Study Says Future Brahmaputra River Floods Due to Climate Change Could Be Underestimated

A new study examining seven centuries of water flow in South Asia’s mighty Brahmaputra River suggests that scientists are underestimating the river’s potential for catastrophic flooding as the climate warms. The revelation comes from tree ring examinations, which showed precipitation patterns that date back centuries before instrumental and historical records.

Many researchers agree that climate warming will intensify the seasonal monsoon rains that drive the Brahmaputra, but the purported baseline of previous natural variations in river flow is mostly based on flowmeter records dating back only to the 1950s. The new study, based on age-old tree rings in and around the river’s watershed, shows that the post-1950s period was actually one of the driest since the 1300s. The rings show there have been much longer periods wet in the past, driven by natural oscillations that occurred over decades or centuries. The takeaway: Destructive floods are likely to come more frequently than scientists thought, even without the effects of man-made climate change. Estimates are likely nearly 40 percent lower, the researchers say. The results have just been published in the journal Nature Communications.

“The tree rings suggest that long-term underlying conditions are much wetter than we thought,” said Mukund Palat Rao, Ph.D. student at Columbia University’s Lamont-Doherty Earth Observatory and lead author of the study. “Whether you look at climate patterns or natural variability, the message is the same. We need to be prepared for a higher frequency of flooding than we are currently predicting.”

The Brahmaputra is one of the most powerful rivers in the world, flowing under a variety of names and intertwining paths for approximately 2,900 miles through Tibet, northeastern India and Bangladesh. Near its mouth, it combines with the Ganges River in India to create the third largest ocean runoff in the world, behind only the Amazon and Congo. (It is related to the Orinoco of Venezuela.) In places it is nearly 12 miles wide. Its delta alone is home to 130 million Bangladeshis and many millions more live upstream.

The river regularly floods the surrounding areas during the July-September monsoon season, when moisture-laden winds sweep from the Indian Ocean and bring rain along its length, from its Himalayan headwaters to the coastal plain. As with the Nile, flooding has a positive side, as the waters release nutrient-rich sediments to replenish farmland and some degree of flooding is essential for rice cultivation. But some years, the floods spiral out of control and lower Bangladesh is hit hardest. In 1998, 70 percent of the country went under water, taking crops, roads and buildings out and killing many people. Other severe floods occurred in 2007 and 2010. As of September 2020, the worst flooding since 1998 was still ongoing, with a third of Bangladesh flooded and 3 million people homeless.

Higher temperatures lead to more evaporation of ocean waters, and in this region water ends up as rain on land during the monsoon. As a result, most scientists believe that warming climate will intensify monsoon rains over the next few decades and, in turn, increase seasonal flooding. The question is: how often could major floods occur in the future?

The authors of the new study first looked at the records of a river flow meter in northern Bangladesh. This showed a median flow rate of about 41,000 cubic meters per second from 1956 to 1986 and 43,000 from 1987 to 2004 (during the big flood year of 1998, the maximum flow more than doubled).

They then examined data from age-old tree rings that the researchers sampled at 28 sites in Tibet, Myanmar, Nepal and Bhutan, at sites within the Brahmaputra watershed, or close enough to be affected by the same weather systems. Most of the samples have been taken from conifer species over the past 20 years by scientists from the Lamont-Doherty Tree Ring Lab, led by study co-author Edward Cook. Because people have long since felled trees in populous areas, Cook and his colleagues sometimes walked for weeks to reach undisturbed places in remote and mountainous terrain. Samples of the straw width were drilled from the logs, without damaging the trees. The oldest tree they found, a Tibetan juniper, was from the year 449.

Back in the laboratory, they analyzed tree rings, which widen in years when soil moisture is high, and therefore indirectly reflect rainfall and the resulting river runoff. This allowed the scientists to assemble a 696-year chronology, from 1309 to 2004. By comparing the rings with modern instrumental records and historical records dating back to 1780, they could see that the wider rings lined up neatly with the major known years of flooding. This in turn allowed them to extrapolate the river’s annual flow in the centuries preceding modern records. They found that 1956-1986 was only in the 13th percentile for river discharge and 1987-2004 in the 22nd.

This, they say, means that anyone using the modern discharge record to estimate future flood risk would underestimate the hazard by 24 to 38 percent, based solely on natural variations; To this should be added man-made warming. “If the instruments say we should expect floods towards the end of the century every four and a half years, we are saying that we should really expect floods to occur every three years,” Rao said.

Tree rings show other relatively dry times, in the 1400s, 1600s and 1800s. But they also show very wet periods of extreme flooding with no analogues in the relatively short modern instrumental period. The worst lasted from about 1560 to 1600, from 1750 to 1800 and from 1830 to 1860.

Climate change will almost certainly affect the flow of other major rivers in the region, though not necessarily in the same way. The mighty Ganges, which flows mainly through India, is also powered mainly by the monsoon, so it will likely behave very similarly to the Brahmaputra. But the Indus, which flows through Tibet, India, and Pakistan, derives most of its flow not from the monsoon, but rather from the winter accumulation of snow and ice in the Himalayan glaciers, and subsequent melting in the summer. In 2018 Rao and colleagues published a tree-ring study that showed the river’s flow has been anomalous in recent years. They suggest that when the climate warms and glaciers undergo accelerated melting, the Indus will provide plenty of water for the necessary irrigation, but at some point, when the glaciers lose enough mass, the seasonal tap will turn the other way and may not there is enough. water.

Human vulnerability to flooding along the Brahmaputra has increased in recent years not only due to the enormous volume of water, but also because the population and infrastructure are growing rapidly. On the other hand, accurate flood warnings have become more advanced and this has helped many villages to reduce economic and social losses. “High discharges will continue to be associated with a higher likelihood of flood risk in the future,” the study authors write. But, they say, this could be countered to some extent by “potential changes in policy, land use or infrastructure that could improve the risk of flooding.”

This story has been republished courtesy of the Earth Institute, Columbia University http://blogs.ei.columbia.edu.