Land cover change - To what degree do human land cover dynamics affect climate change? [Present]

Historic changes in land use have altered the land surface significantly. For example, since the early 19th century, there has been a substantial increase in the area of cropland in the middle latitudes of the Northern Hemisphere. The pronounced tropical deforestation during the 20th century has paralleled the large-scale development of urban settlements and irrigated agriculture. The land-cover changes have resulted in a number of alterations in the regional and global climate system, primarily by: 1) Changing the surface albedo; 2) Changing the surface evapotranspiration; 3) Modifying winds, heat wave resilience, vulnerability to floods and other such factors in the proximity of human settlements; and 4) Modifying atmospheric CO2 uptake.

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Figure 1: Effects of land use and CO2 forcings on temperature change from pre-industrial to present-day in two heavily deforested areas (central North America and central Eurasia) as simulated with seven atmosphere-land models (de Noblet-Ducoudré et al. in press). Most simulations suggest that the propagating land use resulted in significant regional cooling, which approximately counteracted the concurrent CO2-related warming in these regions.

Changes in the albedo and evaporation have likely had a discernible effect on global mean temperatures since the late 19th century, although models show varying results of the net effects on climate (Pitman et al. 2009). Decreased forest cover has generally increased the surface albedo, thereby reducing the net energy available at the surface. This has possibly led to a downward modulation of the global mean warming rate (approximately 0.7°C since instrumental measurements began; IPCC 2007) by 0–0.1°C (de Noblet-Ducoudré et al. in press). Local land-atmosphere feedbacks generate large spatial variability of the land-use effects. In general, land-use-induced temperature changes are relatively small in the tropics, but increase significantly while moving to the equator. In areas with large deforestation (e.g. USA, central Eurasia) the local cooling has likely more than compensated for the global mean warming induced by elevated greenhouse gas concentrations (de Noblet-Ducoudré et al. in press; Fig. 1), although this finding needs to be balanced with the fact that deforestation itself has significantly contributed to the increase in CO2 (Pongratz et al. 2010). Net effects of land use on evaporation are more uncertain than those on albedo. Higher evaporation may be alternatively found over forests or grassland depending on the local conditions (Teuling et al. 2010).

Apart from the direct impacts on the physical climate system, large-scale deforestation has resulted in a significant release of carbon to the atmosphere, adding to the CO2-perturbation caused by fossil fuel burning. On top of the estimated 9.1±0.5 Gt carbon released from fossil resources in 2010, another estimated 0.9±0.7 Gt carbon was released by land-use change (Peters et al. 2011). Through the combination of CO2 and biophysical effects, deforestation is expected to lead to a net climate warming in tropical regions, but possibly to a net cooling in boreal regions (Betts et al. 2007, Bonan 2008). However, human management could also play a role, because areas that are deforested tend to have higher carbon content and less snow cover (Pongratz et al. 2011). Another marked effect of land-use change on climate is an increase in vulnerability to climate extremes, both because of the potential inability of forest areas to dampen temperature extremes during the early heatwave stages, and because of the increased exposure to extreme events like floods.

In the context of the 5th Coupled Model Intercomparison Project (CMIP5), many Global Circulation Model projections have been carried out for a number of future socio-economic scenarios, including land-use change. Early results indicate that the overall magnitude of projected land-use change (that is, the conversion of natural vegetation to cropland) is generally smaller than observed during the 20th century in all future scenarios. The regional differences, however, are pronounced. Sub-Saharan Africa is projected to experience a significant increase in agricultural area in most of the scenarios, even in the low-emission scenario targeted to meet the 2-degree global warming criterion. The local expression of land-use interaction with climate and the large spatial variability of the nature and degree of land-use change calls for an increasing focus on assessing impacts of land-cover change at a regional level.

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