Scientific methods to study how forests affect climate are distinguished as environmental monitoring, experimental manipulation, or modeling. Meteorological measurements of air temperature and wind speed in forests and adjacent clearings characterize microclimates. More complex measurements of energy, water, and carbon dioxide fluxes obtained using principles of eddy covariance required sophisticated instruments on tall towers extending above the forest canopy. In situ measurements of leaves and individual trees reveal physiological functioning and can be extrapolated to an entire forest. Whole-ecosystem manipulations that warm the soil or enrich the air with carbon dioxide provide insight to ecosystem responses to environmental change. Ecosystem studies monitor carbon and elemental stocks and fluxes, and watershed studies monitor water flows. Remote sensing instruments that acquire radiative signatures of the land provide an indicator of vegetation type, health, and productivity. Numerical models of terrestrial ecosystems and climate provide a means to test theories and develop understanding of the biosphere-atmosphere system.

Limiting planetary warming requires reducing emissions of carbon dioxide, and even removing carbon dioxide to lower the atmospheric concentration. Principal among the climate services of forests is their carbon storage. This is the basis upon which forest advocates call to protect, restore, and manage forests to mitigate the harmful effects of climate change. Large areas of new forests must be planted to offset anthropogenic carbon dioxide emissions. Yet the climate services of forests extend beyond just carbon. Forests influence climate through many biogeophysical and biogeochemical mechanisms, often in ways that augment the carbon benefits and sometimes to the contrary. To the carbon benefits of forests must be added their influence on temperature and precipitation through albedo, surface roughness, evapotranspiration, and biogenic aerosols. If the potential of forests to lessen planetary warming over the coming century is to be realized, their many influences on climate must be synthesized into an integrated understanding. In this, tropical forests are readily recognized as beneficial for climate.

There is an enduring connection within the human consciousness between forests and climate, whereby forests are understood to influence climate and that clearing the woods or planting trees changes climate. From several centuries, this idea exploded onto public awareness with the belief that clearing forests improved climate. The drive for climate betterment gave way to concern for a decline in rainfall as the forests were cleared, and the nineteenth century saw repeated calls in all reaches of the world to reforest denuded lands to increase rainfall. Meteorologists, however, dismissed an influence of forests on climate, and the science of forest meteorology was forgotten. Now, forests are again recognized for their climate benefits. Like our forebears, we again talk of purposely using forests to improve climate. Protecting existing forests, restoring degraded forests, and planting new forests are seen as critical to solving the climate problem. That the biosphere is fundamental to, not separate from, climate is a core tenet of the newly emerging Earth system science. This realization is not new. It is borne from the long, controversial chronicle of forests and climate change.

Forest microclimates are the climate within a forest in contrast to a nearby open area created by a clearing or pasture. A robust observation is that less sunlight reaches the ground in a forest than in open areas, but this depends on the size of the clearing, the height of trees surrounding the clearing, and the location in the clearing (relative to the edge) where measurements are obtained. Daytime temperature is generally lower in forest than in open areas, with greatest differences during the growing season; but this, too, is subject to edge effects and whether the canopy is sparse or dense. Wind near the ground is less than in exposed areas or above the canopy. Another point is that there are various microclimates within a forest canopy. The environment of the forest overstory differs from that of the understory, and this has important consequences for species conservation and protection from climate change. Forest microclimates are now recognized as an indispensable ecological service of forests. The forest canopy moderates under-canopy air temperature and buffers the understory from extreme temperatures, possibly lessening the ecological impacts of a warmer world.

There is a distinct poleward zonation of climate defined by gradations of progressively colder annual mean temperature in tropical, subtropical, temperate, boreal, arctic, and polar latitudes. Additional climate zones are defined based on annual precipitation and the seasonality of temperature and rain. The climate at large spatial scales extending over thousands of kilometers is known as the macroclimate. It is determined by geographic variation in solar heating of the planet, which sets in motion large-scale atmospheric circulations that transport heat poleward from the tropics, and also by proximity to oceans, which similarly transport heat in ocean currents. Mountains and large lakes create a regional climate that can deviate from the macroclimate. Climate at this scale, generally up to a several hundred kilometers , is referred to as mesoclimate. Variation in topography, soils, and vegetation creates local climates at a spatial scale ranging from a few to tens of kilometers, known as microclimates. A south-facing slope has a different microclimate than a north-facing slope. Forests have a different microclimate compared with open land.

The processes by which forests influence large-scale climate are well known, but the specific response to changes in forest cover varies with background climate (e.g., tropical, temperate, boreal), the extent of forest change and type of conversion, and time of year, and differs between day and night. Forests remove carbon dioxide from the atmosphere, thereby lessening planetary warming. Forests also affect climate through exchanges of energy, water, and momentum with the atmosphere, which can warm or cool the climate depending on geographic location, time of year, and time of day. There is a distinct latitudinal pattern from tropical to temperate to boreal forests, with different influences on temperature and different underlying mechanisms. In general, forests cool the daytime surface climate during the growing season through evapotranspiration and other non-radiative processes, and they warm the nighttime climate. Outside of the growing season, forests are generally warmer than open fields, especially in locations where snow is present. Further climate influences occur through chemical emissions that produce aerosols. Much of this understanding comes from models of Earth’s climate.

