Greenhouse gas emissions caused by human activity has led to rising temperatures in India, a study by scientists at the Indian Institute of Technology Delhi has proved. The landmark study said the increase could have been higher if there weren’t a few mitigating factors.
Average temperatures in the country have risen by as much as half a degree Celsius between 1956 and 2005, according to the study published on June 12 in Scientific Reports, a journal of the Nature Group. The rise in annual and seasonal surface air temperature in India can be attributed mainly to greenhouse gases that are emitted due to human activity, said the study titled Human influence on sub-regional surface air temperature change over India.
“The reason for the rise in temperatures is greenhouse gases,” Krishna AchutaRao, Professor at the Centre for Atmospheric Sciences in Indian Institute of Technology (IIT) Delhi, and co-author of the report, told indiaclimatedialogue.net.
The rise in temperatures is particularly noticeable over the Himalayan regions as well as the east and west coasts of India, the study found. Much of the warming has occurred over the second half of the 20th century, it said.
Ironically, the temperature rise has been offset to some extent by the cooling influence of other pollutants released by human into the atmosphere above India, the study found. The main pollutants responsible for this partial cooling effect are aerosols, the authors said. Changes in land use and land cover have also mitigated temperature rise to some extent, they added.
This partial offset should not make policymakers complacent, since aerosols have serious adverse effects on human health.
Western Himalayas affected the worst
“The largest warming rates are seen in the western Himalayas, a mountainous region that is likely experiencing elevation-enhanced warming,” said the IIT Delhi study. “This has implications for the health of glaciers in this region, with continued anthropogenic forcing leaving them more prone to mass loss and resulting downstream effects.”
“We found that in the western Himalayas, the observed change is 1.7 degrees Celsius,” AchutaRao said. “We found that greenhouse gases account for 3 degrees Celsius warming and other anthropogenic reasons account for 1.5 degrees Celsius cooling, and (there is) no contribution from natural factors.”
“The glacier melt is a direct consequence of higher temperatures,” he told indiaclimatedialogue.net. “There is also a possibility of precipitation at higher altitudes coming down as rain rather than snow, leading to lower accumulation on glaciers.”
The study found that temperatures have gone up by about 0.5 degrees Celsius on the east coast and by 0.7 degrees in the west coast of India.
Climate change can be caused by natural factors as well as human factors such as emission of greenhouse gases, aerosol emissions, and changes in the land surface such as deforestation and urbanisation. The role of human beings in the global rise of temperatures has been documented by various assessment reports of the Intergovernmental Panel on Climate Change (IPCC).
However, these changes are harder to separate from natural variations — known as climate noise — over smaller geographical regions such as the Indian subcontinent. Numerous researchers have documented temperature changes over India, but before now, the responsibility of humans for the warming in the country had not been established conclusively by a scientific study.
There has been earlier research that attempted to attribute the warming over India to human causes. For instance, a 2016 study by P. Sonali and D. Nagesh Kumar of the Indian Institute of Science Bangalore — which analysed variations over time of annual and seasonal maximum and minimum temperatures in India during the second half of the 20th century — found that in most cases, natural variability may not be the major cause of changes in average minimum temperatures.
The IIT Delhi study, authored by AchutaRao, R. Dileepkumar and T. Arulalan, applied a statistical process called detection and attribution on two sets of temperature data collected over 100 years and found that the rise in average temperatures can be robustly attributed to anthropogenic causes. “Ours is the first study to actually quantify the human influence,” AchutaRao said.
“We used two observed temperature data sets, one from the Indian Institute of Tropical Meteorology (IITM) Pune, and the other from Climate Research Unit (CRU), University of East Anglia,” AchutaRao told indiaclimatedialogue.net. “We also used climate model output from the Coupled Model Intercomparison Project Phase 5 (CMIP5) that include simulations with individual forcings that include natural forcings, greenhouse gases only, all anthropogenic forcings, and of course, all of natural and anthropogenic.”