in search of job opportunities in distant locations and remit money back (Adger et al. Reference Adger, Kelly, Winkels, Huy and Locke2002, Connell and Conway Reference Connell and Conway2000). Massey et al. (Reference Massey, Arango, Hugo, Kouaouci, Pellegrino and Taylor1993) suggest that this type of migration strategy is more likely to be prevalent in areas where local income-making opportunities are scarce and other options for managing risk are unavailable, such as insurance, lending institutions, and financial and commodity markets. As an example, Rain (Reference Rain1999) describes in rich detail how rural households in West Africa send young adults to cities each dry season to reduce the number of mouths to feed while at the same time hoping they will find wage labor and send home cash remittances (Box 1.7). In that example, the migration is cyclical in nature; when the rainy season returns, so too do the migrants, as their labor is once again needed at home. In other examples in other places, household members may migrate away indefinitely, on the understanding that the migrant will repay the costs of their migration and absence from the household workforce by remitting money home (Erdal Reference Erdal2022).

1.3.3 Social Processes and Structures That Influence Migration

Some types of migration are inherently driven by social motivations, such as the desire to live near friends, to be reunited with family members, to get married, or to move into a retirement community of people of similar age and need. Social and economic considerations often go hand in hand when making a migration decision, so they are not entirely discrete factors, but from an analytical point of view, it is useful to distinguish social factors because of their potentially larger and longer-term effects in initiating and perpetuating migration movements. Even over short distances, migration has direct financial costs for the migrant and their household, such as transportation, visa fees (if applicable), tuition (of an international student), and the need to find accommodation at the destination. Intermediaries such as labor recruitment agencies, matchmaking services, and smuggling organizations (in instances where legal migration options do not exist) may also require up-front payments. There are also the indirect costs of migration to consider, such as the household’s loss of the migrant’s labor, the migrant’s absence from participation in family life, and the opportunity costs associated with giving up a known economic and social system for a less well-known one. Social networks can help migrants and those considering migration overcome some of the direct and indirect financial costs, particularly once an initial group of migrants has successfully established itself in a new destination and is able to facilitate the subsequent migration and incorporation of family, friends, and acquaintances from the sending community (Massey and Espinosa Reference Massey and Espinosa1997, Nee and Sanders Reference Nee and Sanders2001).

Examples of the benefits social networks provide include detailed information about how to cross borders, assistance in securing transportation over long distances, introductions to potential employers in the settlement area, and assistance in finding accommodation upon arrival (Massey and Espinosa Reference Massey and Espinosa1997, Palloni et al. Reference Palloni, Massey, Ceballos, Espinosa and Spittel2001). International migrant networks lead to the formation of what are known as transnational communities through which people, money, resources, and ideas flow between origin and destination countries, examples being the transnational Kurdish community connecting western Europe to Turkey (Faist Reference Faist2010) and the transnational community connecting the Dominican Republic to New York City (Georges Reference Georges1992). Although not a truly transnational community – for Puerto Ricans are US citizens – pre-existing migrant networks linked to cities in New York and Florida were critical in helping people in Puerto Rico recover from widespread destruction caused by Hurricane Maria in 2017 (see case study in Chapter 3). Not all migration movements automatically give rise to strongly interconnected transnational communities (Riccio Reference Riccio2001). The types and nature of social connections between sending and receiving areas can vary significantly from one case to another depending on the cultural group and the degree of assimilation or incorporation into the destination culture. Deeply ingrained social, religious, and class structures can have a significant influence on where migrants and would-be migrants perceive they can and cannot move and the extent to which receiving communities are willing to accept newcomers and facilitate their incorporation (Box 1.8) into the community. For example, in India large numbers of low caste workers, many heavily in debt to landlords, migrate each year to perform difficult and dangerous work in cities where they know they may be exploited and subject to abuse (Acharya Reference Acharya2021). They do so because the caste structure allows them few alternatives to earn a livelihood.

