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The Present Threat to Coastal Cities From Antarctic and Greenland Melt
19
August, 2017
Seas
around the world are rising now at a rate of about 3.3 millimeters
per year.
This
rate of rise is faster than at any time in the last 2,800 years.
It’s accelerating. And already the impacts are being felt in the
world’s most vulnerable coastal regions.
(Rates
of global sea level rise continue to quicken. This has resulted in
worsening tidal flooding for coastal cities like Miami, Charleston,
New Orleans and Virginia Beach. Image source: Ice
Melt, Sea Level Rise, and Superstorms.)
Sea
Level Rise and Worsening Extreme Rainfall are Already Causing Serious
Problems
Last
week, New
Orleans saw pumps fail as a heavy thunderstorm inundated the city.
This caused both serious concern and consternation among residents.
Begging the question — if New Orleans pumps can’t handle the
nascient variety of more powerful thunderstorms in the age of
human-caused climate change, then what happens when a hurricane
barrels in? The pumps, designed
to handle 1.5 inch per hour rainfall amounts in the first hour and 1
inch per hour rainfall amounts thereafter were
greatly over-matched when sections of the city received more than 2
inches of rainfall per hour over multiple hours.
Higher
rates of precipitation from thunderstorms are becoming a more common
event the world over as the hydrological cycle is amped up by the
more than 1 degree Celsius of temperature increase that has already
occurred since 1880. And when these heavy rainfall amounts hit
coastal cities that are already facing rising seas, then pumps and
drainage systems can be stressed well beyond their original design
limits. The result, inevitably, is more flooding.
(Dr
Eric Rignot, one of the world’s foremost glacial scientists,
discusses the potential for multimeter sea level rise due to
presently projected levels of warming in the range of 1.5 to 2 C by
mid to late Century.)
New
Orleans itself is already below sea level. And the land there is
steadily subsiding into the Gulf of Mexico. Add sea level rise and
worsening storms on top of that trend and the crisis New Orleans
faces is greatly amplified.
All
up and down the U.S. East and Gulf Coasts, climate change driven sea
level rise and a weakening Gulf Stream are
combining with other natural factors that can seriously amplify an
ever-worsening trend toward more tidal flooding.
It’s a situation that will continue to worsen as global rates of
sea level rise keep ramping higher. And how fast seas rise will
depend both on the amount of carbon that human beings ultimately dump
into the Earth’s atmosphere and on how rapidly various glacial
systems around the world respond to that insult (see discussion by
Dr. Eric Rignot above).
Presently
High and Rising Atmospheric Carbon Levels Imply Ultimately
Catastrophic Sea Level Rise — How Soon? How Fast? Can We Mitigate
Swiftly Enough to Prevent the Worst?
Presently,
atmospheric carbon forcing is in the range of 490 parts per million
CO2 equivalent. This heat forcing, using paleoclimate proxies from 5
to 30 million years ago, implies
approximately 2 degrees Celsius of warming this Century and
about 4 degrees Celsisus of warming long term. It also implies an
ultimate sea level rise of between 60
and 180 feet over the long term.
In other words, if atmospheric carbon levels are similar to those
seen during the Miocene, then temperatures are also ultimately headed
for those ranges. Soon to be followed by a similar range of sea level
rise. In the nearer term, 1.5 to 2 C warming from the 2030s to late
Century is enough to result in 20 to 30 feet of sea level rise.
Of
course, various
climate change mitigation actions could ultimately reduce that larger
heat forcing and final related loss of glacial ice.
But with carbon still accumulating in the atmosphere and with Trump
and other politicians around the world seeking to slow or sabotage a
transition away from fossil fuels, then it goes to follow that
enacting such an aggressive mitigation will be very difficult to
manage without an overwhelming resistance to such harmful policy
stances.
(Antarctic
ice loss through 2016. Video source: NASA.)
That
said, warming and related sea level rise will tend to take some time
to elapse. And the real question on many scientists’ minds is —
how fast? Presently, we do see serious signs of glacial
destabilization in both Greenland and West Antartica. These two very
large piles of ice alone could contribute 34 feet of sea level rise
if both were to melt entirely.
Meanwhile, East
Antarctica has also recently shown some signs of movement toward
glacial destabilization.
Especially in the region of the Totten Glacier and the Cook Ice
Shelf. But rates of progress toward glacial destabilization in these
zones has, thus far, been slower than that seen in Greenland and West
Antarctica. Present mass loss hot spots are in the area of the
Thwaites Glacier of West Antarctica and around the western and
southern margins of Greenland.
(Greenland
ice loss through 2016. Video source: NASA.)
With
global temperatures now exceeding 1 C and with these temperatures
likely to exceed 1.5 C within the next two decades, it is certain
that broader heat-based stresses to these various glacial systems
will increase. And we are likely to see coincident melt rate
acceleration as more glaciers become less stable. The result is that
coastal flooding conditions will tend to follow a worsening trend —
with the most vulnerable regions like the U.S. Gulf and East Coasts
feeling the impact first. Unfortunately, there is risk that this
trend will include the sudden acceleration of various glaciers into
the ocean, which will coincide with rapid increases in global rates
of sea level rise. In other words, the trend for sea level rise is
less likely to be smooth and more likely to include a number of melt
pulse spikes.
Such
an overall trend including outlier risks paints a relatively rough
picture for coastal city planners in the 1-3 decade timeframe. But on
the multi-decade horizon there is a rising risk that sudden glacial
destabilization — first in Greenland and West Antarctica and later
in East Antarctica will put an increasing number of coastal cities
permanently under water.
Rapid
Mitigation Required to Reduce Risks
The
only way to lower this risk is to rapidly reduce to zero the amount
of carbon hitting the atmosphere from human sources while ultimately
learning how to pull carbon out of the atmosphere. The present most
rapid pathway for carbon emissions reductions involves an urgent
build-out of renewable and non-carbon based energy systems to replace
all fossil fuels with a focus on wind, solar, and electrical vehicle
economies of scale and production chains. Added to various drives for
sustainable cities and increasing efficiency, such a push could
achieve an 80 percent or greater reduction in carbon emissions on the
2-3 decade timescale with net negative carbon emissions by mid
Century. For cities on the coast, choosing whether or not to support
such a set of actions is ultimately an existential one.
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