Pushing and Pulling

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Science  18 Jun 2004:
Vol. 304, Issue 5678, pp. 1719
DOI: 10.1126/science.304.5678.1719a

The global average surface temperature increased by approximately 0.4°C between 1960 and 1990, even though incident shortwave radiation (sunlight) at land surfaces decreased significantly over the same period. This apparently contradictory pair of observations requires either an increase in the downward longwave radiation (heat) that outweighs the decrease in shortwave radiation, or a decrease in surface cooling due to reduced surface evaporation. Wild et al. used a global climate model to calculate how much downward longwave radiation may have changed but could explain a third or less of what is needed to account for the upward temperature trend. The authors suggest instead that reduced evaporative cooling at the land surface is responsible for the divergence of the trends in surface radiative heating and temperature. Recent independent reports also show that the rate of pan evaporation (how fast water evaporates from an open container or body) has decreased during that time. Thus, increased advection of moist air from ocean to land areas may explain the apparent contradiction. — HJS

Geophys. Res. Lett. 31, L11201 (2004).

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