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OP-ED: The story of flooding in the language of trees

  • Published at 01:18 am December 4th, 2020
Brahmaputra Bogra
The waters of the Brahmaputra in Bogra BIGSTOCK

Even leaving aside climate change, we can expect a higher frequency of flooding in the future

We all know climate change is massively disrupting weather patterns all over the world, with especially devastating effects in our South Asian countries. But a new study released earlier this week by Columbia University scientists suggests even our baseline assumptions have been badly skewed by inadequate data which fail to register longer-term trends and variations.

Writing in the journal Nature Communications, in their paper headlined “Seven centuries of reconstructed Brahmaputra River discharge demonstrate underestimated high discharge and flood hazard frequency,” 11 co-authors led by Mukund Palat Rao say: “While most climate models predict an intensified monsoon and increase in flood risk with warming, robust baseline estimates of natural climate variability in the [Brahmaputra river] basin are limited by the short observational record.” 

Rao et al endeavoured to dig much deeper, using “a new seven-century (1309–2004 CE) tree-ring reconstruction of monsoon season Brahmaputra discharge to demonstrate that the early instrumental period (1956–1986 CE) ranks amongst the driest of the past seven centuries (13th percentile). Further, flood hazard inferred from the recurrence frequency of high discharge years is severely underestimated by 24–38% in the instrumental record compared to previous centuries and climate model projections.”

Put more simply, this novel research looked at rings in the trunks of ancient trees from the Brahmaputra river basin, and discovered there was much greater variation in flood patterns than instrumental records (which only began in the 1950s) even begin to indicate. In fact, the current era is actually amongst the driest since the 14th century. Bottom line: Even leaving aside climate change, we must expect a far higher frequency of flooding than previously predicted, because the prevailing models are off by at least 40%.

This is very bad news for everyone who lives in the giant Brahmaputra river basin, which extends across vast territories spanning India, Tibet, Bhutan, Nepal, and Bangladesh that are home to over 600 million people. 

The researchers from Columbia’s excellent Lamont-Doherty Earth Observatory first looked at data from a river-flow gauge at Bahadurabad in northern Bangladesh, and then contrasted it with tree-ring samples -- which are bored out without damaging the trees -- from adjacent old-growth forests (the oldest was a Tibetan juniper from 449).

Using this method, they compiled a chronology going back 696 years, from 1309 to 2004, which lines up neatly with what we already know about flood years, but also shows there were extensive periods -- such as between 1560 and 1600, and again from 1750 to 1800 -- that were far more extreme than anything we have learned over the past several decades of instrumental records.

Via email from New York, Rao told me this research tells us “the past conditions were wetter than the present, and with climate change we expect the future to be wetter as well. This information did not exist previously. I think the main implication is that it reinforces that we need to continue to invest in flood preparedness.”

Rao explained: “As trees grow they incorporate information about the environmental conditions they are living in in their annual growth rings. Trees in the region grow more, and put on wider 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. These cores that we take are really small and do not injure or harm the trees. This field of study is known as dendrochronology.”

In their paper, the co-authors write: “We find that recent decades underestimate the frequency of high discharge and in turn flood hazard from natural variability by 24.37–37.93% and climate change impacts by 42.53–50.11%.” 

Rao elaborated: “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.”

Praemonitus, praemenitus said the Romans, which translates from Latin as “forewarned is forearmed.” This new study about Brahmaputra flooding unambiguously predicts more -- and wetter -- imminent monsoon seasons than any other data has ever indicated. But while we know things are going to get very much wetter, this does not mean disaster is inevitable.

Rao et al also write that we do not have to assume “high discharges will continue to be associated with an increased likelihood of flood hazard in the future.” There are many actions that can be taken to prepare for and mitigate the effects of what we now know is definitely on the way. They conclude, “potential changes in policy, land use or infrastructure [can] ameliorate ‘flood risk.’”

Vivek Menezes is a writer based in Goa, India.

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