others, like Indonesia’s Jakarta, are also sinking
fast. Some spots in Jakarta are sinking at a rate of
20 to 28 centimeters a year.
“For an individual, it doesn’t matter if the water
is coming from sea rise or a storm surge or the
clouds, a flood is a flood,” Hallegatte says. “Cities
should be looking … at one-meter sea level rise, at
least. Because the cost of failure is so big, you need
to have a plan for the worst-case scenario.”
An ambiguous picture
On a Sunday evening in June, the promenade
along Mumbai’s iconic Marine Drive is packed.
Families stroll eating ice cream, children chase
street vendors peddling cotton candy, and friends
squeeze together for selfies framed against the
blue-gray waters of the Arabian Sea. Dark, roiling
monsoon clouds loom over the horizon, as waves
crash a meter away against the concrete barricade.
The promenade was built a century ago when
India was part of the colonial British Empire.
The walkway’s days may be numbered. Mumbai’s
coastal waters rose at least nine centimeters during the 20th century, according to tide gauge data.
Today, seawater regularly spills over the promenade during high tide.
It’s not clear how much farther seas will rise
around Mumbai. A variety of factors, including
tides, gravity and Earth’s rotation, influence local
area sea rise in complex ways. And a lack of detailed
data on Mumbai’s coastal geography available to
scientists leaves questions on how future local
water levels will affect specific areas of the city.
The state of Maharashtra, where Mumbai is
located, acknowledged this data deficit in its 2014
climate change plan. Nevertheless, the state has so
far ignored a 2017 Indian Supreme Court order to
release maps demarcating future flood lines.
Maharashtra’s environment secretary, Anil
Diggikar, told Science News that the mapping
is being done, though he did not say when the
maps might be made public. But the state does
recommend that rainfall and sea level trends be
considered in new construction projects and
public infrastructure. “This is especially important for [the] economic hub of Mumbai and surrounding districts,” he says, while also touting
plans for restoring coastal stands of protective
Marine scientist Mani Murali of the National
Institute of Oceanography in Goa, India, has tried
to work out Mumbai’s future flood risk using
low-resolution 2011 topographic data from NASA.
That work, under peer review, doesn’t tell the
detailed story he knows the city needs. “But I
thought something is better than nothing.”
He may have a point, with the rate of global sea
rise fast accelerating — from a yearly average of
1. 8 millimeters in the last century to about
3.0 millimeters per year today, according to a
report in the Feb. 27 Proceedings of the National
Academy of Sciences.
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 sys-
tem 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.
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