Simulations have shown that the direct effect of increasing levels of carbon dioxide on the Amazon rainforest would result in a decrease in rainfall equal to or greater than the effect of total substitution of the forest with pasture.An increase of 50% in carbon dioxide (CO 2) in the atmosphere could result in a reduction in rainfall in Amazon. This would be as significant as, or more than, the substitution of entire forests by pasture. A rise in CO2 would result in a decrease in water vapor from the forest. This would lead to a 12% annual drop of rainfall. Total deforestation would reduce rainfall by 9 percent.These estimates were presented in a Biogeosciences study by scientists from the National Space Research Institute, the University of So Paulo and the University of Campinas (UNICAMP), respectively in Brazil and Germany.The basic input to photosynthesis is CO 2. As such, when CO2 is increased in the atmosphere, it can affect plant physiology. This can lead to a cascade effect in the transfer of moisture from trees into the atmosphere [transpiration], formation of rain in that region, and forest biomass. David Montenegro Lapola is the last author of this article.Lapola is a professor in UNICAMPs Center for Meteorological and Climate Research Applied to Agriculture, (CEPAGRI), and the principal investigator for a project that was funded by the FAPESP Research Program on Global Climate Change. The study was also part a Thematic Project that FAPESP funded and was supported by a postdoctoral Fellowship to the penultimate writer.Researchers set out to study how rising atmospheric CO2 affects the rainy season. As the CO2 supply increases, plants transpire less and produce less rain.However, most predictions about the rise in atmospheric CO2 do not separate its physiological effects from its effects upon the balance of radiation within the atmosphere. The greenhouse effect is a warming phenomenon caused by the blocking of some of the Sun's reflected energy from entering the atmosphere.The Intergovernmental Panel on Climate Change (IPCC) has presented its latest report. It takes into account changes to the atmospheric radiation balance and the physiological effects on the plants. They have already predicted a reduction in Amazon's annual rainfall of as much as 20%.The researchers used simulations to create the study. They ran them on the INPEs Center for Weather Forecasting and Climate Studies in Cachoeira Paulista (state of So Paulo). To determine how these changes would affect the physiology and ecology of the forest over a 100 year period, they projected scenarios where the atmospheric CO2 level rose 50% and the forest was completely replaced by pasture.Surprised, the only physiological effect on the forest leaves would result in a 12% annual drop in rainfall [252 millimeters more per year], while total deforestation would result in a 9% fall [183 mm]. These numbers are much higher than the natural variation of precipitation between years, which is 5%. Lapola stated.These findings highlight the need to take local actions to reduce deforestation within the nine countries sharing the Amazon basin, and to take global action to reduce CO2 emissions from the atmosphere, such as factories, power plants, cars, and vehicles.Lapola is one the AmazonFACE coordinators. The acronym stands to Free-Air Carbon dioxide Enhancement. The experiment, which is located north of Manaus in Brazil, will increase the CO2 level over small areas of rainforest and then analyze the changes in plant physiology as well as the atmosphere. This experiment could help to predict the climate change scenario for this century (more information at: agencia.fapesp.br/32470).Transpiration in pasture and forestComputer simulations projected that the reduction in rainfall was caused by a decrease of 20% in leaf transpiration. However, the reasons for each reduction may be different.Stomata, microscopic portals found in plant leaves, control the gas exchange necessary for photosynthesis. They are able to absorb CO2 and also emit water vapor. If there is more CO2 in the atmosphere, the stomata stay open longer and emit less water vapour, which reduces cloud formation and precipitation.Another reason is total leaf area shrinkage. The area of the forest would shrink by 66% if it were all replaced with pasture. The forest has several layers of leaves that are superimposed on each other. This means that the leaf area per square meter can be six times greater than what is found on the ground. Deforestation and rising CO2 levels also have an impact on wind and air mass movement, which are key factors in the formation of precipitation.The forest canopy is composed of tall and low trees, leaves, branches, and other vegetation. This is known as canopy surface roughness. Lapola stated that wind causes turbulence. This creates vortices and eddies that then produce instability which in turn leads to heavy equatorial rain. Pasture is a smooth area where the wind flows forward. Forests don't create vortices. As a result, the wind becomes stronger and carries most of the rain away westward. However, much of central Amazonia (the Brazilian part) has less rain.Temperature increases of up to 2 degrees can be caused by CO2 levels rising. This is because there are less water droplets to heat the atmosphere. This causes a series of phenomena that lead to less rain due to the inhibition of deep convection (very high rain clouds, heavy with water vapour).Lapola stated that testing other models would be the next step. More experiments like FACE are another important initiative, because only they can provide data that will allow us to refine and verify modeling simulations such as the ones we did.###About So Paulo Research Foundation, (FAPESP).
