So the destruction of vegetation leads to the emission of carbon dioxide. At the same time, the Earth’s flora renders part of the carbon dioxide released into the atmosphere harmless again. Plants absorb carbon dioxide from the atmosphere through photosynthesis and bind the carbon it contains in organic compounds, releasing oxygen in the process. In the 1990s, for example, each year, the continents reabsorbed around one gigaton (one billion tons) of the approximately 6.4 gigatons of carbon that were released annually through the combustion of coal, oil, and gas. The continents thus actually store 15 percent of fossil emissions each year. This phenomenon is referred to as the land carbon sink. In this way, the vegetation on the continents can counteract the increase in the global temperature, as global warming is directly linked to the increase in the concentration of carbon dioxide in the atmosphere: carbon dioxide reduces the permeability of the atmosphere to the thermal back-radiation of the Earth, and the lower atmospheric layers warm up as a result. Therefore, by absorbing carbon, the land carbon sink mitigates the increase in temperature that might otherwise be expected as a result of the combustion of fossil fuels and the expansion of agricultural areas.
However, vegetation is also relevant for climate in another respect. The different types of vegetation influence the exchange of energy, water and momentum between the atmosphere and the Earth’s surface. This affects particularly the regional climate. For example, grassland typically looks brighter than forest from a bird’s-eye view – scientists refer to this as a higher albedo. Grassland thus reflects sunlight better and warms up less. At the same time, forests evaporate more water through their leaves and needles, as they often have deep roots and can therefore cool themselves better than shallow-rooting grasslands. Which of these effects predominates – warming through solar radiation or self-cooling through evaporation – depends, among other things, on the position of the Sun, the availability of water in the soil, the level of atmospheric humidity and the type of vegetation involved. The vegetation cover thus shapes local climate in combination with the prevailing mean solar radiation, wind direction and precipitation. This is why we at the Max Planck Institute for Meteorology examine how strongly changes in vegetation influence the absorption of solar radiation and their consequences for the carbon-dioxide exchange between the land masses and the atmosphere.
Climate has changed naturally since the end of the last ice age 10,000 years ago. As a result, new plant communities have formed and spread. But on top of these natural changes, human activities, such as agriculture, forestry and urbanization, have had a substantial influence on the interaction of the atmosphere and the vegetation on the continents. Calculations have shown that, today, around 24 percent of global plant growth is controlled by humans. In the millennia between 9,000 and 5,000 years before the present day, agriculture and livestock farming developed independently of each other in at least four regions: the so-called Fertile Crescent of Asia Minor, parts of China, and Central and South America. From there, the cultures that practiced agriculture spread and gradually replaced the historically older hunting and gathering societies. Unfortunately, there are very few detailed records available on how much land in a region was used for agriculture at a particular point in time. This lack of data has hindered the study of changes in global vegetation distribution and their role in climate events to date.