Water evaporated from trees cools global climate

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. These findings, published September 14 in Environmental Research Letters, have major implications for land-use decision making.

Evaporative cooling is the process by which a local area is cooled by the energy used in the evaporation process, energy that would have otherwise heated the area's surface. It is well known that the paving over of urban areas and the clearing of forests can contribute to local warming by decreasing local evaporative cooling, but it was not understood whether this decreased evaporation would also contribute to

The Earth has been getting warmer over at least the past several decades, primarily as a result of the emissions of from the burning of coal, oil, and gas, as well as the clearing of forests. But because water vapor plays so many roles in the , the 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 vapor acts as a in the atmosphere. Also, the energy taken up in evaporating water is released back into the environment when the condenses and returns to earth, mostly as rain. Globally, this cycle of evaporation and moves energy around, but cannot create or destroy energy. So, evaporation cannot directly affect the global balance of energy on our planet.

The team led by George Ban-Weiss, formerly of Carnegie and currently at Lawrence Berkeley National Laboratory, included Carnegie's Long Cao, Julia Pongratz and Ken Caldeira, as well as Govindasamy Bala of the Indian Institute of Science in Bangalore. Using a climate model, they found that increased evaporation actually had an overall cooling effect on the global climate.

Increased evaporation tends to cause clouds to form low in the atmosphere, which act to reflect the sun's warming rays back out into space. This has a cooling influence.

"This shows us that the evaporation of water from trees and lakes in urban parks, like New York's Central Park, not only help keep our cities cool, but also helps keep the whole planet cool," Caldeira said. "Our research also shows that we need to improve our understanding of how our daily activities can drive changes in both local and global climate. That steam coming out of your tea-kettle may be helping to cool the Earth, but that cooling influence will be overwhelmed if that water was boiled by burning gas or coal."

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Sep 14, 2011
Any serious study of climate change would consider the large amount of heat that can be acquired or released in phase changes of water (H2O)

a.) Liquid H20 <=> Gaseous H2O (water vapor)

b.) Liquid H20 <=> Solid H2O (ice)

c.) Solid H2O <=> Gaseous H2O (water vapor)

Unfortunately world leaders used government funds to finance many less-than-serious climate studies.

See discussions about the resignation of Physics Nobel Laureate Dr. Ivan Giaever from the American Physical Society despite Al Gore's last-ditch effort to revitalize the global warming story.



With kind regards,
Oliver K. Manuel
Former NASA Principal
Investigator for Apollo

Sep 15, 2011
These Ecologist are more like shamans!

Sep 15, 2011
So I suppose the clear-cutting of the Amazon rain forest,and other forested areas, is likely having a significant effect.

Sep 15, 2011
Water vapor rises because it makes the air that contains it less dense. As it rises, it experiences cooling from the surrounding air (which is cooler at higher altitudes). When the water vapor is cooled sufficiently, it condenses into clouds, releasing the latent heat of condensation it absorbed when it evaporated to form water vapor at the lower altitude. This means that heat from lower altitudes is transported to higher altitudes in the atmosphere, where it has a greater chance of radiating off into space.

When the latent heat of condensation is released, the air it is in is warmed, and lighter moist air rising below this cloud rises higher into the atmosphere until it is finally cooled into more cloud. This process continues until the water vapor supply runs out.

We can watch this process in action on days when puffy little cumulus clouds becom thunder-bumpers. These thunderstorms are actually heat engines, transferring heat from the surface to the upper atmosphere.

Sep 15, 2011
The process of evaporation from plants is called evapotransporation, and has been studied for about 150 years. Records go back to the 1860s.

Mankind exploits the forests and makes savanas out of them to feed cattle. The heat engine effect is destroyed, and eventually we get desertification. Look at the African desertification history for demonstrated proof.

We should learn not to poop in our mess-kits.

P.S, I was an Air Force Wx forecaster and member of the AMS.

Sep 15, 2011

"Evapotranspiration"... Typo... my bad

Sep 15, 2011
"The claim is that the temperature has changed from ~288.0 to ~288.8 degree Kelvin in about 150 years, which (if true) means to me is that the temperature has been amazingly stable, and both human health and happiness have definitely improved in this 'warming' period," - Ivar Giaever, AGW "denier", Nobel Prize in physics -1973

Sep 21, 2011
How can evaporation of water lead to climate cooling?

The evaporation process would have water abosorb latent heat locally. But when it condenses it would give up this heat. SO the net effect is heat redistribution not lost.

I have seen no studies to indicate that there is any recent increase in radiation leaking back in to space as a result of this increase in water vapour.

Now if the presence of water vapour resulted in formation of more clouds, and hence lower the amount of solar energy reaching Earth then this is another matter. But does this mean that a large chunk of water vapour previously taken as part of the CO2 feed back will have to be reasigned?

Sep 21, 2011
@Truthforall: The clouds are blackbody radiators, and absorb incoming long wavelength sunlight (or ground radiation) only to re-radiate the energy. This is why clouds keep temps higher at night than cloudless nights. This works mainly in the absorbtion spectrum of the cloud, so a great deal of short wavelength (visible light) solar energy is just reflected back out into space.

Now, as to the loss of latent heat of condensation (long wavelength energy). At higher altitudes, the density of greenhouse gases is less, so more long wavelength energy radiates into space. This is easy to see in the infra-red satellite pictures that show more infra-red in the denser cloud areas. The satellite shows the heat loss from clouds into space because it is using that heat to take the picture.

I doubt that there are any studies showing an increase in longwave radiation. More likely a decrease due to CO2, etc. That does not invalidate the process.

Sep 22, 2011
Thank you for your kind explanation, however those facts are known to me already and my question referred to quite a different topic.
The radiation that leaks back out to space, thus cools the Earth, has been well studied and the recent revelation of evapotransporation did nothing to affect the record of the energy budget measured except in interpretation of contribution from different factors in the budget and to indicate that part of the atmospheric water vapour that was previously attributed to feed back from CO2 forcing was in fact from plants.
This fact must then be reflected in future climate model.

Sep 22, 2011
As for the effect of cloud cover. Yes, the water vapour would reflect long IR back to the ground giving the impression that it is warmer at night with cloud cover. However if cloud cover persists into day time, the cloud would reflect more sunlight back to space. With the ground getting less heat in day time there will be less heat to reflect by the cloud at night hence the following night will be cooler. So persistent cloud cover will in fact lead to cooling not warming, unless weather cooperates and provides clear (or next to clear) sky in the day and clouds at night. Thats why we had prolonged winter after major asteroid impact or cooler climate after volcanic eruption.

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