Land use as climate change mitigation.

As the international community works to develop a postKyoto framework for responding to climate change over the next several decades, policymakers should give serious consideration to broadening the range of management strategies beyond those conventionally defined as “mitigation”. According to the Intergovernmental Panel on Climate Change (IPCC) mitigation concerns “implementing policies to reduce greenhouse gas (GHG) emissions and enhance sinks,” thus strategies assume reductions in the atmospheric concentration of GHGs to be the sole mechanism through which ongoing changes in climate can be slowed or arrested (1). Under the current mitigation framework, non-emissionsrelated responses to climate change are characterized as adaptive to rather than preventive of warming phenomena and, as such, generally have not been prioritized in national and international climate change policy. However, an established body of evidence suggests that land use is playing a measurable and significant role in ongoing climate change at multiple geographic scales and through a set of mechanisms independent of GHG emissions (2, 3). In light of this evidence, a more comprehensive and, ultimately, effective framework for climate change management must respond to both the atmospheric and land surface drivers of warming. The development of such a framework will require not only a redefinition of the terminology employed in national and international agreements but a fundamental reassessment of the governmental structure through which the climate problem is best monitored and managed. The significance of land use change to emissions of GHGs is well recognized, with approximately one-third of anthropogenic CO2 emissions since 1850 attributed to land use activities (4). However, recent work suggests alterations in surface fluxes of moisture and energy resulting from land use activities may hold more direct implications for regional scale climate phenomena than associated changes in emissions. The extensive conversion of forested areas to cropland in the Amazon basin, for example, has been linked to markedly drier and warmer climates in that region (5-7); the impact of land use change on temperature was found to be comparable to that of GHG accumulation (7). Likewise, extensive deforestation in both tropical and higher latitude forests has been associated with reduced rainfall, reduced cloud formation, and enhanced shortwave radiative forcing and temperature (8-10). At larger geographic scales, analyses of surface and atmospheric temperature trends across the U.S. and China find land use change to have played an approximately equal or greater role in warming trends over the latter 20th century when compared to changes in atmospheric composition (11, 12). As concluded by a review in Science of this growing body of evidence “[a]long with the diverse influences of aerosols on climate. . . land use effects may be at least as important in altering the weather as changes in climate patterns associated with greenhouse gases” (2). The influence of land use on climate is most pronounced at the scale of urbanized regions. Characterized as the “urban heat island effect”, alterations in surface energy and moisture fluxes, combined with anthropogenic heat emissions, can enhance near-surface air temperatures by several degrees Celsius relative to proximate rural areas (13). Recent work has found the conversion of land from forest or cropland to urban uses to be associated with a greater average increase in minimum and maximum temperatures than rural land conversions (14). Further, cities have been found not only to exhibit higher temperatures than proximate rural areas but also to be warming over recent decades at a significantly higher rate (11, 15-17). While urbanized land accounts for only a modest fraction of the global land surface, a rapidly expanding urban populationsnow accounting for the majority of the global population (18)sis increasingly vulnerable to rates of warming exceeding that of the planet as a whole. The influence of land use on climate change at the urban scale is clearly observed in temperature trend data for large U.S. cities. Figure 1 presents temperature anomalies (1951-1980 base period) for paired rural and urban weather N AS A Environ. Sci. Technol. 43, 9052–9056