Trees, water and climate: Cool scientific insights, hot implications for research and policy

* Any views expressed in this opinion piece are those of the author and not of Thomson Reuters Foundation.

The CGIAR Research Program on Forests, Trees and Agroforestry (FTA) recently organized a two-day virtual symposium entitled Trees, forests and water: cool insights for a hot world to share live online the findings of a recent review paper by David Ellison and 21 other scientists, including four from FTA, and discuss their implications for research and policy.

The findings shed brand new light on the role of forests and trees in the climate debate.

Since Rio 1992 and the climate convention, climate policy has put a “mitigation” sticker – ­with the associated pressure – on forests and land use for their role in climate, emphasizing the carbon stored in forests and peat soil, and the capacity of trees to sequester carbon.

This triggered a lot of scientific work and publications, including by FTA scientists, on the best pathways to strengthen the role of forests and trees in climate mitigation. Attention was given to integration with other dimensions (‘cobenefits’ and ‘safeguards’) beyond carbon measures, especially social and economic dimensions.

It also triggered many debates, in science and in policy, as to whether or not this was the right approach, such as the recurrent debates about land-use and forestry-related loopholes in the necessary climate action in the energy sector.

However, this perspective on forests and climate change might well change radically. And the change may not come from carbon, but from water.

What is this new light? The findings of the review, with the level of confidence of underlying scientific evidence – assessed in the symposium – are the following:

1. Trees influence local temperature through high transpiration rates, and remote sensing and infrared tools exist for visualizing this effect (very high confidence).
2. Forests recharge atmospheric moisture and regional evapotranspiration responds to tree-cover transitions (very high confidence).
3. Vegetation and trees influence cloud formation and trigger rainfall via bioprecipitation effects (high confidence).
4. Historical evidence from tree rings and their isotopic composition shows decadal variation and local influences of land use on local rainfall and climate (medium confidence).
5. Rainbow (atmospheric) water bookkeeping combined with prevailing winds shows continental-scale teleconnections on all tropical continents. Forests enable cascading transport of water vapor over distant locations, and therefore rain, far from the sea (high confidence).
6. Forests, as biotic pumps, attract air and moisture flows, and the loss of forests can create a tipping point turning wet climates to dry climates (medium confidence).
7. Trees and forests can improve groundwater recharge, with the existence of an optimum level of tree cover for that effect (high confidence).

To sum up: forests and trees are drivers of key mechanisms that govern the water cycle, atmospheric moisture, precipitation and climate at the local, regional and continental levels.

So what? 

Altogether, these findings have significant implications for policy and action, and for research – particularly for FTA research – and what it can do or the tools it can provide to inform and underpin this new agenda.

In climate policy, it calls for a change of narrative, in which forests (looked at as carbon reservoirs) will play a fundamental role in the world’s adaptation, to preserve the water cycle for agriculture and for food security. Forests and trees can be a cornerstone of the adaptation article (Article 7) of the Paris Agreement.

In land restoration, it gives perspective on how to better understand which trees to plant where and how or what natural regeneration to assist.

In water policy, we should not only look to where the rain goes (the watershed) but to where it comes from (the “precipitationshed”), which opens up a whole new perspective on institutions and incentives for action (including the repartition of costs and benefits).

It is not often in research that efforts lead to considering possible “changes of theory” and such-wide ranging implications. The next challenge for research is both internal and external: for us to better quantify and assess the magnitude of these relations, providing clarity about uncertainties and validity domains (including on the diversity of local-specific situations), and to avoid oversimplification.

We still need to better understand the question of “where does rain come from?” and our models need to better handle the processes that determine winds and rain. 

This will enable us to pursue our quest for strengthening the knowledge base, while being credible enough not to delay the opening of new avenues for action, with clarity on current evidence, (un)certainties, and policy and development implications.

The CGIAR Research Program on Forests, Trees and Agroforestry (FTA) aims to enhance the management and use of forests, agroforestry and tree genetic resources across the landscape from forests to farms. CIFOR leads FTA in partnership with Bioversity International, CATIE, CIRAD, INBAR, Tropenbos International and the World Agroforestry Centre.

This work is supported by CGIAR Fund Donors.