http://www.ecowatch.com/sea-level-rise-inevitable-2195929828.html
[links in on-line article]
Sea Level Rise Inevitable, Even With Dramatic Cuts in Carbon Emissions
January 17, 2017
By Tim Radford
Even supposing the world does make dramatic reductions in carbon dioxide
emissions, the fate of the small island states remains uncertain.
New research suggests that other, more short-lived greenhouse gases in
the atmosphere could drive the thermal expansion of the oceans for
centuries to come.
This is grim news for those low-lying coral atoll nations in the Pacific
already threatened by sea level rise.
They fought at the Paris summit in 2015 for world agreement on a
reduction of emissions that would keep global warming to 1.5 C above
historic levels. But a new study, in the Proceedings of the National
Academy of Sciences, suggests that may not be enough.
"If you think of countries like Tuvalu, which are barely above sea
level, the question that is looming is how much we can emit before they
are doomed. Are they already slated to go under, even if we stopped
emitting everything tomorrow?" said Susan Solomon, professor of
atmospheric chemistry at the Massachusetts Institute of Technology.
Inevitable Rise
"It's all the more reason why it's important to understand how long
climate changes will last and how much more sea-level rise is already
locked in."
Sea levels are rising as glaciers the world over retreat and Arctic ice
caps melt, in response to rising carbon dioxide levels in the
atmosphere, as humans burn fossil fuels.
But this time the MIT team applied their climate model not to carbon
dioxide, but to the other greenhouse gases, among them methane and the
chlorofluorocarbons. They then fed into the simulation the effect of
heat on water, which expands with temperature.
And the short-lived gases are potent: the finding says that even if the
production of these ceased entirely, at the end of 100 years
three-fourths of the thermally-driven sea level rise would still be
there, washing over low-lying coasts, drowning estuaries and river
deltas, and seeping into the coral bedrock of islands too small even to
register on the global climate maps.
And this slow enduring threat is what the small island states have
feared most. They know that their citizens cannot survive if the sea
levels rise to the heights predicted in the worst-case scenarios, but it
isn't clear where they could go to escape. They have already been warned
that even the Paris Agreement may not be enough to save them.
Carbon dioxide is an enduring molecule: even if emissions ceased now,
the quantities already emitted will go on warming the world for another
1,000 years. So the effects of fossil fuel combustion are irreversible
in the scales of human lifetimes.
But the new research shows that the short-lived gases have long-term
consequences as well. Chlorofluorocarbons were banned by the Montreal
Protocol nearly three decades ago: had the ban been delayed to 2050,
then by 2100 they would have added 14 cms to the overall sea level rise,
the authors calculate.
The scientists also tested methane in their simulation: suppose the
world went on emitting at the current rate until 2050, 2100 or 2150 and
then stopped completely?
In all three scenarios the methane cleared quickly and atmospheric
warming slowed. But ocean warming continued for centuries and with it,
sea level rise. And the later the emissions ceased, the longer the seas
stayed high.
"Amazingly, a gas with a 10-year lifetime can actually cause enduring
sea-level changes," Prof. Solomon said. "So you don't just get to stop
emitting and have everything go back to a pre-industrial state. You are
going to live with this for a very long time."