Evaporated water can cool surface of earth: Report

According to scientists, evaporated water helps in cooling earth as a whole, not just the local area of evaporation, a study said.

A new study by scientists at the Indian Institute of Science, the Carnegie Institution for Science at Stanford University and Lawrence Berkeley National Laboratory have demonstrated that evaporation of water from trees and irrigated crop areas could cool the planet which could have major implications on decision making for land use.

These findings were published online on September 14 in Environmental Research Letter.

"In the Indian backdrop of proposed Land acquisition Bill, this findings could have major implications for land-use decision making," he said.

"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," Govindasamy  Bala of the Centre for Atmospheric and Oceanic Sciences, IISc Bangalore told PTI.

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 Bala "it is the feedback loops in the climate system. 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," he said.

"The policy implications of this study are enormous. The recent Food and Agriculture Organisation estimate showed 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. Conversely, planting trees in the tropics could remove CO2 from the atmosphere as well as cool the planet through enhanced transpiration,"  Bala said.

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, he said.

For this study, the authors used a global climate model developed at the National Center for Atmospheric Research (NCAR), USA and is freely available to the climate modelling community around the world.

"The model has full representation for atmospheric processes such as air circulation and transport of heat and water vapour, clouds, evaporation from oceans and soils, transpiration from plants, 'radiative' heating by solar radiation and greenhouse warming effect by carbon dioxide and other trace gases in the atmosphere," Bala said.

Scientists from both the institutions  used a configuration of the model that has representation for the surface layer of the oceans as well as sea ice in the polar regions. This  type of climate model is also known as a general circulation model.

Bala said the climate model was run on supercomputers to study the effect of enhanced surface evaporation on the climate system.  
In one of their experiments, the authors increased the surface water evapotranspirtation to mimic increased tree cover on the planet.

Simultaneously, they also reduced the heat transfer from the surface, as in real world, so that the total energy addition into the atmosphere is zero. No global-mean warming is anticipated in this experiment. But, to their surprise, they found a cooling triggered by increased cloud cover in the lower levels of the atmosphere.

Bala pointed out that in the past, many climate modelling 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.