Deforestation–Rainfall Feedback

Deforestation-rainfall feedback is the process by which forest loss reduces the return of water to the atmosphere and can weaken rainfall that helps sustain the remaining forest. In tropical rainforest regions, moisture recycling occurs when water from land returns to the atmosphere through evaporation and plant transpiration, then contributes to rainfall locally or farther downwind.

How rainforests recycle moisture

Rainforests receive moisture transported by winds, including moisture originating over oceans. They also return part of their rainfall to the atmosphere, allowing water to contribute to successive rainfall events across a region.

• Through transpiration, roots take up soil water and leaves release water vapour into the air; deep roots can help sustain this process during relatively dry periods.
• Evaporation from wet leaves, soil and other surfaces combines with transpiration to form evapotranspiration.
• Winds transport this water vapour, which can contribute to cloud formation and rain when moist air rises, cools and reaches saturation.

How clearing disrupts the cycle

Replacing rainforest with pasture, cropland or bare ground generally reduces leaf area, canopy water storage and access to deeper soil moisture. These changes can reduce the supply of water vapour to the atmosphere.

• Lower evapotranspiration leaves less recycled moisture available for rainfall, including in areas downwind of clearing.
• Reduced evaporative cooling shifts more available energy towards heating the surface and overlying air, increasing sensible heating.
• Forest removal also changes surface roughness and atmospheric circulation, influencing moisture transport and the conditions for cloud development.
• The rainfall response depends on the extent and location of clearing, the season and large-scale circulation; rainfall does not necessarily decline uniformly at every cleared site.

Why the disruption can reinforce forest loss

Where reduced moisture recycling weakens rainfall, the remaining forest faces greater water stress. This can create a positive feedback: forest loss promotes drying, and drying makes further forest degradation more likely.

• Drier vegetation and litter become more susceptible to fire when ignition occurs, potentially causing additional canopy loss.
• Further vegetation loss can reduce moisture recycling again, reinforcing regional drying rather than merely producing a one-time local change.
• Conserving extensive forest cover helps maintain regional moisture recycling; restoration can support recovery, although rainfall also depends on oceanic moisture supply and atmospheric circulation.