And while global sea level projections up to 2050 are considered reliable, the situation beyond midcentury is less clear. Much depends on whether humankind can limit global emissions of carbon dioxide and other heat-trapping atmospheric gases. Princeton University climatologist Michael Oppenheimer is not optimistic.
“This is a battle that we are currently losing,” says Oppenheimer, a coordinating lead author of the Intergovernmental Panel on Climate Change’s special report on oceans, cryosphere and climate change, due out in September 2019. “Sea level rise and the flood heights are only going to increase …for the foreseeable future.”
The annual monsoon, the seasonal shift in winds that brings flooding rains to Mumbai, adds an extra layer of uncertainty to projecting how much flooding will accompany sea rise, he says. The future of this South Asian weather system has been difficult to predict, thanks in part to the mysterious influence of the Indo-Pacific Warm Pool. It’s Earth’s largest region of warm surface seawaters spanning the midocean region between the western Pacific and the eastern Indian oceans. That warmth partly fuels monsoon storm clouds.
Still, most studies suggest that the monsoon rains will increase. “Uncertainty is not an excuse [for inaction] at this point,” Oppenheimer says. “People need to get moving.”
Land where it shouldn’t be
Lakshmi Murali lives with her husband and son in a quiet, gated community, lush with jackfruit trees and flowering hibiscus in Mumbai’s flood-prone neighborhood of Andheri. Every June, as the rain starts falling, she unplugs the electronics in their ground floor apartment and moves her silk saris out from under the bed.
Across the city, the rains rage against the glass windows of luxury high-rises. Public transportation and street commerce come to a halt. Water pounds the tin roofs of slum shanties where about half of Mumbai’s 21.4 million people live. A sewage-tainted slurry burbles out of the city’s outdated and often-clogged drainage system, backing up into rivers and creeks that then overflow into homes and businesses.
Last year was particularly bad: In 24 hours, about 33 centimeters of rain fell. “You had to see it to believe it,” says Murali, a 54-year-old lawyer who is not related to the marine researcher of the same name. Her building’s plumbing system failed, and the toilets overflowed. Residents turned off their power for fear of getting electrocuted. As water rose inside their homes, Murali and a few neighbors used an iron rod to smash a hole through the wall surrounding their backyard to let the water flow out.
“Today, we are young, and we say, ‘Yeah, it’s OK,’ ” Murali says. Even as such flooding worsens, she has what some might call misplaced faith that things will work out. “The state will work on building enough infrastructure to keep the city alive and will not allow the city to drown. Man will work against what nature is proposing to do.”
Mumbai’s current predicament is partly due to the power of engineering over nature. Large parts of the city are built on land that, 300 years ago, was mostly underwater. When the Portuguese settled the region in the 16th century, they maintained Mumbai as a sleepy collection of coastal islands. But the British, who took over in 1661, reimagined Mumbai as a contiguous landmass and created a peninsula by filling in land gaps to connect the islands even in the wet season.
Much of Mumbai is built atop landfill (black) that connects several islands (green) in the middle of Bombay Harbor. Those passages once allowed water to flow through the system at high tide and during monsoon rains.
Source: T. Riding/J. Hist. Geography 2018
“So many of these megacities are built on land that is only artificially higher than sea level, in places where landfilling took place,” says Washington D.C.–based Susmita Dasgupta, the lead environmental economist for the World Bank’s Development Research Group.
Dasgupta was involved in the World Bank’s first report in 2007 on how sea level rise might affect national economies. The aim was to trigger discussion and preparation for a possible future economic catastrophe. She and her team offered guarded impact estimates based on hypothetical scenarios of between one and five meters of global sea level rise, using satellite images of coastal outlines and local elevations.
In estimating potential economic losses, the team considered an affected area’s population multiplied by the country’s gross domestic product per capita, but not infrastructure or property assets. That report projected that one meter of sea rise would cost the world 1.3 percent of the global economy. Applied to the forecast global GDP for 2018, that comes to about $1.3 trillion, not far from the estimates by Hallegatte’s team.
“But we wanted to raise the issue,” Dasgupta says. She faced a wave of hostility and derision for the effort. “Even bank colleagues were unhappy about it, saying we were being alarmist and that this kind of research was premature.” Eleven years later, no one doubts the sea is rising.
Juggling the numbers
Amid the confusing tumble of scientific studies on how climate change might raise flood risks, some scientists have built online visual apps to help the public understand what’s at stake.
One tool, by the U.S. National Oceanic and Atmospheric Administration, shows past global sea level trends based on tide gauges. But the app does not give projections. And it relies on sometimes patchy data. For example, there are no readings for Mumbai’s water levels from 1994 to 2005 or after 2010. The Maharashtra government says local sea levels are rising 1.2 millimeters a year, based on those incomplete data.
In 2017, a team from NASA’s Jet Propulsion Laboratory, or JPL, launched an app to demonstrate how melting ice sheets would affect 293 major port cities across the globe. The scientists measured the melt using NASA’s GRACE satellites, which detect gravity changes from the ice loss. To boost accuracy, the team recently added a component to the app that accounts for the fact that water expands as it warms.
Still, true sea level rise projections involve complex computer modeling of overlapping systems. The JPL app doesn’t do that. “So it’s risky” to put too much stock in the numbers it spits out, says JPL sea level and ice supervisor Eric Larour. “But the real risk is that people underestimate that this is going to get worse.”
For Mumbai, the JPL app foresees at least another 2.9-centimeter rise in coastal water in 10 years — and 14.4 centimeters in the next 50 years.
Those estimates could soon be revised upward. Larour’s team plans one more update to include research published in the June 13 Nature showing that Antarctic ice sheets are melting three times as fast as they were 25 years ago (SN: 6/7/18, p. 6). That much melting, Larour says, is “a big, big deal.”
The JPL team hopes to have a single, detailed modeling app for the world within two years, using NASA’s high-resolution satellite images of water levels and of land gradients, “so that people can use it in active mitigation policy,” Larour says. “A lot of areas at risk in South Asia — India, Bangladesh, Sri Lanka — and across Asia don’t have the information to do this.”
Economic gains lost
It’s not easy to find a coastal megacity taking decisive and effective action against future flood risks. Bangladesh has long built coastal sea walls of stacked mud, which may help prevent ocean storm surges from cascading inland to Dhaka. Fast-sinking Jakarta is working on its own giant sea wall as well. But walls won’t help Mumbai; they would prevent rain-driven freshwater floods from draining out after the monsoon.