1.3.4 Cultural Influences on Migration

Cultural norms and understandings about migrants and migration can be influential at all stages of the migration process, from decision to migrate to the incorporation of migrants into the destination population. The relationship between culture and migration varies considerably across cultural groups. Relevant factors arise from the migrant’s own culture as well as from the culture of the dominant population at the destination, assuming they differ. For some groups and peoples, migration is a fundamental element of their culture. This is particularly the case for many of the estimated 200 million people worldwide who practice pastoralist livelihoods centered on moving herds of livestock on a seasonal or annual basis across diverse geographical settings, from Central Asia’s steppes to Sudano-Sahelian Africa to Scandinavia’s Lapland region (Kaufmann et al. Reference Kaufmann, Hülsebusch, Krätli, Ferranti, Berry and Anderson2019). The cultural practices, values, and beliefs of pastoralists are often markedly different from those of other, more politically dominant groups in the same geographical regions, and the mobility of the pastoralist lifestyle does not mesh well with private property regimes favored by the political majority in many countries. This has sometimes led to political instability and occasionally violent conflicts between pastoralists and their non-pastoralist neighbors, and between pastoralists and governments that have sought to restrict their movements or force them to move into fixed settlements (Fernandez-Gimenez and Le Febre Reference Fernandez-Gimenez and Le Febre2006, Penu and Paalo Reference Penu and Paalo2021).

In other cultures and communities, undertaking migration at a particular life stage, typically young adulthood, is an encouraged or at least widely accepted practice (Gabriel Reference Gabriel2006, Geisen Reference Geisen, Bekerman and Geisen2012) (Box 1.9). The underlying reasons may be primarily economic, such as seeking out higher-paying employment elsewhere and remitting money home. In some cultures and communities, households that receive remittances acquire not only greater wealth but greater social status within their communities, and this dynamic can place pressure on other young people to migrate as well (Sana Reference Sana2005, Suksomboon Reference Suksomboon2008). In other cases, the motives for youth migration may be primarily lifestyle related, with young people seeking out adventures and new experiences. As just one example, the ski resort industry depends heavily on large numbers of young workers originating from a wide swathe of countries, whose primary goal is to have ongoing access to skiing and snowboarding opportunities (Thorpe Reference Thorpe2017). Young people who migrate with economic motives may be eager to integrate into the destination culture, thereby enhancing their employment prospects, but those motivated by lifestyle considerations may be less concerned about integration and more interested in the experience of learning about new places and people (Samuel Hall Reference Hall2023). Youth culture around the world is changing rapidly in the era of social media, and so, too, are youth attitudes and experiences with respect to migration in terms of undertaking it themselves and in their views of migrants arriving in their own communities from elsewhere.

Marriage-related migration is an important feature of many cultures. The dynamics of how marriages are arranged or facilitated, the expectations of the families involved, and the desired social and economic outcomes vary considerably from one cultural group to another (Yeung and Mu Reference Yeung and Mu2020), and so it is important to be careful about making generalized statements about it from the perspective of a culture where it is not widely practiced. Marriage-related migration can lead people to move within countries and internationally, and in most cultures where it is widely practiced the intention is for young people to enter into a heterosexual marriage with a partner from a family with a similar cultural background, religious faith, and/or economic status (Ullah and Chattoraj Reference Ahsan Ullah and Chattoraj2023). Although not universal, in most cultures, it is the woman who is expected to move to join the husband, although arrangements may differ if the woman is working abroad in a higher income location and is able to bring her new spouse to join her. Higher levels of vulnerability in terms of personal safety for women and girls in societies can result from marriage-related migration, especially those moving from low-income households where the marriage is seen as an opportunity to raise the family out of poverty. It is estimated that 22 million people worldwide, two-thirds of them female, have been coercively forced into marriages they did not want, most often by their parents (International Labour Organization, International Organization for Migration and Walk Free 2022).

Marriage migration is just one aspect of the wider gender dimensions of migration of all types and motivations (see further discussion in Chapter 7). Although too numerous to summarize here, there are trends distinctive to female migration that warrant mention. Roughly half of all international migrants are female, and although this percentage has not changed much in recent decades, the proportion of women international migrants who move in search of employment has grown significantly (Global Migration Data Portal 2024). There are, however, notable variations between countries and regions. In many high-income countries of Europe and North America, the migrant labor workforce is disproportionately female, with migrant women making up growing numbers of workers in sectors such as domestic work and health care. By contrast, labor migration to oil-rich states in the Middle East is disproportionately male, the key employment sectors being industry and construction. The expansion of light manufacturing and garment, textile, and footwear production to low-income countries has in many instances led to increased female migration to fill such jobs (Jones Reference Jones2020). This has in many cases run contrary to traditional migration practices in such countries, where it was once primarily men that moved and stimulated changes in the perceived role of women in society.

