Tropical forests generate rainfall worth billions
Tropical forests help to generate vast amounts of rainfall each year, adding weight to arguments for protecting them as water and climate pressures increase, say researchers.
A new study led by the University of Leeds has put a monetary value on one of forests’ least recognised services as a source of rainfall to surrounding regions, finding that each hectare generates 2.4 million litres of rain each year - enough to fill an Olympic-sized swimming pool.
Satellite observations were combined with simulations from the latest generation of climate models, to reduce long‑standing uncertainty around the magnitude of forest‑driven rainfall. The researchers then applied a simplified economic valuation to estimate what that rain is worth to society and the economy.
Their conclusions are stark. Using this approach, they calculate that rainfall generation provided by forests in the Brazilian Amazon is valued at roughly US$20bn (£16bn) a year to regional agriculture. By comparison, current financial incentives aimed at protecting or restoring the Amazon amount to only a fraction of that figure.
The study entitled Quantifying tropical forest rainfall generation is published today in the journal Communications Earth & Environment.
Tropical deforestation is increasing, despite international efforts to halt forest loss. Demonstrating the financial benefits that tropical forests provide will unlock investment and strengthen arguments for forest protection.
Tropical forests help to sustain the regional and continental rainfall systems that underpin agriculture, water supplies and energy production via a process known as evapotranspiration where the sun’s energy is used to transfer moisture from the leaves into the atmosphere. The study estimates that, across the tropics, each square metre of forest contributes about 240 litres of rainfall annually, rising to around 300 litres in the Amazon.
“This is the most comprehensive and robust evidence to date of the value of tropical forests’ rainfall provision,” said lead author Dr Jess Baker, from the University of Leeds’ School of Earth, Environment and Sustainability.
“Tropical deforestation is increasing, despite international efforts to halt forest loss. Our work highlights the vital role of tropical forests in producing rain. We estimate that the Amazon alone produces rainfall worth US$20 billion each year. Demonstrating the financial benefits that tropical forests provide will unlock investment and strengthen arguments for forest protection.”
The study finds that producing enough rainfall to support some major crops requires moisture generated by more forest area than the crops themselves occupy. For instance, cotton uses 607 litres of moisture per square metre which is equal to the amount of water produced by two square metres of intact forest. Soybean crops need 501 litres of moisture – equal to 1.7 square metres of intact forest.
Forest loss has already imposed significant costs. The researchers estimate that deforestation over recent decades, which is around 80 million hectares in the Amazon, may have reduced rainfall‑generation benefits by almost US$5bn annually, with knock‑on effects for food production, hydropower and water security.
Brazil’s economy is particularly exposed. About 85% of the country’s agriculture is rain‑fed, and reduced rainfall and delayed wet seasons have already affected soy and maize yields in regions with high levels of deforestation.
Recognising that crucial connection could ease tensions between agricultural and conservation interests whilst building broader support for protecting forests overall.
Beyond agriculture, declining rainfall linked to forest loss also threatens drinking water supplies, river transport in remote regions, hydropower generation and even the carbon‑storage capacity of remaining tropical forests.
Despite repeated international pledges to halt deforestation by 2030, forest loss continues across much of the tropics. The authors of the study argue that failing to account for rainfall generation in economic and legal frameworks has obscured one of the strongest practical arguments for forest protection.
Co-author of the study, Dr Callum Smith of Leeds’ School of Earth, Environment and Sustainability, added: “Tropical forests make it rain, supplying water that is essential for agriculture. Recognising that crucial connection could ease tensions between agricultural and conservation interests whilst building broader support for protecting forests overall.”
Further information
Top image: Amazon rainforest canopy. Credit: Dominick Spracklen
Quantifying tropical forest rainfall generation is published in the peer reviewed journal Communications Earth & Environment.
