Trees return water to the atmosphere by the processes of evaporation from wet surfaces and transpiration – when water vapour escapes from leaves as CO₂ gas enters through the pores (stomates). Through this ‘evapo-transpiration’ process rainforest canopies moisturise the air like fine mist fountains. One square meter of the ocean surface evaporates one litre of water. A tree releases 8-10 times more moisture into the atmosphere than the equivalent area of the ocean.

Evapotranspiration is one of the most important regional ecosystem processes, helping to generate rain over vast distances. Up to half of the rainfall in the Amazon basin is recycled by the forest, rather than brought in from the Atlantic Ocean to the east. The percentage of rainfall derived from recycled water increases from east to west across the Amazon basin, and by the time it reaches the foot of the Andes an estimated 88 per cent of the water has fallen twice as precipitation.

Although there is uncertainty in how much water tropical forests recycle, climate models suggest that deforestation may have significant impacts on global weather patterns and climate. Click the image to view a global atmosphere model of cloud and precipitation developed by The Visualization & Enabling Technologies Section of the National Centre for Atmospheric Research.

Complex chemistry (Volatile Organic Compounds) released by tropical canopies to the atmosphere helps generate the rainfall that stabilises local and regional weather patterns. New research has shown that coastal tropical forests may act as a ‘biotic-pump’ drawing moist air over the sea far inland. Coastal deforestation may weaken this pump, leading to a decline in rainfall inland.

NASA’s TRMM satellite data show that Brazil’s billion dollar soya, beef and bio-fuel industries all depend on rain recycled by the Amazon. Amazonian forests store 3 trillion tons of water. 70% of Brazil’s electricity is sourced by hydropower, also dependent on Amazonia’s rain.

Bibliography:

R. Avissar & D. Werth (2005). Global hydroclimatological teleconnections resulting from tropical deforestation. Journal of Hydrometeorology 6, 134-145.

P. Meir, P. Cox & J. Grace (2006). The influence of terrestrial ecosystems on climate. Trends in Ecology & Evolution 21, 254-260.