Rising CO2 could reduce Amazon rainfall more than deforestation

A drastic rise in CO2 would reduce rainfall in the Amazon more than deforestation, according to a new study from the University of Sao Paulo and the National Space Research Institute.

The experts report that a 50-percent rise in atmospheric carbon dioxide could reduce rainfall in the Amazon as much as or even more than substitution of the entire forest by pasture. 

The research shows that a 50 percent increase in CO2 levels would lead to a 12 percent annual drop in the volume of Amazon rainfall by reducing the amount of water vapor emitted by the forest. By comparison, complete deforestation would reduce rainfall by nine percent.

Study co-author David Montenegro Lapola is a professor at UNICAMP’s Center for Meteorological and Climate Research Applied to Agriculture (CEPAGRI) and the principal investigator for a project funded via the FAPESP Research Program on Global Climate Change (RPGCC).

“CO2 is a basic input for photosynthesis, so when it increases in the atmosphere, plant physiology is affected and this can have a cascade effect on the transfer of moisture from trees to the atmosphere (transpiration), the formation of rain in the region, forest biomass, and several other processes,” said Lapola.

For the investigation, the researchers ran simulations on the supercomputer at INPE’s Center for Weather Forecasting and Climate Studies (CPTEC) in Cachoeira Paulista, state of São Paulo. They used the projections to examine how each of the two scenarios would affect the physiology of the forest over a 100-year period.

“To our surprise, just the physiological effect on the leaves of the forest would generate an annual fall of 12 percent in the amount of rain, whereas total deforestation would lead to a fall of 9 percent. These numbers are far higher than the natural variation in precipitation between one year and the next, which is 5 percent,” said Lapola.

The decrease in Amazon rainfall projected by the computer simulations was caused by a reduction of about 20 percent in leaf transpiration. In each scenario, the reasons for the reduction in leaf transpiration are different.

With more CO2 in the air, the stomata in plant leaves would emit less water vapor, reducing cloud formation and rainfall. On the other hand, in the deforestation scenario, total leaf area would shrink by 66 percent.

“The forest canopy has a complex surface made up of the tops of tall trees, low trees, leaves and branches. This is called canopy surface roughness. The wind produces turbulence, with eddies and vortices that in turn produce the instability that gives rise to the convection responsible for heavy equatorial rainfall,” said Lapola. 

“Pasture has a smooth surface over which the wind always flows forward, and without forest doesn’t produce vortices. The wind intensifies as a result, bearing away most of the precipitation westward, while much of eastern and central Amazonia, the Brazilian part, has less rain.”

The study is published in the journal Biogeosciences.

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By Chrissy Sexton, Earth.com Staff Writer