Scientists confronted with this perspective have always denied such effects, or at best been ‘agnostic’, as it seemed impossible in their data to find evidence. New evidence on credible mechanisms for forest and tree effects on rainfall is, however, emerging. It can revolutionize current climate negotiations that all focus on greenhouse gas emissions.

Key findings:

• Atmospheric moisture transport (‘precipitable water’) in relation to prevailing winds define precipitation sheds (source area of rainfall in a target area, typically including ocean + land areas).
• Forests and trees can trigger rainfall by releasing ice nucleating bacteria and volatile organic carbon compounds, but systematic comparison between vegetation types and species has yet to be made.
• A century after ‘Biological Nitrogen Fixation’ was clarified, the outlines are becoming clear of a ‘Biological Rainfall Generation’ mechanism, influencing atmospheric moisture transport and triggering rainfall events.
• The predicted shortages of freshwater in many areas of the world may not be unavoidable
• The geopolitics of anthropogenic impacts on rainfall as key climate variable will call for new types of collective action at (sub)continental scale, different from current UNFCCC debate

van Noordwijk, M., Bruijnzeel, S., Ellison, D., Sheil, D., Morris, C.E., Sands, D., Gutierrez, V., Cohen, J., Sullivan, C.A., Verbist, B., Murdiyarso, D., Gaveau, D.L.A., Muys, B., 2015. ASB Policy Brief No. 47: Ecological Rainfall Infrastructure: Investment in Trees for Sustainable Development. ASB Policy Brief 47. Nairobi: ASB Partnership for the Tropical Forest Margins.