We turn now to the third of the three branches of science that underpin our changing climate: radiant energy – the energy carried by light and other electromagnetic waves – and the ways it interacts with the gases and particles in our atmosphere. In this chapter we will outline the necessary science, and especially the laws of thermal emission. It turns out that we are bathed in electromagnetic radiation that we can’t see. We will then show how these laws lead to the counter-intuitive outcome that the ‘temperature’ of our planet is actually well below freezing! Fortunately, however, our Earth has a ‘good’ atmosphere, containing some very important gases – greenhouse gases – which act as a blanket, keeping the surface at a suitable temperature for liquid water, and hence for life. A simple model will help you gain a basic understanding of the greenhouse effect.

For centuries, humans have been using the atmosphere as a dumping ground, starting with our first use of fire. After the Earth came out of the Last Glacial Maximum ~11,000 years ago, things began to ramp up, as people congregated in settled communities. The arrival of the Bronze Age, about 5,000 years ago, added the waste products (and their odours) from the smelting of copper and other ores to the (local) atmospheric burden. The first voices of complaint were probably heard in the ancient societies of the Nile and Mesopotamia. The Greeks and Romans expected their leaders to keep the air in their towns clean.

Thermodynamics is the science that governs most of the basic, or ‘static’, properties of a system such as a gas. In this chapter we will be focusing on thermodynamics of the dry atmosphere. Water vapour is sufficiently important to deserve a separate chapter. A thermodynamic system is characterised by a small number of state variables, primarily density, pressure, and temperature. After defining these, we will see how they vary as we move up through the atmosphere. Two key equations – the hydrostatic equation and the ideal gas equation – interconnect these variables and their profiles. The First Law of Thermodynamics tells us how the atmosphere responds to an input of heat, as well as a change in pressure, which may come about with elevation (vertical motion). This is the key to the atmosphere’s vertical stability, and (ultimately) to cloud formation.

The Earth’s radiation balances are being altered by a number of changes in the composition of the atmosphere, and as a consequence the climate system is being ‘forced’, almost certainly in the direction of higher temperatures. In earlier chapters we examined the physics of the processes involved. What are likely to be the effects of such changes? The only way to answer questions such as this is to model the Climate System in sufficient detail. Firstly, we need to model the atmosphere, something we have been doing for half a century to forecast the weather. Because of the significant exchanges of both heat and water between the ocean and the atmosphere, it is clearly necessary to couple an ocean model. Ice sheets are likely to be affected by warming, as they are one of the key feedback processes just mentioned. The land surface also has significant interactions on various timescales.

We experience weather in the air around us, and that implies climate: the statistics of weather. But if we are to understand climate, and especially how it changes, we need to examine all its interactions with the planet. In the previous chapter we looked at the role of sea-surface temperature. If we are to go further, we need to look at other components of our planet and their atmospheric interactions. When we see the range of interconnections, we realise that we are dealing with a complex system, with multiple interactions between components. Whenever we see such a system, we must realise the potential for one interaction between components to end up impacting back on another: this is feedback. If we are to understand climate, we must understand these feedbacks. In the last sections we will illustrate one of the most important of these processes with a ‘relatively simple’ model.

Over the past century, average temperatures have risen a little over 1°C. This may not seem like much: after all, temperatures vary from one day to the next by much more than that, and we take it in our stride. However, over the past couple of decades we have become more aware of the rising incidence of what we call extreme events: heatwaves, droughts, wildfires, floods, severe storms. These are the signs of the times; signs, perhaps, that Mother Nature is not happy. Or is this all simply part of the natural unpredictability of the world we live in? In this chapter we look at recent extremes, along with the recent branch of climate science, Event Attribution, where we endeavour to assess any human contribution to these events. We illustrate both the nature of extreme events, and our growing understanding, with several detailed case studies.

Apart from a tiny amount of energy generated in the core by radioactive decay, and tidal energy from the gravitational interaction of the Moon, the Earth generates no energy of its own. Yet this is a planet full of life, both biological, and physical – assuming we may stretch the language, and call atmospheric circulation, and the hydrological cycle, forms of ‘physical life’. (James Lovelock’s Gaia hypothesis treats the entire planet as a living organism.)

One of the fundamental facts about our planet is that there is a major geographic gradient in the inflow of solar radiant energy, which is manifested in the large temperature difference between equatorial and polar regions. However, this difference would be significantly greater if it were not for the considerable transport of energy by the Earth’s two fluid components, primarily the atmosphere. We start with a semi-qualitative discussion of the forces that act on a fluid, remembering that we live on a rotating Earth. While it is pressure differences/gradients that move the air, it is temperature gradients that sustain these pressure gradients. The general circulation of the Earth’s atmosphere, the dominant influence on the weather and climate most of us experience, is the result. It governs the transport of heat from the tropics to higher latitudes, and also explains why the world’s deserts are where they are.

This book was written primarily because of concerns that our climate is changing, and many of those concerns centre on changes in the composition of our atmosphere, especially the rapid increase in the carbon dioxide concentration. Methane is also a concern. In this chapter we will examine the major exchange processes that cycle key elements through our environment, from the atmosphere and oceans, to the biosphere, and even the ‘solid’ Earth, and back again. Our primary focus will, of course, be on the key element carbon, as this encompasses both carbon dioxide and methane. We will also look at the fascinating element nitrogen, and the greenhouse gas nitrous oxide. While our primary focus will be on the natural biogeochemistry of these gases, we will present some of the latest data on how these gases have been increasing in our atmosphere in recent decades.

