Effects of Deforestation on Monsoon Rainfall

          Our recent research on this topic is "published in PNAS on 2 March 2015". This study shows that compared with local effects, remote effects of large-scale deforestation have a greater influence on precipitation in monsoon regions. Previous studies have suggested a dominant role for local effects on temperature changes. Our research team (Prof. G. Bala, Dr. Devaraju and A.Modak) at the Indian Institute of Science, Bangalore used idealized climate model simulations to investigate the remote and local effects of large-scale deforestation on precipitation in monsoon regions. We found that large-scale deforestation in the northern mid- and high latitudes resulted in significantly reduced precipitation in the Northern Hemisphere monsoon regions of East Asia, North America, North Africa, and South Asia and increased precipitation in the Southern Hemisphere monsoon regions of South Africa, South America, and Australia. The magnitude of the changes in monsoon precipitation varied depending on the location of deforestation, with remote effects showing a larger influence than local effects. For the idealized scenario of global scale deforestation, the South Asian monsoon region was the most affected, with an 18% decline in precipitation over India. The results might have implications for assessing the net benefits of afforestation and reforestation as climate change mitigation strategies. According to the authors, such assessments should evaluate not only carbon sequestration and local impacts on temperature but also remote effects on monsoonal precipitation.

The following Schematic diagram illustrates the southward movement of the tropical rain bands when there is large scale deforestation in the northern hemisphere (NH) mid- and high-latitudes. The tropical rain bands are usually associated with the inter-tropical convergence zone (ITCZ). The southward shift of ITCZ causes less rainfall in NH monsoon regions and more in southern hemisphere (SH) monsoon regions.Our new study finds that this southward movement of ITCZ is larger for mid- and high-latitude deforestation than for tropical deforestation, indicating that the remote effects of deforestation on monsoonal rainfall are larger than local effects. The green arrow shows the atmospheric flow of energy from SH to NH that is associated with the ITCZ shift. This flow is required because NH is losing energy due to the cooling caused by deforestation. The ITCZ movement is exaggerated for illustrative purposes.

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