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Radiative Forcing, Albedo in Land-Use Scenarios

Source:  Andy Jones and Dan Hawkes

Land-use and land-cover changes (LULCCs) are known by recent studies to influence climate directly, by altering surface physical properties such as the amount of reflected sunlight (albedo) or the amount of water transpired from soils to the atmosphere. These effects are most pronounced at regional scales, but some scenarios of future LULCC have been shown to affect global mean quantities, such as temperature and precipitation.

Jones_collinsIn a paper recently published in Climatic Change, a team of climate scientists led by ESD’s Andy Jones (and including ESD Climate Sciences Department Head Bill Collins) demonstrated the effectiveness of a new method for quantifying radiative forcing—a commonly used metric of human influence on the climate system that quantifies changes in the planetary energy balance—from land-use and land-cover changes (LULCC) within an integrated assessment climate model (the Global Change Assessment Model—GCAM). The new method relies on geographically differentiated estimates of radiative forcing from albedo change associated with major land-cover transitions derived from the Community Earth System Model. The team used these calculations to characterize the scale of non-CO2 climate effects for various scenarios of future LULCC, and to explore the implications of including these effects in hypothetical climate-change mitigation policies.

The team found that conversion of 1 km2 of woody vegetation (forest and shrublands) to non-woody vegetation (crops and grassland) yielded between 0 and −0.71 nW/m2 of globally averaged radiative forcing determined by the vegetation characteristics, snow dynamics, and atmospheric radiation environment characteristic within each of the 151 regions they considered globally. Across a set of scenarios designed to span a range of potential future LULCC, they found that LULCC forcing ranged from −0.06 to −0.29 W/m2 by 2070, depending on assumptions regarding future crop yield growth and whether climate policy favored afforestation or bioenergy crops. This range is similar in magnitude to historic forcing from several factors, including the greenhouse gas nitrous oxide.

Inclusion of this previously uncounted forcing in the policy targets driving future climate mitigation efforts led to changes in fossil fuel emissions on the order of 1.5 PgC/yr by 2070, corresponding to a 12–67% change in fossil fuel emissions depending on the scenario. In all scenarios considered, inclusion of albedo forcing in policy targets increased forest and shrub cover globally.

To read the paper, go to:
Citation: Jones, A.D., K.V. Calvin, W.D. Collins, and J. Edmonds (2015), Accounting for radiative forcing from albedo change in future global land-use scenarios.  Climatic Change, DOI 10.1007/s10584-015-1411-5.