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Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties
that is rarely considered in developing such strategies. One specific concern is the possibility that reducing aerosol concentrations could enhance radiative warming. In addition, the policy community has focused primarily on global mean radiative forcing and the associated response in surface temperature. Given the increasing realization of the significance of geographically dependent climate forcings, the policy community will need new forcing metrics and guidance on how to apply them.
Integrate Climate Forcing Criteria in Environmental Policy Analysis
Policies designed to manage air pollution and land use may be associated with unintended impacts on climate. For example, aerosol and ozone have significant impacts on human health, ecosystems, and climate. Emissions of aerosols, aerosol precursors, and ozone precursors are already regulated in the United States and other industrialized nations. Increasing evidence of their health effects makes it likely that aerosols will be the target of further regulations to reduce their concentrations in the future. To date, these control strategies have not considered the potential climatic implications of emissions reductions. Regulations targeting black carbon emissions or ozone precursors would have combined benefits for public health and climate. However, because some aerosols have a negative radiative forcing, reducing their concentrations could actually accelerate radiative warming. Understanding of the effect of aerosols on the hydrological cycle and vegetation is still incomplete, making it is difficult to predict the total effect on climate of reducing aerosol emissions.
The assumptions made about the magnitude of climate sensitivity are an important consideration associated with regulations that attempt to reduce aerosols. Several modeling studies have suggested that aerosol direct and indirect forcing may have offset as much as 50 to 75 percent of the greenhouse gas forcing since preindustrial times. At the same time, the IPCC Third Assessment Report and climate modeling studies attribute the large warming witnessed during the recent decades to the increase of concentrations of carbon dioxide (CO2) and other greenhouse gases. These two findings taken together reveal the possibility that climate sensitivity due to radiative forcing is in the upper range of the 1.5 to 4.5 K global-averaged surface warming for a doubling of CO2. This implies that attempts to regulate air pollution, which would reduce aerosol abundances, could inadvertently trigger a strong acceleration of global surface warming in the coming decades.
Policies associated with land management practices also need to consider their inadvertent effects on climate. The continued conversion of landscapes by human activity, particularly in the humid tropics, could have