People have been transforming natural ecosystems to grow food, and to obtain firewood and timber, for millennia. Today, about 40% of the ice-free landsurface is covered by crops or pastures, and in many parts of the world we continue to expand these areas because the world’s population is growing, and this growth requires resources we obtain from the land.
When investigating the effects of transforming natural ecosystems, scientists often distinguish land-cover change from land-use change. Land-cover change
describes the transformation of an ecosystem type, for instance the replacement of a natural forest or natural grassland with agricultural crops. Land-use and land-use change describes the way that crops, pastures, or forests are managed. This can include a change in the amount of fertiliser or irrigation applied, or animal grazing density, or a change in the tree species composition of a managed forest.
Land-cover change and land-use change both interact with climate change, and in this booklet we will not differentiate between the two; it is, however, important to be aware that both are aspects of what is termed here solely "land-use change".
Land-use change has many effects on climate change. The best known of these are identifiable via the greenhouse gas content of the atmosphere. Greenhouse gases like carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4) affect the earth's climate and these gases have increased in the atmosphere due to human activities, especially over the last 100-200 years. The greenhouse gas concentration in the atmosphere can be measured directly from a set of observation stations around the world, but also, for instance, in air-bubbles trapped in glaciers - and these air bubbles can give us a very good record of greenhouse gas levels hundreds and thousands of years ago. The exact chemical signature of the greenhouse gas molecules (the so-called isotopic composition) helps to identify human activities as the main source for this increase.
When forests are replaced by pasture or crops, a large amount of CO2 enters the atmosphere, most of it directly (if the forest is burned), or over the ensuing years (when the wood products are out of use and are subsequently burned or beginning to decompose). A lot of carbon is stored in the tree stems, but in addition the remaining tree roots die and are decomposed to CO2 by soil organisms. Since crops and grasses do not have stems, and have less root biomass than trees, agriculture and pasture ecosystems contain less carbon in total than a forest - carbon is thus "lost" to the atmosphere upon deforestation. Carbon can also be "retaken" from the atmosphere if forests replace crops, but the area, globally, where forest increases is relatively small. It has been estimated that around one third of the total anthropogenic CO2 in the atmosphere today originates from land clearance over the last decades to centuries.
Like CO2, N2O and CH4 are important greenhouse gases. Around 50% of the N2O that can currently be measured in the atmosphere may originate from agriculture, mostly from fertiliser use. Nitrogen-containing fertiliser is partially taken up by plants, to support growth. Part of it, however, remains in the soil, where microbes transform it into various N-containing gases, including N2O, which then diffuses back into the atmosphere. CH4 is also a by-product of soil microbial activities in rice paddies, produced by so-called methanogens. These microbes use dead plant material to "feed" themselves and to grow in conditions of low oxygen (found in rice because it is often grown in flooded soils), and CH4 is the end-product of their metabolism. And, CH4 is produced in the stomachs of ruminants, particularly cows. At present, rice paddies and livestock jointly contribute nearly half of the total annual man-made methane emissions. These greenhouse gas emissions contribute to climate warming. Their effect is important for climate globally, because these gases are chemically low-reactive and long-lived, and therefore have plenty of time to become mixed in the atmosphere. They remain in the atmosphere for decades to many centuries, until they are eventually removed by physical or chemical processes.
Land-use change also affects climate by processes unrelated to the emission of greenhouse gases. These processes are often summarised as "biophysical", and they operate by affecting radiation and evapotranspiration. In short, when sunlight hits the land surface, a proportion of this light is directly reflected back to the atmosphere, and the remain deris absorbed. The amount of reflection is called albedo, and the albedo of a dark surface is lower than that of a light surface. A forest landscape, therefore, has a lower albedo than a cropland or grassland, and this affects the forest's surface temperatures, as the absorbed sunlight is turned into heat. In ecosystems that are managed for food production, more sunlight is reflected compared to a forest, and thus their landsurface temperature is relatively lower than that above a forest.
This is not where the story ends, however, because the absorbed radiation is only partially turned into heat; another part is used to move water vapour from ecosystems into the atmosphere. This process is known as evapotranspiration, and consists of watervapour loss from soils (evaporation) and from plants via their green leaves (transpiration). High evapotranspiration leads to cooling. Whether or not a natural forest ecosystem will have higher rates of evapotranspiration than a crop or a pasture system is difficult to say: it depends, for example, on the global region in which the plants grow, the rooting depth of the natural versus the managed vegetation, and whether or not the crop is irrigated. In some regions, the occurrence of droughts has been linked to biophysical land-use change processes, but in other regions, land-use change can even yield a local cooling.
Land-use change is also an important consideration in relation to emissions of trace gases that act as a precursor to the formation of ozone in the lower parts of the atmosphere. In the lower air layers, ozone is a greenhouse gas which contributes to climate warming. Finally, aerosols and their precursors are also climate-change agents, with either a warming or cooling role. In contrast with greenhouse gas emissions, the effects of biophysical processes and of reactive trace gases and aerosols are therefore mostly restricted to the region where the change occurs, and can either contribute to a warming or cooling effect.
These many climate-related aspects of land-use change, and the fact that they operate over different scales of time (days to centuries) and space (regional to global), pose a large challenge when aiming to understand all the effects of past, present and future land-use change on climate. Furthermore, emissions and biophysics are not only determined by the way we use our land, but actually respond to climate change themselves; warmer temperatures will enhance the underlying biological and chemical processes, leading to enhanced emissions and biophysical processes. In this way, they feed back to climate change. But does climate change also affect land use and land-use change? Clearly so; climate in a given region is an important determinant of the type of food or timber grown there, as it determines the available water for irrigation, and impacts yields (through, for example, droughts, floods, and frost).
Climate change will thus affect harvests, both locally and regionally, either positively or negatively, which is one factor that influences how people choose to manage land. Climate is, nonetheless, only one aspect of such decisions, and other factors, such as economic, social or political change, are fundamental to the understanding of land-use change.
How agriculture and forestry change climate, and how we deal with it (Booklet, EU-Project LUC4C)