Some of the most important data on the state of the environment, and how it may be changing, are supplied by satellites, which have the enormous advantage of near-global coverage. They supply some of the most important data used in quantifying environmental change. What sort of measurements do satellites make, and how are we able to obtain meaningful, quantitative information about the atmosphere, or the Earth’s surface, from a sensor more than 500 km above? One key to this question is found in the previous chapter. There we saw that molecules have discrete absorption lines/bands. With a suitable space-based instrument we can observe these, using either thermal emission or absorption. The details of these processes may depend on temperature, so this information may also be inferred. By contrast, aerosol particles (mainly) scatter light, with relatively little wavelength variation: this presents a different challenge.

Virtually all of the energy at the surface of the Earth comes to us from the Sun. This is the energy that, in combination with the greenhouse gases in the atmosphere, maintains our current surface temperature in the range where liquid water is present. In this chapter we will examine solar radiant energy, and its most important interactions with the various constituents of the Earth’s atmosphere. Most of it reaches the lower troposphere, some to be absorbed by water vapour, and some to be scattered by clouds, molecules, or aerosols. The rest (roughly half) reaches the ground. Human activity has the power to alter these inflows, in a number of ways. We devote one section to the potentially disastrous Nuclear Winter scenario, and another to the potentially useful idea, known as Geoengineering, of reflecting some of the incoming sunlight in order to re-balance the Earth’s energy budget.

Over the past century or so, and especially since the end of World War II, our climate has been changing at an unprecedented rate. Human activity has been changing the composition of our atmosphere, and has also been responsible for significant changes to the land surface. And we have seen that the Earth’s surface temperature has risen by around 1°C. Are these changes connected? There are two steps in answering this question. The first is to quantitatively connect the changes just mentioned with any changes in the radiant energy fluxes into and out of the Earth–atmosphere system: the fluxes which ultimately govern our climate. This is the thrust of this chapter. The second of these steps, determining the response of the Earth System to any flux changes, requires a ‘full-scale’ 3D model of the system: that will be the subject of the next chapter.

Aerosols, and rising levels of certain aerosol types, are now recognized as important players in our environment, including climate change. Aerosol, or ‘particulate matter’ (PM), is the collective name for small particles and droplet solutions, with sizes ranging from ~1 nm to ~20 μm, suspended in the air. Aerosols vary enormously, in many ways, and in this respect, they differ radically from gases. In Section 4.7 we will look at their optical properties (i.e. their ability to scatter and absorb radiation) and the effects these may have on the energy flows that are central to climate. Aerosol particles are also the seeds of all cloud droplets, as will be discussed in Chapter 6, and so are a key component of the hydrological cycle. Finally, aerosols may be a pollution issue, and are a central component of air quality standards. For interested readers, we close with a section on aerosol research.

In the first three parts of this book, we have developed the three branches of science – chemistry, physics, radiant energy – that are central to understanding the climate of planet Earth. In doing so, our primary focus has been the atmosphere, with occasional brief excursions into the oceans. It is now time to take a broader look, and start by asking some important questions.

Human activities, principally through emissions of greenhouse gases, have unequivocally caused global warming, with global surface temperature reaching 1.1⁰C above 1850–1900 in 2011–2020.’ These words are from the Summary for Policy Makers of the Synthesis Report of the IPCC’s 6th Assessment Report. It is the aim of this book to help you understand exactly how, and why, the collective body of scientists has come to these, and many other, conclusions. Addressing this aim requires that you understand two facets of the nature of our planet. Firstly, you need to understand how our planet ’behaves’ under what we might choose to call ’normal conditions’. Secondly, you need to understand how human activity has moved us away from that normal. The following chapters will take you through both of these in the necessary detail. In this chapter we present a quick overview, as well as introducing you to the IPCC.

We know that the Earth’s climate has changed in the past, including times when humans had yet to put in an appearance. These changes must, therefore, have been the result of ’natural processes’. So how do we know that current changes are not similarly natural? The challenge this poses for science is to understand the processes that were at work in the past, and compare them with the climate drivers, both natural and anthropogenic, at work today. There are two reasons to try to unravel the climatic history of our planet. The first might be described as its intrinsic interest. The more important reason is to see what lessons we can learn that might help us think about potential future climate change, and especially the much slower response of sea level to changes in temperature. This is the reason that it is now an important component of the IPCC process.

The inflow of solar radiant energy into the Earth System is relatively straightforward. The outward flow of terrestrial/long-wave radiation is a different kettle of fish. The majority of the energy radiated by the surface is absorbed by gases in the atmosphere, again mostly in the troposphere. Much of this is then radiated back to the surface, warming it. This, of course, is the greenhouse effect. So now we must turn our attention to the central questions that we have so far glossed over. Why is it that some gases are radiatively active – i.e. greenhouse gases – while others are not? Exactly how do these gases absorb, and re-emit, long-wave radiation? This latter question is central to the whole field of global warming – also known as the enhanced greenhouse effect – as it is the key to being able to quantify the effects of any increases in these gases.