Tree planting’s water impact may depend on the climate it grows into
Reforestation is often promoted as one of the clearest nature-based responses to climate change. Plant more trees, store more carbon, and help stabilize the planet. But new modeling work suggests the hydrological side of that story is far less straightforward. According to a study using CMIP6 Earth system simulations, the effect of large-scale reforestation on water availability can shift dramatically depending on how much warming the world experiences.
The research, led by scientists at the Institute of Atmospheric Physics at the Chinese Academy of Sciences and published in One Earth, compared the same reforestation activity under two future pathways: a lower-warming scenario, SSP1-2.6, and a higher-warming scenario, SSP3-7.0. The team focused on land water availability, defined as the balance between precipitation and evaporation.
One intervention, opposite outcomes
The central finding is unusually stark. Under the lower-warming pathway, reforestation slightly increased global water availability overall, but it also widened the divide between wetter and drier regions. In the researchers’ summary, wet places tended to get relatively wetter, while dry places fell further behind.
Under the higher-warming pathway, the pattern flipped. Reforestation reduced total global water availability, yet made water distribution more equal across regions. That means the same tree-planting strategy can produce nearly opposite aggregate results depending on the surrounding climate background.
For policymakers, this complicates a common assumption that scaling up forests has broadly similar co-benefits regardless of future emissions. The climate context, in this study, is not a side variable. It is a determining factor in whether reforestation slightly lifts water resources overall or pushes them downward.
Population amplifies the pressure
The study also examined water availability per person, not just total supply. Here again, the higher-warming scenario looked more difficult. Because SSP3-7.0 assumes a much larger global population than SSP1-2.6, the per capita water losses in wetter regions under high warming became even more pronounced.
That matters because water stress is ultimately experienced socially and economically, not only hydrologically. A change that appears manageable in total volume can become far more severe when distributed across more people, agriculture and infrastructure systems.
Why the response changes
To understand the split outcome, the researchers conducted a moisture budget analysis. They found that the divergent responses were tied to differences in atmospheric circulation, especially how moisture converges over wetter regions under different warming levels.
The authors say a full mechanistic explanation still needs more work. But the study does offer a plausible route for resolving earlier disagreements in the literature. Previous research has reported conflicting effects of reforestation on water resources, with some studies showing gains and others showing losses. This paper suggests those disagreements may partly reflect different climate baselines rather than simple methodological contradiction.
A reminder that climate solutions interact
The findings do not argue against planting trees. They argue against treating tree planting as a uniform tool with fixed side effects. Reforestation remains important for carbon removal, ecosystem recovery and land restoration. But when it is framed as a water strategy, the broader climate trajectory appears to matter enormously.
That creates a more demanding planning challenge. Large-scale restoration projects may need to be assessed not only by how many hectares are planted or how much carbon is stored, but also by how regional hydrology responds under plausible warming paths. In some places, the trade-offs could shift over time as the world moves farther from a lower-emissions future.
The study’s broader significance is that it links two agendas often discussed separately: mitigation through land use and adaptation to water stress. It suggests they cannot be treated independently. Forest restoration may still be indispensable, but its benefits for water will depend heavily on whether global warming is contained or allowed to intensify.
This article is based on reporting by Phys.org. Read the original article.
Originally published on phys.org