Land-use change has accompanied human population growth for many years, but particularly the last few centuries. Before 1850, deforestation occurred mostly in the temperate regions of Europe, Asia and North America on the land considered most suitable for farming. Since 1900, most deforestation has been occurring in the tropical regions of South and Central America, South-east Asia and Central Africa, reaching a rate of 16 million hectares per year during the 1990s.
As with temperate deforestation, forest clearance in the tropics occurs mainly to acquire land suitable for agriculture, in particular for food and fuel crop growth in Africa and South-east Asia, and cattle ranching in South and Central America. Pressure on the land from mineral mining (to obtain gold, copper and tin), coal mining, and oil drilling is also particularly prevalent in parts of South America, Africa and South-east Asia. Since 2000, the rate of tropical forest loss has slowed, with approximately 13 million hectares of forest removed globally each year between 2000 and 2010. For the same period, the global net rate of forest area change is far lower than the rate of forest loss, due to the natural regrowth of forests on previously managed lands and extensive tree planting, particularly in China.
Deforestation has many impacts on the climate; the net effect of deforestation in any given location will be determined by how these different impacts combine. As a result of the large amount of carbon stored within terrestrial ecosystems, a considerable emission of carbon can occur during deforestation, predominantly through the burning of vegetation. Over the period 2002-2011, it’s estimated that roughly 1 petagram (1 billion tonnes) of carbon was emitted per year due to land-use and land cover change; this is equivalent to around 11% of the total anthropogenic emission. These emissions increase the concentration of carbon dioxide in the atmosphere which has a warming effect on the climate.
Most trees are darker in colour than other surface types; this means that a smaller proportion of incoming solar radiation is reflected by forest covered surfaces, imposing a warming on the Earth system (when compared to more reflective surface types such as grass and cropland). At high northern latitudes (above 60°N), snow covers the ground for a substantial fraction of the year; if the snow is lying on short vegetation, it will completely cover it and the reflectivity of the surface will be high, but if trees are present, they will protrude from the snow and the reflectivity of the surface will be much lower. In terms of deforestation, the dark colour of forests means that replacing them with a more reflective surface would have a cooling effect on the climate. LEAF scientists in the School of Earth and Environment are trying to use satellite observations to see just how big this difference in reflectivity really is after land has been deforested.
Deforestation also affects the transfer of moisture to the atmosphere (see Plants and Water). LEAF scientists showed that a return to the former high levels of deforestation in the Amazon would have substantial impacts on regional rainfall.