The cooling power of trees ‘may be more important than their carbon storage’

In a new research paper, scientists say that the cooling effects of trees may be more environmentally important than their capacity for storing carbon.

Trees, the researchers say, directly affect our climate via rainfall. They help to retain water on the ground and produce cooling moisture.

In the paper, entitled “Trees, forests and water: Cool insights for a hot world”, the researchers suggest that there needs to be a turnaround in thinking. In discussions and actions about climate change, more attention needs to be paid to the relationship between trees and water, they say.

Given the major role they play in water cycles and in cooling temperatures, forests and trees contribute to food security and to climate change adaptation, the scientists say.

They state in their research paper: “Our call to action targets a reversal of paradigms, from a carbon-centric model to one that treats the hydrologic and climate-cooling effects of trees and forests as the first order of priority.

“For reasons of sustainability, carbon storage must remain a secondary, though valuable, by-product.”

The new paper, which was published in the journal Global Environmental Change, has 22 co-authors from the United States, Australia, Britain, France, Sweden, Norway, the Netherlands, Peru, Indonesia, Ethiopia, the Czech Republic, Italy, and Belgium.

In recent decades, in the discourse about climate change, forests and trees have been considered mostly as carbon stocks and carbon sinks.

The lead author of the new paper, David Ellison, said: “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.”

The paper’s authors are participating in a two-day virtual symposium organised by  the Center for International Forestry Research (CIFOR)¹ and the World Agroforestry Centre (ICRAF) as part of the CGIAR Research Programme on Forests, Trees and Agroforestry (FTA).

The symposium is being held on the International Day of Forests today (Tuesday) and World Water Day on Wednesday (March 22).

Attendees are discussing the findings in the new paper and new areas of research about the links between forests, water, and climate.

FTA director Vincent Gitz says scientists are seeing that trees have a wider role than is generally recognised.

“This is very important in the context of the Paris Agreement, which recognised that climate change is not only about mitigation, but also about adaptation.”

Gitz says those setting global agendas for food security and climate change adaptation need to be taking into account the important influence of trees on water cycles.

“With trees, there is no trade-off between adaptation and mitigation, but a synergy.”

Using the sun’s energy, individual trees can transpire hundreds of litres of water daily. This represents a cooling power equivalent to 70 kWh for every 100 litres of water transpired, which is enough to power two average household air-conditioning units for a day.

With deeper roots, trees can maintain their cooling function even during long-lasting heat waves, the scientists say.

“Trees likewise reduce temperatures in urban settings. Urban areas with greater tree and vegetation cover and fewer impervious surfaces tend to exhibit lower temperatures than those blanketed by solid surfaces.”

Ellison says that the finer details of how forests affect rainfall are still being discussed among scientists from different disciplines and backgrounds.

“However, 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.”

Climate collaboration

CIFOR points to the fact that those conducting new research about trees and water are combining knowledge from many different fields of science: biology, chemistry, climate science, geology, hydrology, and social science.

The research paper and the symposium are a good example of the interdisciplinary research and collaboration that are needed to address the issue of climate change adaption from multiple scientific and policy angles, CIFOR says.

The new research paper says that forest-driven water and energy cycles are poorly integrated into regional, national, continental, and global decision-making about climate change adaptation and mitigation, land use, and water management.

“This constrains humanity’s ability to protect our planet’s climate and life-sustaining functions,” the researchers state.

“The substantial body of research we reviewed reveals that forest, water, and energy interactions provide the foundations for carbon storage, for cooling terrestrial surfaces and for distributing water resources.”

Forests and trees must be recognised as prime regulators within the water, energy, and carbon cycles, the researchers say.

“If these functions are ignored, planners will be unable to assess, adapt to or mitigate the impacts of changing land cover and climate.

“The effects of tree cover on climate at local, regional, and continental scales offer benefits that demand wider recognition.”


Primary rainforest in Danum Valley in Sabah, Malaysia. Photo by M. Edliadi for CIFOR.

The researchers say that their understanding of the way trees and forests influence water, energy, and carbon cycles has important implications for the structure of planning, management, and governance institutions, and for how trees and forests might be used to improve sustainability, adaptation, and mitigation efforts.

“Billions of people suffer the effects of inadequate access to water and extreme heat events,” the researchers state. “Climate change can exacerbate water shortages and threaten food security, triggering mass migrations and increasing social and political conflict.”

Strategies for mitigating and adapting to such outcomes are urgently needed, they say.

“For large populations to remain where they are located without experiencing the extreme disruptions that can cause migrations, reliable access to water and tolerable atmospheric temperatures must be recognised as stable ingredients of life.

The maintenance of healthy forests is a necessary pre-condition if this is to be achieved, the researchers say.

Deforestation, they say, can impact rainfall for reasons beyond its impact on precipitable water.

“And the combination of warming and altered rainfall patterns due to climate change can lead to feedback effects on remaining vegetation, reduced biomass accumulation, drought, die-off and fires.

“Forest and land fires resulting from the increased incidence of drought, agricultural land conversion, clearing, and other causes likewise play havoc with rainfall. Aerosol particles from fires can scatter solar radiation, disrupt water vapour uplifting, alter regional circulation and otherwise disrupt rainfall patterns.”

With increasing deforestation, the researchers say, locations further from upwind coasts are likely to feel the strongest impact of change in land-atmosphere interactions and experience reduced predictability, extent, and quantity of rainfall.

“In borderline regions, reduced predictability, seasonal timings and feedback effects may even trigger a switch from wet to dry climates.”

The scientists say that land conversions from forests to agriculture have downwind impacts on water availability and alter the land surface energy balance.

“On the other hand, enforcing upstream or upwind forest protection for the benefits of downstream or downwind agriculture can potentially restrict the freedom of choice and the livelihood options of upstream and upwind communities.”

The researchers point to the importance of communication with, and the incorporation of, local communities in decision-making practices.

The role of forests in water and climate regulation and, consequently, food production must meanwhile be better integrated into all levels of land-use management and governance, they say.

“A new and radically improved mitigation and adaptation agenda designed for the new millennium could learn to marshal land-atmosphere carbon, water and energy cycles in ways that optimize their potential.

“Building on synergies and avoiding or minimizing trade-offs represents the key to a more sustainable and productive future with improved adaptation and thereby mitigation potential.”

The scientists say that a call to action on forests, water, and climate is emerging on many fronts.

“Consideration of the effects of forests on water and climate suggests this call is urgent. Stimulating regional and continental approaches may help develop more appropriate governance, thereby improving the chances for success.”

A forest in eastern Cameroon. (Photo by M. Edliadi.)

  1. CIFOR is based in Bogor in Indonesia and the Kenyan capital Nairobi.

Photos of forest view and fern close-up (Central Kalimantan) by Nanang Sujana for CIFOR.

Article updated on 22/3/2017