Gray wave of the great transformation: A satellite view of urbanization, climate change, and biological productivity

Background/Question/Methods

Urban land transformation is being increasingly recognized as an important process in global climate change. Large urban conglomerates and their populations alter the biosphere locally by changing surface parameters and globally through increased demand for products of photosynthesis. We show results from several studies using satellite and statistical data to reveal relationships between city size, density, and biome context on the formation of urban heat islands (UHI) and estimate the impact of the combined increase in both population and per capita consumption on global Net Primary Production (NPP). We use Landsat and nighttime lights satellite products from the DMSP OLS sensor to map the fractional area of impervious surfaces (ISA) and we compare city size and ISA density to land surface temperatures from MODIS across biomes globally. We also look at urbanization’s impact on the global biosphere through human consumption of NPP. We use AVHRR and MODIS to estimate annual NPP on land and compare it to the per capita human requirement for food, fiber (fabrication), and vegetation-based fuel derived from the UNFAO database to produce a global balance sheet of NPP-carbon ‘‘supply’’ and ‘‘demand’’.

Results/Conclusions

We find that urban heat islands increase in amplitude with both city size and increasing ISA but are modulated substantially by ecological setting. Globally we find that cities in forested regions have the highest UHI’s (8ºC higher on average than non-urban areas nearby) whereas cities in desert areas can become heat sinks. The average amplitude of the UHI is remarkably asymmetric with a 4.3 ^oC temperature difference in summer and only 1.3 ^oC in winter across all biomes.  Our results show that as the global population becomes more urban, the per capita NPP requirement appears to be increasing. We estimate that as of 2005, humans require more than 25% (14.5 Pg C) of Earth’s total average net primary production on land (56.8 Pg C) annually (a 5% increase from the 1995 estimate). Regionally, the NPP-carbon balance percentage varies from 6% to over 90% and locally from near 0% to over 30,000% in densely populated urban areas. Scenarios modeling per capita consumption, population growth, and technology suggest that global NPP demand will continue to trend upward while the 20-year record of global NPP shows no trend.