Reforestation effects on water resources depend on global warming level

Planting trees is widely promoted as a natural solution to climate change. But a new study led by researchers from the Institute of Atmospheric Physics at the Chinese Academy of Sciences finds that the hydrological consequences of reforestation depend critically on how much the world warms.

Using the latest Earth system model simulations from the CMIP6 project, the research team compared the effects of the same large‑scale reforestation activity under two contrasting future scenarios: a low‑warming pathway (SSP1‑2.6) and a high‑warming pathway (SSP3‑7.0). They focused on land water availability — the balance between precipitation and evaporation — which is essential for ecosystems, agriculture, and human water use.

“The same trees, planted at the same scale, lead to nearly opposite outcomes depending on the background climate,” said Dr. Tao Tang, lead author of the study published in One Earth. “Under low warming, reforestation slightly increases global water but widens the gap between wet and dry regions — the rich get richer. Under high warming, it reduces overall water availability but makes water distribution more equal.”

The study also examined water availability per capita and found that population differences further amplify these contrasting patterns. Because SSP3‑7.0 scenario assumes a much larger global population than SSP1‑2.6 scenario, the per‑capita water loss in wet regions under high warming is even more pronounced.

To find out what caused the divergent outcomes, the team conducted a moisture budget analysis. The analysis reveals thatdivergent responses of water availability to reforestation under different warming scenarios are driven by differences in atmospheric circulation — specifically, how moisture converges over wet regions under the two warming levels. The authors note that a full mechanistic explanation of why circulation responds differently under low versus high warming requires further investigation.

Importantly, the new findings resolve earlier conflicting studies—some showing reforestation increases water, others showing decreases—by proving that both outcomes can be correct, depending on the background climate state that past work largely ignored.

The findings have direct policy implications. “Reforestation is not a one‑size‑fits‑all solution,” said Dr. Junji Cao, one of the co‑authors of the study. “Policymakers need to consider not just how many trees to plant, but also where and when — because the same reforestation effort may benefit water resources under a low‑emission future but reduce them in a hotter world.”