Using MODIS data to examine the influence of land cover on climate extremes

The influence of land cover on climate extremes, which can have adverse societal, ecological, and economic consequences, is largely unknown. Climate models predict more frequent heatwaves in the future, as a result of increases in both mean temperature and temperature variability. Recent studies have attributed changes in temperature extremes to anthropogenic forcings, but the influence of land cover remains largely unquantified. The effects of land cover change on climate include changes in emissions of carbon dioxide and other trace gases (biogeochemical effect), and direct changes to surface radiation and turbulent fluxes (biophysical effect). The biogeochemical effects of land cover change are global in nature, as greenhouse gases are well mixed in the atmosphere. In contrast, the biophysical effects of land cover change are highly localized, and vary in sign and magnitude geographically due in part to local background climate, natural ecosystem structure, and land management strategies. The biophysical effects of land cover change include changes to surface albedo, surface roughness, and evapotranspiration (ET). The surface temperature response to these competing biophysical processes is complex, but a latitudinal pattern has been observed through climate model simulations and surface air temperature measurements. Generally, deforestation produces a warming effect due to the lower ET rates of cleared land, while deforestation in high latitudes recues average air temperature due to changes in surface albedo and roughness. A latitudinal asymmetry exists such that the reduction in average air temperature in boreal zones is greater than the warming effect in the tropics.

While knowledge of the influence of land cover change on mean climate conditions is important, understanding the causes of temperature extremes is arguably more important to society than temperature means. Additionally, identifying the anthropogenic causes of temperature extremes in the current climate will help us understand how extremes are likely to change with increasing atmospheric greenhouse gas concentrations. The overall objective of this research is to assess the impact of land cover on temperature extremes. More specifically, this research will investigate (1) how land cover influences the statistical distributions of maximum temperature (Tmax), and minimum temperature (Tmin) across a latitudinal gradient using MODIS land surface products, and (2) identify the mechanisms behind spatial and temporal variation in extreme temperatures using a global climate model.

This research uses MODIS land surface products derived from sensors aboard the Aqua satellite to characterize the effect of land cover on extreme temperatures across North and South America. The 8-day composite products available from Aqua at 1:30 and 13:30 local time serve as a close approximation of Tmin and Tmax. Additional MODIS products including albedo, ET, and land cover classification are used to determine the causes behind variations in temperature extremes. This research is focused on building statistical distributions of Tmax and Tmin for different land cover classifications, and investigating the causal relationships behind observed changes. This study will take a space-for-time approach in quantifying the effect of land cover change. Rather than comparing surface climate before and after land use change, a comparison between adjacent sites in the same background climate conditions is conducted. Finally, NCAR's earth system model, CESM is utilized to identify the mechanisms that contribute to spatial and temporal variations in extreme surface and air temperatures. The forthcoming results of this research have important implications for the attribution of climate extremes to anthropogenic activities. Further, due to the negative socioeconomic impacts associated with climate extremes, the results of this research will be of great interest to the public and policy makers.