Reduced rainfall in tropics linked to deforestation

Deforestation is resulting in reduced rainfall across large parts of the tropics, according to new research.

People living in tropical forest communities have often complained that the climate gets hotter and drier once trees are cleared but until now scientists have not been able to identify a clear link between the loss of tree cover and a decline in rainfall.

A research team at the University of Leeds combined satellite data of deforestation and rainfall to show that the loss of tree cover in the tropics over the last 14 years was associated with reductions in rainfall.

They estimate that by the end of the century, if the rate of deforestation in the Congo was to continue, rainfall in the region could be reduced by between 8% and 12%, with a major impact on biodiversity and farming.

Reduced rainfall could also threaten the viability of the Congo forests, which are among the world’s largest stores of carbon.

Callum Smith, a doctoral researcher in the School of Earth and Environment at Leeds and the lead author in the study, said the investigation provides “compelling evidence” to protect forests from uncontrolled clearing.

He added: “Tropical forests play a critical role in the hydrological cycle through helping to maintain local and regional rainfall patterns. The reduction in rainfall caused by tropical deforestation will impact people living nearby through increased water scarcity and depressed crop yields.

“Tropical forests themselves rely on moisture to survive and remaining areas of forest will be impacted by a drier climate.”

The research paper ‘Tropical deforestation causes large reductions in observed precipitation’ has been published in the scientific journal Nature.

Amazon Forest seen from the air. Picture credit: Dr Jess Baker, University of Leeds

The researchers looked at the impact of forest loss in three areas of the tropics - the Amazon, Congo and Southeast Asia - which have all experienced rapid land-use changes.

The study involved analysis of satellite observations from 2003 to 2017, to identify locations where forests had been cleared. Rainfall data in these areas, also measured by satellites, was compared to rainfall from nearby locations where forests had not been lost.

The study revealed that tropical forest loss caused reductions in rainfall throughout the year, including in the dry season when any further drying will have the biggest impact on plant and animal ecosystems. The greatest absolute decline in precipitation was seen in the wet season with up to a 0.6 mm a month reduction in rainfall for every percentage point loss of forest cover.

Writing in the paper, the researchers warn that climate change will lead to increased drought and that could be exacerbated by continued deforestation.

Link between forest cover and rainfall

It is believed the loss of tree cover disrupts the process where moisture from leaves - through a mechanism called evapotranspiration - is returned to the atmosphere where it eventually forms rain clouds.

As well as impacting natural ecosystems, a reduction in rainfall would be detrimental to agriculture and hydropower plants. That would have an affect on the healthy functioning of the forests and on local communities.

The research team say, on average, crop yields decline by 0.5% for every 1% reduction in rainfall.

Tropical forests sustain rainfall

Professor Dominick Spracklen, from the School of Earth and Environment at Leeds who supervised the project, said: “Local people living near deforested regions often report a hotter and drier climate after the forests are cleared. But until now this effect had not been seen in rainfall observations.

“The study shows the critical importance of tropical forests in sustaining rainfall. Although there have been efforts to halt deforestation, the loss of forest cover in the tropics has continued. There needs to be renewed efforts to stop forests being lost and to regenerate lost and degraded areas.”

The scientists warn that a decline in rainfall has a negative impact on biodiversity, increases the risk of forest fires and reduces carbon sequestration, where nature removes carbon from the atmosphere and stores it.

The research was funded by the European Research Council under the European Union's Horizon 2020 research and innovation programme and the Newton Fund, through the Met Office Climate Science for Service Partnership Brazil.

The paper was authored by Callum Smith, Dr Jess Baker and Professor Dominick Spracklen, all from the University of Leeds.