Planting trees for climate services – storing carbon, cooling surface climate, enhancing rainfall, providing aerosols that reflect solar radiation, creating favorable microclimate refuges, or other benefits – is not small-scale or immediate. It requires vast tracts of healthy and thriving forests and setting aside the land to grow forests for 50 to 100 years or longer. Achieving the climate benefits of forests requires a permanent forest presence over many decades. Climate will change during that time, and a forest planted today may not thrive in the climate of tomorrow. The forests of the future will grow in a climate different from today's and likely in regions of the world that differ from today's. They will be stressed by climate change, increased wildfires, disease, and insects. Asking forests to solve the climate problem requires a long-term commitment to and investment in forests and their health. Forest growth, too, is not one-directional. Wildfires, droughts, insects, and wind storms continually reset forests back to young stages of development. An old-growth forest that has accumulated enormous stores of carbon in its trees and soil becomes a young, regenerating forest.

Forests regulate climate through exchanges of energy and materials with the atmosphere. The idea that forests affect climate is not new. A vigorous debate about deforestation, reforestation, and climate change began during European settlement of the Americas, spreading to all regions of the world before collapsing in the early 1900s. The story of forests and climate change is told as being scientifically wrong and advanced for political, economic, or cultural reasons, but it has not been told from a modern scientific perspective. In fact, it represents the foundation for the interdisciplinary study of Earth as a system. Many of the questions posed in today’s study of climate change and climate solutions have their origins in the forest-climate question. The multicentury controversy over forests and climate change is a narrative in which purposeful modification of climate is longstanding, but by felling or planting trees. Earth system science is a centuries old idea, conceived in the long-held belief that forests influence climate and doomed to fail by the disciplinary specialization of the sciences. Narrow-mindedness prevented a vision of the world as an interconnected system.

Climate is but one way to perceive forests. In our comparatively short time on Earth, we humans have forged an inextricable bond with forests. Trees have inspired human imagination for tens of thousands of years. Forests are central to the cultural evolution of humankind and have molded our beliefs and values and societies. Our world would be impoverished without forests. The forest-climate question was of much common knowledge during the eighteenth and nineteenth centuries. Many of the scholars in the debate also opined on the aesthetics of trees and their contributions to the human experience. How we view forests, and how we value them, must be considered from a multifaceted standpoint that recognizes their many contributions to humanity and planet Earth, not just as public utilities that influence climate. The rationalism of science must be balanced with the romanticism of forests.

Planting trees to increase rain was the grand climate controversy of the nineteenth century. Some European scientists with diverse backgrounds in physics, meteorology, forests, and soils developed a new science of forest meteorology that blended meteorology, forest ecology, and forest hydrology. They sought answers in direct measurement of forest influences on climate and installed meteorological observatories in forested and open lands to obtain the necessary data. They explained forest influences in the laws of physics, fused with interdisciplinary knowledge of meteorology, forest hydrology, and forest ecology, and gathered the data to further their theories. It was an understanding based on observations of microclimates, but upon which was layered a dynamical framework applicable to macroclimates. Many of the findings have withstood the test of time, and the questions posed are still relevant to today’s scientists.

This chapter delves into the science of forests and streamflow. The amount of water that runs off the land into a stream is known as water yield. It is the water available for human uses. Evapotranspiration is a loss of water that reduces streamflow. Forests increase annual evapotranspiration and reduce annual streamflow compared with grasslands and other types of vegetation. The science, however, is not precise and our understanding is more qualitative than quantitative. Water yield and the climate services of forests represent conflicting demands for water. Forests cool the surface climate through evapotranspiration and remove carbon dioxide from the atmosphere during growth, but in doing so they consume water and reduce water yield for human usage. Consideration of spatial scale further muddies the policy implications because any precipitation benefits of forests occur at large spatial scales covering vast regions of land or entire continents, while the water cost of forests is felt at the scale of the watersheds that supply towns and cities.

The period from the mid-nineteenth century to the end of the century saw heightened interest in the harmful effects of deforestation on rain. There was growing fear that deforestation was turning prosperous lands into deserts, accompanied by efforts to conserve remaining forests or replant denuded lands. In India, Australia, New Zealand, South Africa, Russia, and the United States, the cry was raised: forests must be protected and managed for rain. These efforts followed prior concern in France and British island possessions. In this, forest conservationists advanced a need for government control of forests and used forest influences on rainfall as justification. Opponents, in turn, attacked the premise of forest influences on rainfall and decried the lack of evidence for climate deterioration. The ensuing debate was a narrative of misunderstanding, misuse of data, and hyperbole. It is this aspect of the forest–climate question, the so-called desiccation theory and its misuse to achieve policy goals, that has formed the historiography of the controversy, but beneath the rhetoric is found a fledgling knowledge of forest influences on climate that can be seen in today’s science.