Education shapes migration behavior and patterns in a variety of ways. The greater availability of educational opportunities can be an incentive for rural people to migrate to cities (van Maarseveen Reference van Maarseveen2021). In some countries, rural people with higher levels of education are more likely to move to cities than are other people, but this is not the case across all countries (Ginsburg et al. Reference Ginsburg, Bocquier, Béguy, Afolabi, Derra, Augusto, Otiende, Odhiambo, Zabré, Soura, White and Collinson2016). Educational attainment is often closely linked to international labor migration flows, with higher skilled workers often migrating in search of better wage opportunities and government regulations tending to make their movement across borders easier than for lower skilled workers. However, migrants’ education and skills credentials may be seen by employers in high-income countries as being less valuable, leading to newcomers to work in jobs below their skill levels and earning lower average wages than established residents as a result (OECD 2022). Many western countries encourage international student migration not so much from a sense of enlightenment but as a way of generating additional revenues for schools, colleges, and universities (international students contribute an estimated US$40 billion to the US economy each year (Greenfield Reference Greenfield2023)).

1.3.5 Political Influences on Migration

Three important ways by which political factors and processes influence migration behavior are through:

• the policies and regulations implemented by states and other political actors to shape migration and citizenship;

• general governance structures and processes that make a given jurisdiction more or less politically stable and economically prosperous relative to others;

• violence carried out by the state against members of its own population or those or its neighbors.

Government regulations on mobility and migration are an obvious constraint on the agency of migrants and potential migrants. States that allow their citizens a high degree of freedom with respect to mobility and travel tend to have higher numbers of people migrating internationally over long distances as compared with states where mobility and travel rights are restricted (Karamera et al. Reference Karemera, Oguledo and Davis2000). Under international law, states have the right to enforce entry across their borders and to make determinations with respect to legal right of residency within their borders and citizenship. Most states maintain entry controls at land border crossings, ports, and airports; some also maintain exit controls in order to have greater influence over anyone who might wish to leave. The passport and the visa are basic tools used by states to regulate the movement of people across borders. A passport is a government-issued identity document for the purpose of international travel that follows standards set by the International Civil Aviation Organization for its appearance and required contents. The passport allows the issuing state to exert some control over its nationals’ ability to travel abroad, and allows receiving states to establish the identity and citizenship of those seeking entry. A visa is a document issued by a state that allows non-nationals the right to appear at its border control points and seek admission. Its effect is to enable the issuing country to pre-screen those who would seek entry, keeping them at a physical distance until the traveler has demonstrated at a consulate, embassy, or authorized agent that they meet admissibility requirements. Such controls do not necessarily prevent unauthorized migration (see Box 1.10 for more on why this terminology is preferred); millions of people each year enter other countries using what are known as “irregular” channels (i.e. entering a country without authorization and without appearing at an official point of entry). The actual criteria for gaining admission and the length of time for which non-nationals are permitted to stay vary from one state to another.

States also determine questions of citizenship which typically includes an automatic right of indefinite residence within a state’s borders. Other rights and benefits accrue with citizenship, which vary according to the state. Common ones include the protection of the state, access to the labor market, and to state-provided services such as health and education and, in democracies, the right to participate in the selection of political leadership. In return, citizens defer to the state’s authority and undertake certain obligations, such as the requirement to pay taxes. Some countries may also require that citizens perform a period of military duty or other form of national service. Citizenship can be obtained through three general mechanisms, the specific details of which vary from state to state. One is consanguinity, whereby a child acquires the citizenship of its parent. Variations of consanguinity exist, with some states having broad rules that allow people with familial ties more distant than simply parent and child to claim citizenship on the basis of their ethnicity. A second way of acquiring citizenship is through place of birth, whereby countries automatically confer citizenship upon anyone born within their territory, regardless of the citizenship or residency status of the child’s parents. Not all countries do so, however, and a child born in a country that does not confer automatic citizenship can become stateless if the parents’ home country does not automatically confer consanguine citizenship (see Box 1.11). The third possibility is naturalization, whereby people who have migrated from another state may acquire citizenship according to some prescribed administrative process. Each state maintains its own rules and regulations regarding naturalization. States that allow little or no possibility of naturalization, such as many oil-rich Middle Eastern states, are committed to a policy of ethnic citizenship only, having no interest in integrating foreign workers.

