RIO DE JANEIRO, Brazil (23 July, 2012)
As the world grapples with rising temperatures, scientists are trying to find ways to utilise forests to influence rainfall patterns in areas experiencing water shortages or severe drought.
"While forests have received a lot of attention for their role in storing carbon, thus helping mitigate climate change, they could also help us to adapt to a changing climate and combat drought by influencing rainfall patterns," said David Ellison from the Institute for World Economics at a Rio+20 side event organised by the Center for International Forestry Research (CIFOR).
Research by CIFOR has shown that tropical forests contribute to regulating river flows both during dry seasons and high rainfall events, thereby minimising risks related to water scarcity and floods.
The relationship between forests and water is complex because, at the local level, catchment studies show that trees actually remove water from the system.
Growing trees take water from the soil and release it into the atmosphere. Tree leaves also act as interceptors, catching falling rain, which then evaporates causing rain precipitation elsewhere - a process known as evapo-transpiration.
By better understanding this process, we may, one day, be able to strategically plant trees that will bring rain to regions that need it most, Ellison said.
"As the climate changes, severe droughts are likely to become more common, and we shouldn't miss the opportunities to influence the hydrologic cycle in a beneficial way using trees," he said.
After oceans, forests are the most efficient sources of precipitation, said Ellison, who studied the world's major river basins to identify what proportion of water vapour came from evapo-transpiration from terrestrial plants as compared with the seas.
"Evapo-transpiration is a very large component of rain generation - on average about 50% in summer across the globe, and 40% on an annual basis," he said.
"We know that trees in forests are the most efficient evapo-transpirators out there. If we compare them to say agricultural land cover, trees can evapo-transpirate twice as much as agricultural crops and about twice as much as water body surfaces."
"So removing forests will have the biggest impact on cross-continental transport of water vapour."
Conversely, by planting forests in strategic locations, we may be able to increase rainfall in key areas, Ellison said.
But more research is needed.
"I've thought a lot about the idea of how much forest do you need in order to produce a rain cloud over the next region, and I don't think there are any simple answers to that problem," he said.
"Obviously you need a lot of forest to be able to have some kind of a positive impact on rainfall patterns, but I do think it's possible to afforest large regions and have a significant impact on precipitation in other locations."
Ellison says one area where this might work is the Nagy Alföld (Hungarian Great Plain) agricultural region of Hungary, the country where he is based.
"As climate change continues, the predictions are not very positive for this region, in terms of much higher temperatures and much lower precipitation in summer," he said.
"So the question is, how will agriculture in that area survive?"
"I think a lot could be done with afforestation, particularly in the southwestern region of Hungary, because that is essentially the way wind currents go," he said, thus carrying the rain clouds were they are most needed.
CIFOR research has also shown that forests play a role in how societies adapt to climate change as forests provide diverse ecosystem services that contribute to human well-being and reduce social vulnerability.
But the use of forests as a climate adaptation tool requires regional, national, and international coordination, Ellison said, because the increased rainfall will likely occur not over the forested region, but elsewhere.
"You can't just think about this at the level of individual catchments, you have to think about much larger regional relationships. You have to think carefully about how all the different areas of a region are interlinked in terms of the cross-continental transport of water vapour."
CIFOR-CIRAD scientist Bruno Locatelli says policymakers need to pay more attention to forests. "We need to include forests in climate change adaptation policies for two reasons," he said.
"Firstly, because they are vulnerable to changes in the climate - and secondly, because they can play a key role in reducing the vulnerability of societies to losses causes by climate change."
According to Ellison planting new forests - in addition to protecting those still standing - may just be one way to help us adapt.
"Across all of the southern regions of Europe we expect to see much higher temperatures and much lower levels of rainfall, and that's a common finding across a lot of areas worldwide," he said.
"In many areas afforestation looks like it could have a very important impact."