Water evaporated from trees cools global climateby Jonathan DuHamel on Sep. 19, 2011, under Climate change
“Scientists have long debated about the impact on global climate of water evaporated from vegetation. New research from Carnegie’s Global Ecology department concludes that evaporated water helps cool the earth as a whole, not just the local area of evaporation, demonstrating that evaporation of water from trees and lakes could have a cooling effect on the entire atmosphere.” –from press release Carnegie Institution.
The abstract from the paper:
Land use and land cover changes affect the partitioning of latent and sensible heat, which impacts the broader climate system. Increased latent heat flux to the atmosphere has a local cooling influence known as ‘evaporative cooling’, but this energy will be released back to the atmosphere wherever the water condenses. However, the extent to which local evaporative cooling provides a global cooling influence has not been well characterized. Here, we perform a highly idealized set of climate model simulations aimed at understanding the effects that changes in the balance between surface sensible and latent heating have on the global climate system. We find that globally adding a uniform 1 W m 2 source of latent heat flux along with a uniform 1 W m 2 sink of sensible heat leads to a decrease in global mean surface air temperature of 0.54 ± 0.04 K. This occurs largely as a consequence of planetary albedo increases associated with an increase in low elevation cloudiness caused by increased evaporation. Thus, our model results indicate that, on average, when latent heating replaces sensible heating, global, and not merely local, surface temperatures decrease.
Read full paper here.
Evaporation uses heat energy to put water vapor into the atmosphere. Water vapor is a strong greenhouse gas. Furthermore, when that water vapor condenses into rain or snow, it gives up its heat back to the atmosphere. The evaporation-condensation cycle is, however, energy neutral. The difference, apparently is that the cycle produces clouds which reflect sunlight, less energy reaches the ground. According to the Carnegie research, the net effect is one of cooling. IPCC climate models do not do well with clouds and assume a positive feedback, whereas the Carnegie research shows a negative feedback. Note that the Carnegie research is also a computer simulation. With all computer modeling, results depend on assumptions. In view of the poor record of the IPCC climate models versus observations, it seems that the Carnegie model is closer to reality.