New Delhi: Large-scale deforestation has had a major impact on the South Asian monsoon, causing an 18% decline in precipitation over India and 12% drop over other parts of the region, according to a study published on Tuesday in the Proceedings of National Academy of Sciences (PNAS).
Deforestation at high latitudinal and temperate regions causes a decrease in rainfall in the northern hemisphere regions such as South Asia, North Africa, North America and East Asia, but causes an increase in precipitation in the southern hemisphere regions like South America, South Africa and Australia.
It is based on first-hand climate modelling. Using a three-dimensional climate model, the researchers performed three experiments related to deforestation in the tropical, temperate and high-latitude areas.
To gain understanding of the two hemispheres, the researchers chose monsoon regions from each for the study aimed at understanding the effects of land use change on precipitation.
About 35% of the global land area consists of croplands today. It has been established that deforestation results in higher emission of carbon dioxide and other greenhouse gases (GHGs), but there are other problems.
“When a climate effect of deforestation is estimated, only the amount of carbon dioxide released to the atmosphere, a biogeochemical effect, and its warming potential is calculated”, said Bala, one of the authors, in a statement by the Indian Institute of Science.
“The changes to surface characteristics such as reflectivity and plant transpiration (biogeophysical changes) and their effect on climate are not accounted.”
Bala explained that several studies have researched the impact of biogeophysical change on temperature, but rainfall has not been studied.
“Rainfall is a challenging climate variable because it is not only affected by the amount of water vapour in the atmosphere, but also by converging circulations of moist air,” he said.
Devaraju, the lead author of the paper, said large scale deforestation leads to a decrease in global mean surface air temperature in the global, boreal, temperate, and tropical deforestation simulations, respectively. The mean precipitation decreases by 3.21%, 1.70%, 1.01%, and 0.50%, respectively.
“This study has huge implications for evaluating the climate benefits of afforestation and reforestation programmes that are promoted by Kyoto Protocol for reducing climate warming. Our study shows that just estimating carbon sequestration benefits of these programmes is not sufficient,” said Bala.
“We need to also account for the biophysical effects like temperature and rainfall as well since they also have large effects. The need for integrated assessment is more pronounced now to tackle the climate impacts of deforestation,” he added.