Every year, the Brahmaputra floods vast areas in India’s Northeast, particularly Assam, and continues its trail of destruction into Bangladesh, from where it finally flows into the Bay of Bengal. For years, scientists have been looking with concern at the river’s potential for catastrophic flooding in the future, especially as the climate warms. It turns out that this potential has been underestimated so far — even without accounting for a warming climate. This is the conclusion of a new study published in the journal Nature Communications.
Existing projections of flooding of the Brahmaputra are based on observations of past rainfall patterns, but they rely on discharge-gauge records that date back only to the 1950s.
The new study, on the other hand, is based on an examinations of tree rings, which provided a picture of rainfall patterns going back seven centuries.
The rings showed that the post-1950s period was actually one of the driest since the 1300s — there have been much wetter periods in the past. Using climate models to simulate for future discharge, the researchers found that destructive floods probably will come more frequently than thought.
“The tree-rings suggest that the recent decades (particularly from the 1950s to 1980s) were unusually dry. Therefore, in general, past conditions were wetter,” lead author Mukund Palat Rao, who recently earned his PhD from Columbia University, said by email.
“Similarly, climate models suggest that the future will likely be wetter due to our emissions of carbon-dioxide. Taken together, this suggests that we might be underestimating the current frequency of ‘wet years’ and in turn of flooding,” Rao said.
If one projects from modern discharge records, the study found, one would be underestimating the danger by 24% to 38%. As Rao explains on the Columbia University website: “If the instruments say we should expect flooding toward the end of the century to come about every four-and-a-half years, we are saying we should really expect flooding to come about every three years.”
Why tree rings
Tree rings grow wider in years when soil moisture is high. Indirectly, wider rings reflect more rainfall and higher river runoff.
“As trees grow they incorporate information about the environmental conditions they are living in in their annual growth rings,” Rao told The Indian Express. “Trees in the region grow more and put on wide rings in wet monsoon years. Conversely in dry monsoon years (or droughts) they grow less and put on narrow rings. Since some of these trees can live for a long time, by taking a small, pencil-thin tree-core from these trees and measuring their rings under a microscope we can learn more about climate conditions for the past several centuries.”
Ancient trees were sampled at 28 sites in Tibet, Myanmar, Nepal and Bhutan, at sites close enough to be affected by the same weather systems as the Brahmaputra watershed. Analysing the rings, the scientists built a 696-year chronology (1309 to 2004).
“The trees are telling us about past river flow by informing how wet the upper part of the basin was. This is obviously closely related to the strength of the monsoon,” Rao said. (The river originates in Tibet.)
From historical records going back to the 1780s, the researchers found that the widest rings coincided with major flood years. From this, they extrapolated the yearly river discharge in the centuries preceding modern records. 📣 Express Explained is now on Telegram
Findings & takeaways
From a river-flow gauge in northern Bangladesh, records showed a median discharge about 41,000 cubic metres per second from 1956 to 1986, and 43,000 cumec from 1987 to 2004.
The tree rings, in contrast, showed that 1956-1986 was in only the 13th percentile for river discharge, and 1987-2004 in the 22nd. The rings did show some other relatively dry times — in the 1400s, 1600s and 1800s — but they also showed periods of extreme flooding with no comparable period during 1956-2004. The worst spell lasted from about 1560-1600, 1750-1800 and 1830-1860.
The researchers said projecting from the existing discharge record would underestimate future flood hazard by 24-38%, without factoring in climate warming — which would only increase the frequency of future flooding.
Higher temperatures drive more evaporation of ocean waters, and in this region that water ends up as monsoon rainfall. That is why scientists believe that the warming climate will intensify monsoon rains in coming decades, and in turn increase seasonal flooding.
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Low-lying areas of Bangladesh are traditionally hit the hardest when the Brahmaputra is flooded. In 1998, 70% of Bangladesh went underwater, while serious floods also came in 2007 and 2010, the researchers noted.
The findings are obviously relevant to Assam and Northeast India too. “Additionally,” Rao said, ”flood risks could be compounded by planned projects in the region.”
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