The rise in CO2 could cause a drop in water vapor that would lead to a 12% annual reduction in rainfall, while total deforestation would reduce rainfall by 9%
A 50% increase in carbon dioxide (CO2) levels in the atmosphere could have an effect on reducing rainfall in the Amazon similar to or even greater than replacing 100% of the forest with pasture. The increase in CO2 could cause a drop in water vapor emitted by the forest, which would lead to a 12% annual reduction in rainfall, while total deforestation would reduce precipitation by 9%.
The estimate was presented in a study published in the journal Biogeosciences by scientists from the National Institute for Space Research (Inpe), the University of São Paulo (USP), the Technical University of Munich and the State University of Campinas (Unicamp).
“As CO2 is a basic input for photosynthesis, when it increases in the atmosphere there is an impact on the physiology of plants, which can have a cascade effect on the transfer of moisture from trees to the atmosphere [transpiration], formation of rain in the region , forest biomass and a series of other processes”, explains David Montenegro Lapola, professor at the Center for Meteorological and Climate Research Applied to Agriculture (Cepagri) at Unicamp, who led the study.
The researcher coordinates a project funded under the FAPESP Research Program on Global Climate Change (PFPMCG). The work is also part of a project supported by the Foundation in the Thematic modality and included a post-doctoral scholarship.
The researchers wanted to know what the influence on the rainfall regime would be just from the physiological effect caused on plants by the increase in CO2 in the atmosphere. It is known that greater availability of gas causes plants to transpire less, emitting less moisture into the atmosphere and, consequently, generating less rainfall.
Normally, however, predictions about the increase in carbon dioxide in the atmosphere do not dissociate the physiological effect of the increase in CO2 from the effect that the increase in this gas has on the radiation balance in the atmosphere. In this second case, the gas prevents part of the heat from escaping from the atmosphere, causing the phenomenon known as the greenhouse effect.
Projections presented in the latest report by the Intergovernmental Panel on Climate Change (IPCC) of the United Nations (UN), taking into account changes in the balance of atmospheric radiation, plus the physiological effect on plants, had already shown a possible reduction of up to 20% in the annual volume of rainfall in the Amazon, showing that a large part of the changes in the rainfall regime in the region will be controlled by the physiological response of the forest to the increase in CO2.
For the current study, the researchers carried out simulations on the supercomputer at Inpe's Center for Weather Forecasting and Climate Studies (CPTEC), in Cachoeira Paulista. Scenarios of a 50% increase in atmospheric CO2 concentration and its impacts on forest physiology were projected over a hundred years of simulation. Another simulation was able to predict the effect of replacing 100% of the forest with pastures. “To our surprise, just the physiological effect on forest leaves would generate a 12% annual decrease in rainfall [252 millimeters less per year], while total deforestation would lead to a 9% reduction [183 millimeters less per year ]. These are values well above the natural variation of 5% in precipitation in the Amazon between one year and another”, says Lapola.
The results draw attention to the need for both local actions – to reduce deforestation in the nine countries that are home to the Amazon – and global ones, in order to reduce CO2 emissions into the atmosphere due to industrial activity, transport and energy generation, for example.
Lapola is one of the coordinators of the AmazonFACE (Free-Air Carbon Dioxide Enrichment) experiment. Installed north of Manaus, it will increase the concentration of carbon dioxide in small areas of forest, in order to verify the physiological and atmospheric changes caused by the increase in carbon dioxide. The experiment can anticipate the climate scenario predicted for this century (read more at: agencia.fapesp.br/32279/ and agencia.fapesp.br/31140/).
Forest and pasture transpiration
In the two scenarios projected by computer simulations, the reduction in rainfall would be caused by a drop of approximately 20% in transpiration through the leaves. The reasons for this decrease, however, are different in each situation.
Leaves have microscopic openings on their surface, called stomata. To capture CO2 for photosynthesis, the stomata open and capture the necessary amount of gas, at the same time as they emit water vapor. In the scenario with more carbon dioxide in the air, leaves spend less time with their stomata open. As a result, they emit less vapor and reduce the formation of clouds and, therefore, rain.
Another reason is the reduction in the total leaf area. If 100% of the forest was replaced by pastures, there would be a 66% reduction in this area. This is because, while a square meter of pasture has a leaf area slightly larger than that same square meter, in the forest, with several layers of leaves overlapping the trees, the so-called leaf area can be greater than six square meters for each meter floor square. Finally, both the increase in carbon dioxide and deforestation also influence winds and air masses, which play a fundamental role in the rainfall regime.
“The forest canopy has tall and short trees, leaves, branches, which bring complexity to the surface, the so-called roughness. The wind hits these places and generates eddies, vortices, which in turn generate instability and are at the origin of the convection responsible for equatorial rains. The pasture, in turn, is a smooth surface, on which the wind flows straight ahead and, without the forest, it does not form these whirlpools. This causes an increase in the wind, which takes much of the precipitation to the west, while a large part of the Eastern and Central Amazon, the Brazilian area, receives less rain”, informs Lapola.
The decrease in sweating caused by the increase in CO2, in turn, increases the average temperature by up to two degrees, since there are fewer water droplets to alleviate the heat. This factor initiates a cascade of phenomena that results in the inhibition of the formation of so-called deep convection (very high rain clouds loaded with water vapor), reducing rainfall.
“A next step would be to test other computational models to compare the results with what we found. Furthermore, it is essential to carry out experiments like FACE, as only they can provide data to verify and improve modeling simulations like the ones we did”, concludes the researcher.
The article CO2 physiological effect can cause rainfall decrease as strong as large-scale deforestation in the Amazon, by Gilvan Sampaio, Marília H. Shimizu, Carlos A. Guimarães-Júnior, Felipe Alexandre, Marcelo Guatura, Manoel Cardoso, Tomas F. Domingues, Anja Rammig, Celso von Randow, Luiz FC Rezende and David M. Lapola, can be read at https://bg.copernicus.org/articles/18/2511/2021/ .
original article published on the Agência Fapesp website.
