Fire and flood are on the rise. Bangladesh and New York face more flooding: the American West may see more forests burn.

LONDON, 14 December, 2020 − More extreme weather is on the way for the hapless residents of Bangladesh, New York and the western US,  facing the prospect of worsening fire and flood.

There is a new future for New York. By the close of the century, thanks to sea level rise and global heating, parts of it could be swept by hurricane-driven catastrophic floods almost every year.

Things don’t look much brighter for much of Bangladesh. Scientists have recalculated the risk of flooding by the Brahmaputra river system to find that, even without the climate emergency, they had under-estimated the likelihood of devastating floods across the crowded, low-lying landscape.

And far away in the American west, US citizens face yet more and more devastating seasons of fire. The area incinerated by severe fires has increased eight-fold in the last 40 years, thanks to intensifying heat and drought. And thanks to climate change, drought will become more extended and more frequent. The temperatures, too, will go on rising.

All this emerged in just another week of routine climate science, as researchers try to gauge the difficulties to come, for national and civic authorities, for foresters and for farmers.

“The increase in these once-in-a-generation floods is so dramatic because the impact of sea-level rise will create greater flooding, even if the storms today stay the same”

In 2012, Hurricane Sandy hit the US to cause $70bn in damages, and even slammed unexpectedly into New York, to devastate parts of the city. It counted as a once-in-500 years event.

Researchers report in the journal Climatic Change that they looked at the probabilities of more flooding in Jamaica Bay, on Long Island, New York as sea levels rose, along with the sea surface temperatures that drive fiercer storm weather, through the century.

Floods that tend to happen every century could, by 2050, occur every nine years. By 2080 to 2100, they could become annual events. And 500-year events like the 2012 superstorm could by the end of the century happen perhaps once every four years.

“Future projections of the hurricane climatology suggest that climate change would lead to storms that move more slowly and are more intense than we have ever seen before hitting Jamaica Bay,” said Reza Marsooli, an environmental engineer at the Stevens Institute of Technology in Hoboken, New Jersey, a co-author.

“But the increase in these once-in-a-generation or even less frequent floods is so dramatic because the impact of sea-level rise will create greater flooding, even if the storms we are seeing today stayed the same.”

Prepare for worse

The hazard that faces Bangladesh − much of which is at sea level, on fertile floodplain created by the Ganges-Brahmaputra river system − is more insidious.

One of the great waterways of the world, it rises in the Himalayan snows and swells in the monsoon season to flood the rice paddies and replenish farmlands with nourishing sediments. Occasionally the floods become devastating: in 1998, some 70% of the nation was submerged. Floods have recurred, in 2007, 2010 and 2020.

Engineers have been monitoring the flow since the 1950s, and thought they knew the flood probabilities. But US, Australian and Chinese scientists report in the journal Nature Communications that they studied the growth rings in ancient trees to find that Bangladeshis have been living in unusual times: for much of the past 70 years, on the evidence told by old trees along the watershed, the river flow has been unusually dry − the driest in the last 700 years.

“The tree rings suggest that the long-term baseline conditions are much wetter than thought,” said Mukund Palat Rao, of the Lamont-Doherty Earth Observatory at Columbia University in New York, who led the research.

“Whether you consider climate models or natural variability, the message is the same. We should prepare for a higher frequency of flooding than we are currently predicting.”

Forests’ future threatened

In the past 40 years, thanks to global heating driven by ever-higher emissions of greenhouse gases from the combustion of fossil fuels, the state of California has experienced a series of droughts that lasted for years. The fire season too has begun earlier and lasted much longer.

Ecologists report in the journal Geophysical Research Letters that they defined high-severity fires as those that killed 95% of all trees. They then counted the most severe episodes of burning in four great regions of the western US from 1985 to 2017.

They found that by 2017, the area wiped out by severe fires had risen eight times, to more than 2,000 sq kms or 800 sq miles. Much of the tree cover of the US west is adapted to episodes of fire. But the frequency and intensity of recent blazes threatens the future of the forests altogether.

“As more area burns at high severity, the likelihood of conversion to different forest types or even to non-forest increases,” said Sean Parks of the US Forest Service Rocky Mountain Research Station, and the lead author.

“At the same time, the post-fire climate is making it increasingly difficult for seedlings to establish and survive, further reducing the potential for forests to return to their pre-fire condition.” − Climate News Network