AsianScientist (May 14, 2018) – Higher levels of carbon dioxide (CO2) could lead to a wetter Indonesia and a drier Amazon, according to a study published in Nature Climate Change.
Researchers at the University of California, Irvine (UCI) in the US have identified an unexpected but major factor in this worldwide precipitation shift: the direct response of the forests themselves to higher levels of CO2.
“People tend to think that most of the disruption will come from heat going into the oceans, which, in turn, will alter wind patterns,” said study corresponding author Professor James Randerson. “We have found that large-scale changes in rainfall can, in part, be attributed to the way tropical forests respond to the overabundance of CO2 humans are emitting into the atmosphere, particularly over dense forests in the Amazon and across Asia.”
In many aspects of Earth system science, the local effects of environmental factors can impact faraway regions through their influence on the circulation and movement of moisture within the atmosphere. The researchers predicted a similar cascade of events, beginning with stomata, small structures on the underside of leaves that open and shut for plants to take in CO2 and release water vapor.
When more CO2 is present, these orifices do not open as widely, which reduces the amount of water evaporated into the atmosphere. According to the researchers, this small process at the plant level, multiplied across the rainforest, will cause changes in the atmosphere, affecting the way winds blow and the flow of moisture coming from the ocean.
“With higher CO2, trees and forests evaporate less moisture into the air, so fewer clouds are formed above the Amazon,” said Randerson. “And rather than [joining with the usually abundant clouds and] raining over the forest, water vapor from the Atlantic Ocean blows across the South American continent to the Andes mountain range, where it comes down as rain on the mountain slopes, with limited benefit to the rainforest in the Amazon basin.”
Randerson said that the reduction in evaporation will lead to warming over the forests on islands such as Borneo, Java and Sumatra, which are surrounded by humid air above warm ocean surfaces.
“You’ll get a stronger contrast in heating over the islands compared to the nearby ocean, and so it will enhance a natural ocean-land breeze, pulling in more moisture from these neighboring ocean systems to increase rainfall over the forests,” he said.
Assistant Professor Gabriel Kooperman, the first author of the study, also highlighted the fact that the resulting droughts and forest mortality in the Amazon and a potential increase in flooding in other rainforests may have an impact on biodiversity, freshwater availability and food supplies for economically vulnerable populations.
The article can be found at: Kooperman et al. (2018) Forest Response to Rising CO2 Drives Zonally Asymmetric Rainfall Change Over Tropical Land.
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Source: University of California, Irvine; Photo: Pexels.
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