Many states impose strict controls on the entry of foreign nationals, and will actively detain and deport those who violate immigration rules. This does not necessarily prevent people from attempting to enter; in a single month (December 2023) patrols encountered nearly 250,000 people trying to enter the US from Mexico at locations other than official border crossings, most of whom were immediately returned to Mexico (Gramlich Reference Gramlich2024). Governments in many western countries are increasingly using remote sensing devices, drones, and other advanced technologies to prevent undocumented entry at their land and sea borders (McLeman Reference McLeman2019). As a consequence, migrant smuggling organizations have become more active in organizing migration into the European Union from Africa, the Middle East, Turkey, and Asia (İçduygu Reference İçduygu2021, Frowd et al. Reference Frowd, Apard and Dele-Adedeji2023, Triandafyllidou Reference Triandafyllidou2022) and into the US via Central America and Mexico (Kim and Tajima Reference Kim and Tajima2022). Migrant smugglers are often embedded within larger organized crime groups whose methods are violent and exploitative, and prey upon the vulnerability of people on the move (Cleaveland and Kirsch Reference Cleaveland and Kirsch2020). This has in turn led to the occasional occurrence of migrant “caravans” – large groups of overland migrants who travel together toward the US border through Central America and Mexico in order to protect themselves from criminals, corrupt officials, and others that might exploit them (Frank-Vitale Reference Frank-Vitale2023).

Some governments impose controls on population movements within their borders to achieve particular political or economic ends (Coutin Reference Coutin2010). For example, the People’s Republic of China, has since the 1950s operated a system of household registration known as hukou (Cheng and Duan Reference Cheng and Duan2021). Under this system, the personal details of each household member are registered with the local government registration office, along with any changes due to births or deaths. Those registered in the hukou are entitled to basic social benefits such as education and primary health care, and are legally eligible to work in that jurisdiction, but they are not officially allowed to change their place of residence without permission. The massive industrial expansion in China that began in the 1990s created employment opportunities far better than those on offer in rural areas, leading hundreds of millions to move without official permission to large cities and industrial centers. They are sometimes referred to as China’s “floating population.”

There are many other ways by which the policies and practices of governments have a direct influence on migration. The legacies of colonialism and past conflicts continue to have impacts on migration and displacement at local, regional, and global scales, and amplify the vulnerability of previously displaced peoples to climate change and other environmental hazards (Sadiq and Tsourapas Reference Sadiq and Tsourapas2021, Farrell et al. Reference Farrell, Burow, McConnell, Bayham, Whyte and Koss2021). In some parts of the world, colonial-style removals of traditional and Indigenous peoples from their lands continue to occur, to make way for such activities as forestry and export-oriented industrial agriculture, in a phenomenon known as land-grabbing (Rude et al. Reference Rude, Niederhöfer and Ferrara2021, Vigil Reference Vigil2022). In politically unstable states that lack responsible government, such as Syria, Somalia, Sudan, and Afghanistan, a lack of personal security and periodic episodes of violence can result in large-scale internal population displacements and displacements across borders. The number of displaced people globally has risen significantly since 2010, exceeding 100 million people by the end of 2022 (Figure 1.10).

Figure 1.10 Estimated global number of displaced people, 1992–2022. Statistics include asylum seekers, refugees, and estimates of all internally displaced people, regardless of reason.

Data source: UNHCR 2024.

