The most advanced set of climate models used in the Intergovernmental Panel on Climate Change Assessment Reports could be underestimating the impacts of climate change on rainfall over India, a new analysis has found.
The sixth Coupled Model Intercomparison Projects, or CMIP6, is a leading group of models that “simulate the physics, chemistry and biology of the atmosphere, land and oceans” to project the future impacts of climate change under various emission scenarios.
They are among the most advanced set of climate models widely used by researchers and policymakers to understand how climate change will affect weather patterns and socioeconomic development pathways across the world.
But while the CMIP6 is able to provide insights at a global level, it is likely to falter at a more regional level. A new study, correcting and downscaling the CMIP6’s projections, finds that at a regional scale over India, the CMIP6 is underestimating the magnitude of long and short duration extreme rainfall events in the future.
“These findings may provide fundamental insights to formulate national climate change adaptation policies for the extreme rainfall events,” the researchers, from the Indian Institute of Tropical Meteorology and the National Institute of Technology, Rourkela, wrote in the paper.
Refining models
The study aims to understand the effects of climate change on the Indian summer monsoon between June and September, across various emission scenarios. To do this, the researchers downscaled and bias-corrected the sea surface temperature and precipitation components of the CMIP6 models.
“Scaling essentially involves bringing the coarse resolution from the CMIP6 models to a higher resolution, and bias correction involves bringing the outcomes as close as possible to observations,” explained Jasti S Chowdary, a scientist with Indian Institute of Tropical Meteorology and co-author of the study.
The baseline period for models and observations used are India Meteorological Department data from 1980 to 2014. While most other downscaled models include only the precipitation component, this study is among the first few to also include sea surface temperature in its projections, which is an important determinant of the Indian summer monsoon’s progress.
The researchers were able to tell that the CMIP6 simulations “strongly underestimate the intensity and spatial distribution of rainfall associated with the extreme rainfall events” because downscaling and bias correction resulted in projections improving by 96 % – meaning they were closer to actual observed outcomes by 96%.
The researchers found that in the baseline period (1980-2014), 8% of the Indian land region is experiencing heavy rainfall associated with the extreme rainfall events during the summer monsoon season, whereas in the near future (between 2030 and 2060), a 14% rise is noticed compared to the baseline period in a high emissions scenario, and an 18% rise is seen in the far future (2060 to 2100).
Further, in the far future, extreme rainfall thresholds are projected to be 21% more than the baseline under the high emissions scenario, whereas it is 6% in the mid-emissions scenario.
It is also found that, before 2060, the number of short duration extreme rainfall events is high compared to the long duration extreme rainfall events. After 2060, this trend switches, “which is a two-fold increase of long duration extreme rainfall events compared to the short duration extreme rainfall events over the monsoon core region”, says the study.
“Long duration extreme rainfall events last from several days. Typically, these events result in a large total accumulation of rainfall over larger geographic areas, often leading to sustained high water levels in rivers and reservoirs. Short duration extreme rainfall events last from several hours to a few days. They are characterised by very high rainfall rates, leading to rapid accumulation of water in a short period. They are usually localised, affecting smaller geographic areas,” said Gopi Nadh Konda, lead author of the study.
The CMIP6 models also underestimated the seasonal mean and extreme precipitation thresholds. According to the analysis, the seasonal mean precipitation extreme thresholds over India is 48.8 mm per day, according to India Meteorological Department rainfall. The uncorrected CMIP6 models assumed they were 25.8 mm per day, whereas corrected precipitation is closer to the observations.
Erratic rainfall in India
The Indian Institute of Tropical Meteorology study adds to previous research which also found that popular climate models were likely underestimating the impacts of climate change on rainfall over the subcontinent.
A 2020 study from researchers from the Indian Institute of Gandhinagar and Michigan State University found that bias-corrected projections of CMIP6 “project a warmer (3-5 degrees celsius) and wetter (13%-30%) climate in South Asia in the 21st century”.
The research comes at a time when India’s monsoon season is becoming more and more erratic. A tehsil level analysis of rainfall patterns by the Council On Energy, Environment and Water found that 55% of tehsils witnessed an increase and 11% witnessed a decrease in southwest monsoon rainfall from 2012 to 2022 by more than 10% each, compared to the climatic baseline (1982-2011).
It also found that 48% of tehsils “saw an increased rainfall in October by over 10 percent, which could be due to the delayed withdrawal of the southwest monsoon from the subcontinent”.
This article was first published on Mongabay.
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