Land-use change is estimated to contribute 10-30% of all current anthropogenic CO2 emissions. It is estimated that humanmade changes in land use have, until now, produced a cumulative global loss from the land of ~200 Pg C. Widespread deforestation has been the main source of this loss, estimated to be responsible for nearly 90%
of losses since the mid-19th century. These losses primarily occur due to the relatively long-term carbon stocks of forests being replaced by agricultural land.
The conversion of land from forested to agricultural land can have a wide range of effects as far as CO2 emissions are concerned. Soil disturbance and increased rates of decomposition in converted soils can both lead to emission of CO2 to the atmosphere, with increased soil erosion and leaching of soil nutrients further reducing the potential for the area to act as a sink for atmospheric carbon. Similarly, land reclamation and changes in land use management can effect an increase in terrestrial carbon uptake. Current estimates suggest that such land-use changes lead to the emission of 1.7 Pg C/year in the tropics, mainly as a result of deforestation, and to a small amount of uptake (~0.1 Pg C) in temperate and boreal areas, thus producing a net source of ~1.6 Pg C/year.
This balance between land-use change being a source and being a sink of CO2 is highly vulnerable to social and economic pressures. Humankind's need for wood - for fuel and construction - and our ever-increasing need for agricultural land have led to systematic clearances of forests across the planet in the past few hundred years. Today the pressure on forested areas is huge, with a rapidly growing human population requiring food and the land necessary for its cultivation. Increased awareness of the most sensitive way to manage land, tied to better agricultural practice and combined with political agreement on food trade and avoidance of deforestation, is required if land-use change is not to become an even greater net source of carbon to the atmosphere in years to come.
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