1.3.6 Environmental Influences on Migration (Non-climatic)

Weather and climate are only one subset of environmental factors that have the potential to influence migration decisions. Some environmental conditions are beneficial for human livelihoods and well-being, and places that feature these can become attractive destinations for migrants and experience population growth. Valleys of wide, slow-flowing rivers, coastal estuaries, and deltas generally make attractive locations for human settlements given the natural availability of wild foods, fertile soils, potable water for human use, and flat surface waters that allow for easy transport of people and goods. It is not a coincidence that the most densely populated regions on the Earth are located within such geographical features. Over 400 million people live in the Ganges River floodplain (GRID 2015), and of the world’s ten largest cities, only Mexico City is not located on a major river or in a coastal area. Other types of environments can also be seen as attractive for certain groups of people and/or for people at certain stages of their lives. For example, wealthy retirees will often move for part or all of the year to locations with pleasant temperatures, especially those with mild winters (earning such migrants the moniker snowbirds). Other people may be attracted to locations of scenic beauty with natural features such as mountains and lakes, a phenomenon known as environmental amenity migration (Abrams et al. Reference Abrams, Gosnell, Gill and Klepeis2012). The term digital nomads has been coined to describe people who earn their living through online work that does not require them to live in any fixed location, and so they move to places that have natural or cultural attributes that appeal to them (Chevtaeva & Deniczi-Guillet Reference Chevtaeva and Denizci-Guillet2021). The emergence of the online economy is just one example of how technological innovations have affected the relative attractiveness of particular locations and environments over time. For example, in the early decades of European settlement and colonization of North America the southern Great Plains were labeled on maps as the Great American Desert (Dillon Reference Dillon1967), mountains were seen as barriers to be overcome and refuges for only the poorest settlers (Case et al. Reference Case, Fowler, Morgan, Schwellenbach, Culbertson, Wescoat and Johnston2008), and the state of Florida – now home to over 20 million people – was lightly settled, seen as being a sweltering, malarial wetland (Faust Reference Faust1951). Technological innovations such as mechanized pumps to power irrigation and drainage systems, air conditioning (and cheap electricity to power it), insecticides, railways, interstate highways, and affordable air travel combined to transform places that were once to be avoided into places now seen as having favorable amenities (Gutmann & Field Reference Gutmann and Field2010), and led to the late twentieth-century growth of large cities in otherwise unlikely locations such as Phoenix, Las Vegas, Denver, and Orlando.

Environmental conditions – or changes in them – can also have adverse impacts on livelihoods and well-being, and thereby act as stimuli for people to leave, or in the worst cases, flee particular locales. Sometimes such changes occur for natural reasons, other times from human carelessness. Potable water is a basic requirement for human settlements, and loss or contamination of it by pollution quickly makes a location unlivable and triggers outmigration and eventual abandonment (McLeman Reference McLeman2011). Fertile soils on which food systems and rural economies depend can be easily degraded by poor land management practices. Land degradation exacerbates the impacts of droughts and can potentially increase the potential for out-migration from affected areas (Hermans & McLeman Reference Hermans and McLeman2021); this dynamic contributed to the large outflow of migrants from the southern US Great Plains during the 1930s Dust Bowl era (McLeman et al. Reference McLeman, Dupre, Berrang Ford, Ford, Gajewski and Marchildon2014). Although the present book focuses on climatic hazards, there are other types of naturally occurring environmental hazards as well, such as earthquakes, volcanoes, and tsunamis, and these cause destruction of homes and infrastructure that can lead to temporary and potentially permanent out-migration from affected areas, depending on the circumstances (Barclay et al. Reference Barclay, Few, Armijos, Phillips, Pyle, Hicks, Brown and Robertson2019, Basile et al. Reference Basile, Centofanti, Giallonardo and Licari2024, Rigg et al. Reference Rigg, Grundy-Warr, Law and Tan-Mullins2008)

A simple way of illustrating the range of potential migration responses to environmental phenomena is shown in Figure 1.11, in which duration of the migration event is represented on the horizontal axis and the nature of the environmental stimulus (i.e. hazards vs amenities) is on the vertical axis, with known examples of climate-related migration from the US given for each of the four quadrants. The two quadrants above the horizontal axis reflect the potential migration outcomes where environmental conditions are beneficial or attractive. The upper right quadrant describes migration undertaken on a permanent basis to take advantage of favorable environmental conditions, an example being high rates of population growth in the Sun Belt states in the era of air conditioning, much of it being fueled by north-to-south migration (Gutmann & Field Reference Gutmann and Field2010). The upper left quadrant reflects temporary or seasonal migration to access favorable environmental conditions, the previously described snowbirds being an easy example. The lower half of Figure 1.11 describes situations where changes in environmental conditions have adverse impacts on livelihoods and well-being, the lower right quadrant describing long-term or indefinite migration out of affected areas such as the 1930s Dust Bowl migration, and the lower left quadrant describing short-term or temporary migration in response to adverse environmental conditions, which often occurs following sudden-onset hazard events such as floods and extreme storms. Hurricane Katrina is one such event we assess in greater detail in the next chapter, and the multiple types of migration responses to it would sprawl across both lower quadrants of Figure 1.11.

Figure 1.11 Environmental influences on migration, with examples.

From McLeman (Reference McLeman, Dupre, Berrang Ford, Ford, Gajewski and Marchildon2014), reprinted with permission from Cambridge University Press.