New agricultural land is typically created by cutting down forests. When trees are cut down and burned or left to decompose, carbon goes into the atmosphere. In the present day, deforestation and the resulting net carbon source to the atmosphere is primarily occurring in the tropics. However, in the last 200 years, forest clearing for agriculture in the middle latitudes of the Northern Hemisphere was also a substantial source of carbon to the atmosphere. Since the mid-1900’s, much of the less-productive agricultural land in the US and Europe has been allowed to regrow as forests – making for uptake of carbon from the atmosphere through carbon accumulation in woody biomass and soils.
Uncertainty in estimates of the land use source are large in part because estimates of deforestation are, themselves, uncertain, and also in part because the amount of carbon stored in the forests is not well-quantified.
The Future of Land Use
A variety of models were used to project future land use for the emissions scenarios use for the IPCC AR4 report (IPCC Special Report on Emission Scenarios, Chapter 6, Figure 6-6b). These models suggest that deforestation will peak by about 2025 and then gradually decline with time. This trend is primarily driven by reduced rates of population growth and improving agricultural productivity. Eventually, several scenarios predict that land use, currently a substantial source of CO2 to the atmosphere, will become a sink of CO2 from the atmosphere as forests grow back.
In this image from the NASA MODIS Satellite, forest fires on the Yucatan Peninsula and northern Central America are highlighted in red. Smoke plumes can also be seen.
These terms are separated to clarify the direct impact of humans in forest clearing and subsequent regrowth (Land Use), and the natural system’s response to anthropogenic addition of carbon to the atmosphere and climate warming (Land Uptake). In many studies, however, it is impractical to precisely distinguish between these two terms and so often some component of the Land Use term (such as afforestation in the mid-latitudes) is effectively subsumed into the Land Uptake term.