Large scale deforestation in the high latitudes – the boreal regions – could affect rainfall in the monsoonal regions of tropics, new research by the Indian Institute of Science published on Tuesday shows.
Deforestation is known to cause temperature increases in local areas. While releasing carbon dioxide into the atmosphere, deforestation also causes changes in how much light reflects off the earth's surface and the amount of moisture in the atmosphere from plants transpiring.
The impact of mid- and high-latitude deforestation is in fact more than the impact of tropical deforestation. The South Asian monsoon is the worst affected, with an 18% decline in precipitation over India.
Deforestation at high latitudinal and temperate regions causes decrease in precipitation in the Northern Hemisphere monsoonal regions like South Asia, North Africa, North America and East Asia. On the other hand, it causes an increase in precipitation in the Southern Hemisphere monsoonal regions like South America, South Africa and Australia.
The study, led by Govindasamy Bala from the Divecha Centre for Climate Change at the Indian Institute of Science and published in the Proceedings of National Academy of Sciences, is based on first hand climate modeling. Researchers used a model simulating atmosphere circulation, as well as photosynthesis, transpiration, warming of the ocean surface and ice melt.
The study quantifies the precipitation and the shift of the Inter-Tropical Convergence Zone (ITCZ) in all monsoon regions. The ITCZ appears as a band of clouds, usually thunderstorms, which circles the globe near the equator and drives current rainfall patterns in the tropics.
By the 1750s, humans had deforested approximately 6–7% of the global land surface area for cultivation. Today, croplands and pasturelands make up approximately 35% of the global land area. Such massive deforestation not only results in the increased emission of carbon dioxide and other greenhouse gases (GHGs), but also causes changes in surface characteristics like "albedo" or surface reflectivity and evapotranspiration.
"Yet, 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.
"The changes to surface characteristics such as reflectivity and plant transpiration (biogeophysical changes) and their effect on climate are not accounted," the professor added.
Furthermore, several studies have researched the impact of biogeophysical change on temperature but rainfall has not been studied, he said.
"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."
In the study, the main idea was to understand the effects of land use change on precipitation and rainfall, by analysing the shift in the ITCZ in monsoonal regions.
The researchers have used a state-of-the-art, comprehensive 3-dimensional climate model for this work. The model can simulate the atmospheric circulation, land surface processes such as photosynthesis and transpiration, surface-ocean warming and ice melt.
"We wanted to get a basic understanding of the effects of large scale deforestation at different locations on monsoon rainfall. So we performed 3 experiments: deforestation in the tropical, temperate and high-latitude areas," said Bala.
To understand the effects of deforestation on precipitation in the two hemispheres, the researchers chose monsoon regions from each hemisphere.
N Devaraju, the lead author of the paper, noted that large scale deforestation leads to a decrease in global mean surface air temperature by 1.50 K, 0.90 K, 0.47 K, and 0.04 K in the Global, Boreal, Temperate, and Tropical deforestation simulations, respectively.
Correspondingly, the global mean precipitation decreases by 3.21%, 1.70%, 1.01%, and 0.50%. According to the model, large scale deforestation leads to decrease in global-mean temperature as well as decrease in precipitation.
As most of the monsoon regions are located near ITCZ, a shift in ITCZ can affect the monsoonal regions. A southward shift was observed in all cases.
"This shift has a huge impact on tropical rainfall. For deforestation in the temperate and high-latitudes, we find that the ITCZ shifts southward and hence all the monsoon regions (South Asia, North Africa, North America and East Asia) in the Northern Hemisphere get reduced rainfall. At the same time, the Southern Hemisphere monsoon regions (Australia, South Africa and South America) receive more rains," said Bala.
Moreover, the high-latitude deforestation displaces the ITCZ southward much more than tropical deforestation -- indicating that the remote effects on rainfall are actually stronger than local effects.
In their global deforestation experiment, the SAs monsoon region is affected the most, with a 12% decline in precipitation across the region.
"This study has huge implications for evaluating the climate benefits of afforestation and reforestation programs that are promoted by Kyoto Protocol for reducing climate warming. Our study shows that just estimating carbon sequestration benefits of these programs is not sufficient. 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," said Bala.
He added that the results from this IISc study can also help scientists to understand monsoon changes in the past and future.