Most Earth system models used by the Intergovernmental Panel on Climate Change (IPCC) project increases in rainfall across tropical Asia, and neutral or drying trends across lowland South America. The cause of this asymmetric rainfall response is not well understood, and many previous researchers have explored climate change effects on sea surface temperatures and changes in ocean and atmospheric circulation as possible drivers. Here researchers from UC Irvine and U. of Georgia, along with collaborators from LBNL, ORNL, NCAR, and U. of Washington, show that much of this pattern can be traced back to the way forests respond to rising atmospheric CO2.
Water loss by plants is regulated by tiny holes on the bottom of leaves (stomata) that open or close in response to changing environmental conditions. Our best understanding of the way these stomata function suggests they will become more restricted as carbon dioxide levels in the atmosphere increase. As this occurs, the plants will become more efficient at using available water in soils, but will evaporate less moisture into the air, causing the land surface to heat up and change the way energy from the sun is transmitted back into the atmosphere. The current study, published in Nature Climate Change, indicates that these changes in surface energy fluxes can have a dominant role in changing future patterns of rainfall across the tropics. This work highlights the importance of improving our understanding of forest responses to rising CO2 as a means to reduce uncertainties in climate projections of future drought stress.
Applying a simulation design in which CO2 increases were isolated over individual continents in the Community Earth System Model (CESM), we demonstrate that contrasts in regional circulation, moisture flux, and stability anomalies arise over each continent from declines in stomatal conductance and transpiration. Our analysis indicates that South American forests may be more vulnerable to rising CO2 than Asian or African forests.