Research Highlights

Can water evaporated from the surface cool the Planet?

- Govindasamy Bala, CAOS

We all know trees provide shading and they also cool the local environment by evapotranspiration. However, scientists have long debated the role of plant transpiration on global climate. The reason is that while evaporation causes a local cooling, condensation of the same water heats up the atmosphere somewhere higher but within the climate system. Globally, this cycle of evaporation and condensation moves energy around, but cannot create or destroy energy. How could then evapotranspiration change global climate if the net heating is zero?

According to a new study by scientists at the Indian Institute of Science, the Carnegie Institution for Science at Stanford University and Lawrence Berkeley National Laboratory, evaporated water helps cool the earth as a whole, not just the local area of evaporation, demonstrating that evaporation of water from trees and irrigated crop areas could cool the planet. These findings, published online on 14 Sept 2011 in Environmental Research Letters, could have major implications for land-use decision making.

It is well known that clearing of forests for agriculture and infrastructure development can contribute to local warming by decreasing local evaporative cooling, but it was not understood whether this decreased evaporation would also contribute to global warming.

When there is no net heat added to the planet, how does a change in surface evapotranspiration cause planetary cooling? “It is the feedback loops in the climate system” says Prof. G, Bala of the Indian Institute of Science in Bangalore. “In this case, it is primarily the cloud feedback. Enhanced surface evaporation causes an increase in the amount of low level clouds in the atmosphere. These clouds scatter more solar radiation back to space and cool the planet”.

In the past, many climate modeling studies on tropical deforestation studies have indeed simulated the local warming from deforestation. But because water vapour plays so many roles in the climate system, the global climate effects of changes in evaporation were not well understood. The researchers even thought it was possible that evaporation could have a warming effect on global climate, because water vapour acts as a greenhouse gas in the atmosphere.

“The policy implications of this study are enormous. The recent Food and Agriculture Organization estimate shows that around 13 Million hectares of forests are converted to other uses or lost each year. In addition to contributing to global warming through CO2 emissions, deforestation in the tropics could be causing more global mean warming through reduced evapotranspirtation” says Prof. Bala.