Large-scale deforestation in India has a debilitating impact on the South Asian monsoon, causing an 18-per cent decline in precipitation levels in India and a 12 per cent drop in other parts of the region, according to a study published today.
While deforestation has long been known to cause temperature increases in surrounding areas, new research shows large-scale deforestation could cause monsoon rains to shift south, cutting rainfall in India by nearly a fifth.
Besides releasing large amounts of carbon dioxide into the atmosphere, deforestation also causes changes in the reflection of light off the earth's surface and the amount of moisture in the atmosphere, says a study published in the Proceedings of National Academy of Sciences (PNAS).
N Devaraju, Govindasamy Bala, and Angshuman Modak from the Divecha Center for Climate Change carried out the study at the Indian Institute of Science, Bangalore, based on first-hand climate modeling.
The researchers used a model simulating atmosphere circulation, as well as photosynthesis, transpiration, warming of the ocean surface and ice melt.
According to Devaraju, the lead author of the paper, 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 per cent, 1.70 per cent, 1.01 per cent, and 0.50 per cent, respectively.
They performed three deforestation experiments, removing all trees in tropical, temperate and high-latitude areas to look at the impacts. "We wanted to get a basic understanding of the effects of large-scale deforestation at different locations on monsoon rainfall," the authors said in a statement.
About 35 per cent 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, the study notes.
Land used for crops and pastures has increased globally from 620 million hectares in the 1700s - or about 7 per cent of the global land surface - to 4,690 million hectares in 2000, about a third of the world's land surface, according to the researchers.
''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,'' Bala, one of the authors, said in a statement issued by the IISc.
''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.
Deforestation in temperate and high latitudes caused changes in atmospheric circulation resulting in a southward shift in the monsoon rains.
This would effect a significant fall in precipitation in the northern hemisphere monsoon regions of East Asia, North America, North Africa and South Asia, and moderate increases in rainfall in the southern hemisphere monsoon regions of South Africa, South America and Australia, according to the study.
The South Asian monsoon region would be affected the most, with an 18 per cent decline in precipitation over India, the scientists wrote in the paper.