Forests and Monsoon rainfall

A recent report titled ‘Effects of large-scale deforestation on precipitation in the monsoon regions: Remote versus local effects’  published by Divecha Centre for Climate Change (IISc) at the PNAS  reaffirms that compared with local effects, remote effects of large-scale deforestation have a greater influence on precipitation in monsoon regions.


Cty:The Hindu

The research team at  the Indian Institute of Science (IISc), Bangalore used idealized climate model simulations to investigate the remote and local effects of large-scale deforestation on precipitation in monsoon regions and 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.

How does it work?

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.

DeforestationThe 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.

What does this mean for us?

The main finding of the study is that deforestation can shift the location of moisture convergence in the tropics, the so-called inter-tropical convergence zone (ITCZ) in the tropical area. This has huge implication for tropical rainfall.

For deforestation in the temperate and high-latitudes, we find that the ITCZ shifts southward and hence all the monsoon regions in the northern hemisphere (NH) get reduced rainfall. The NH monsoon regions are in South Asia, North Africa, North America and East Asia. At the same time, the southern hemisphere monsoon regions such as Australia, South Africa and South America receive more rains.

The surprise is that the mid- and high-latitude deforestation displaces the ITCZ southward much more than tropical deforestation. This indicates that the remote effects on rainfall are stronger than local effects. This is exactly opposite to what we see in the case of temperature changes where local effects dominate.

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