As parts of the world, including India, face extreme weather events such as increased intensity of rainfall, scientists have warned that global warming could bring about changes to the rainfall pattern. In a new study, researchers from the Indian Institute of Science (IISc) have warned that increase in greenhouse gases increases rainfall over land as well as the ocean.
In the study, which was published in Nature Research in July, researchers Chetankumar Jalihal, Jayaraman Srinivasan and Arindam Chakraborty have explored the evolution of precipitation over India in relation to that over the Bay of Bengal.
The results, the paper says, highlight the fact that global monsoons should not be treated as a homogeneous entity driven by incoming solar radiation over both land and ocean. While there are factors that affect land and ocean uniformly as well as differently, greenhouse gases were found to affect both land and ocean uniformly, unlike solar insolation or radiation. This would result in increased rainfall.
Mr. Jalihal, PhD scholar, IISc, said monsoons were traditionally defined over land, but rainfall over land and ocean are both part of the hydrological cycle“We have used a simple model for monsoon rainfall that depends on two parameters: water vapour and total energy going into atmosphere from the top and surface. An increase in greenhouse gases increases rainfall over land as well as ocean. This is due to the effect of water vapour. Greenhouse gases raise the surface temperature and therefore the amount of water vapour over both land and oceans, resulting in increased rainfall,” he explained.
Though incoming solar radiation influences the energy going into the atmosphere from the top, this has a direct effect on rainfall over land, but the rainfall over ocean decreases due to a chain reaction from surface energy, he said.
These results explain the evolution of Indian monsoon rainfall and the rainfall over the Bay of Bengal since the peak of the Ice Age, around 22,000 years ago, the researchers said.
“When the earth came out of the Ice Age, carbon dioxide and incoming solar radiation increased for the first 10,000 years leading to rapid global warming. During this period the effect of carbon dioxide was dominant and monsoon rainfall over India and the Bay of Bengal increase in a similar manner. Over the next 10,000 years, carbon dioxide concentration was nearly constant, and the effect of solar radiation was dominant, resulting in opposite trends in rainfall over India and the Bay of Bengal,” said Mr. Jalihal, adding that in the future, other factors such as deforestation, and aerosol emissions can have an impact on monsoon rainfall.
“Even though there is substantial evidence that the current global warming is due to the emission of carbon dioxide, sceptics propose that global warming is due to increase in incoming solar radiation. If this were true, it should result in a decrease in rainfall over oceans. Current observations indicate that rainfall has increased over both land and oceans. Thus, our results provide another line of evidence that current global warming is indeed due to carbon dioxide emissions,” he added.
