- Extreme one-day rainfall events in parts of India and Bangladesh increased by four times over four decades due to the effects of climate change.
- Future scenarios project extreme one-day events to double annually in the 2050-2079 period.
- Anthropogenic factors such as infrastructure development have multiplied the effects of flooding over the region, experts say.
Extreme one-day rainfall events over the northeastern parts of Bangladesh and India region, including the state of Meghalaya, quadrupled over the four decades from 1979 onwards, due to the effects of climate change, a new study from researchers across India, Bangladesh and the U.S. has found.
Published in the Quarterly Journal of the Royal Meteorological Society, the study found the quadrupling of extreme one-day rainfall events could spread to the Southeast Bangladesh region over the medium- to long-term. “We have discovered that the worst cases of extreme rainfall events have already started occurring in this region due to climate change,” Abdullah Al Fahad, assistant research scientist at the NASA Goddard Space Flight Center and lead author of the paper, told Mongabay-India over email. “Unfortunately, it will be very challenging to revert to previous climatological conditions, even with decreased CO2 emissions in the near future.”
Parts of the Indian northeast region have been receiving rainfall deficits in recent years, along with unevenly distributed rain and longer dry spells. Now, Fahad and his co-researchers have ascertained the role climate change plays in producing extreme rainfall and flooding in the region, which is on the rise.
In June 2022, the India-Bangladesh region witnessed extreme rainfall and flooding, killing over 170 people in Assam alone and displacing millions of others. “Many studies focus on data from only one of these countries, which does not fully capture the scenario, given the close geographical relationship between the Meghalaya and Assam regions of India and northern Bangladesh. Both Bangladeshi and Indian policymakers must consider meteorological data from both countries when assessing the impact of climate change on local communities and devising strategies to mitigate disaster scenarios.”
Changing climate systems
The researchers were able to establish that the northeast Bangladesh and India region (NEBI), southeast Bangladesh (SEB) and northwest Bangladesh (NWB) have historically experienced the most extreme one-day events, with most occurring between the monsoon months of May and October.
Seasonal monsoon rainfall occurs because low level jets – streams of relatively strong winds in the lower part of the atmosphere – carry moisture from the Bay of Bengal to the north inland, which turns into heavy precipitation when the sudden transport of moisture over a short period of 1 to 5 days, is met with the topography of the region.
The researchers used gridded meteorological data sets from the meteorological departments of both India and Bangladesh to observe trends in extreme rainfall over a 72-year period. They found that compared to the baseline period (1950-1979), the frequency of summer monsoon extreme events between 1979 and 2021 and has quadrupled over Meghalaya during the May to October months, driven by variabilities in the low level jet and warmer sea surface temperatures.
“Previous climate change studies focused on this specific region, concentrating solely on extreme rainfall events, and relied on only a handful of station data. More importantly, these studies utilized smaller temporal datasets, limiting their ability to separate climate change attribution from natural variability. We employed a large-scale gridded dataset based on multiple station data, as well as data spanning from the 1950s to the present, which enabled us to conclusively determine the influence of climate change on extreme precipitation events with statistical confidence,” said Fahad.
The study also used a type of climate model, the CMIP6, to project likely future trends of rainfall and precipitation in the region. It finds that by 2050-2079, the frequency of one-day events “nearly doubles annually” over the Northeast Bangladesh and India (NEBI) region, which includes the Sylhet division of Bangladesh and Meghalaya Plateau of India. This, the study, says, is due to “both seasonal mean increased humidity and a shift in the Bay of Bengal low level jet towards the north.”
Preparing for floods
A climate vulnerability index by the Council on Energy, Environment and Water (CEEW) had found that Assam was the most vulnerable to hydro-meteorological disasters in the country. But apart from the frequency of extreme events, vulnerability to climate change related disasters is also made worse by human activity.
“There has been an unpragmatic expansion of infrastructure projects in the region that multiply the effects of flooding. Deforestation in the hills, which leads to more runoff, also causes the water to carry more sediment as it comes down, which makes the flooding worse,” said Partha Jyoti Das, an environmental scientist and head of the water, climate and hazards division at Aaranyak, an Assam based environmental NGO working on conservation and climate related issues.
Aaranyak is working on an integrated flood management plan that proposes inter-state cooperation and coordination, focussing on keeping the watersheds of rivers intact along with downstream impacts. “States must work towards improving their forecasting and early warning systems so that they reach the most vulnerable groups.”
Banner image: Flooding in Dibrugarh, Assam. Photo by Arunabh0368/Wikimedia Commons.
