Trees are giant air conditioners with no power bills

Photo credit: Gavin Golden via Flickr (CC BY 2.0).

Anyone who has walked outside on a sunny day knows that forests and trees matter for temperature, humidity and wind speed. Planting trees speaks to concerns about climate change -- but the directly important aspects of the tree-climate relationships have so far been overlooked in climate policy where it relates to forest. That, at least, is the conclusion of a new review. The authors suggest that the global conversation on trees, forests and climate needs to be turned on its head: the direct effects via rainfall and cooling may be more important than the well-studied effects through the global carbon balance. Yet, current climate policy only recognizes the latter. While farmers understand that trees cool their homes, livestock and crops, they had to learn the complex and abstract language of greenhouse gasses and carbon stocks if they wanted to be part of climate mitigation efforts. This will be a thing of the past if the new perspectives get widely accepted.

In the review, published in the journal Global Environmental Change, the authors provide examples for the planet-cooling benefits of trees. Scientists found evidence for the widespread perception that trees and forests also influence rainfall. As such, the review insists that water, and not carbon, should become the primary motivation for adding and preserving trees in landscapes. The scientific review was written by Focali members Aida Bargués Tobella and Ulrik Ilstedt together with 20 other authors from different universities, organisations and institutes; e.g. ICRAF, CIFOR, CIRAD and FAO.

“Carbon sequestration is a co-benefit of the precipitation-recycling and cooling power of trees. As trees process and redistribute water, they simultaneously cool planetary surfaces”, says Dr. David Ellison, lead author of the study. “Some of the more refined details of how forests affect rainfall are still being discussed among scientists of different disciplines and backgrounds. But the direct relevance of trees and forests for protecting and intensifying the hydrologic cycle, associated cooling and the sharing of atmospheric moisture with downwind locations is beyond reasonable doubt.”

Trees are giant air conditioners with no power bills. They use solar energy to convert water into vapour, thereby cooling their surroundings. On a hot day the surface temperature of a forest - in an example discussed in the paper - is similar to that of a nearby lake, while a dry patch of meadow or a tarmac road in the vicinity are more than 20 °C hotter. The cooling power equivalent is around 70 kWh for every 100 liter of water transpired, similar to the output of two home air-conditioning units.

“There are important implications for practice, as we can no longer simply focus on carbon sequestration to mitigate or adapt to climate change”, says Dr. Victoria Gutierrez, Chief Science Officer of the WeForest NGO that supports forest landscape restoration efforts in tropical countries, and coauthor of the study. “For organizations and agencies working to restore forest ecosystems for climate and people, it is crucial that we pay greater attention to the sustainability of the water processing and cooling aspects of the trees,” Gutierrez says.

On average 40% of rainfall over land is recycled from evapotranspiration over land surfaces. The authors point out that there is a strong basis for a hydro-climate policy that involves forests and trees. This policy would be much wider than what has so far been shaped by scientific understanding of the greenhouse-gas dominated climate and been incorporated in international agreements.

The review concluded with a cry to action on forests, water and climate. “Climate policy must take these water-processing, cooling and rainfall-generating effects of trees and forests more explicitly into account.” Significant revision of national, regional and continental climate change mitigation and adaptation strategies are urgent as next steps.

Read the full blog post here.

Read the scientific review article here.

Ellison D, Morris CE, Locatelli B, Sheil D, Cohen J, Murdiyarso D, Gutierrez V, van Noordwijk M, Creed IF, Pokorny J, Gaveau D, Spracklen D, Tobella AB, Ilstedt U, Teuling R, Gebrehiwot SG, Sands DC, Muys B, Verbist B, Springgay E, Sugandi Y, Sullivan CA. 2017. Trees, forests and water: cool insights for a hot world. Global Environmental Change