[Vision2020] 8-15-13 "The inevitability of sea level rise" by Anders Levermann, Professor of Dynamics of the Climate System

[Ted Moffett]

http://www.realclimate.org/index.php/archives/2013/08/the-inevitability-of-sea-level-rise/#more-15633

Academic website for Anders Levermann:

http://www.pik-potsdam.de/~anders/

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The inevitability of sea level rise
Filed under:

   - Climate Science<http://www.realclimate.org/index.php/archives/category/climate-science/>

— stefan @ 15 August 2013

*Guest post by Anders Levermann <http://www.pik-potsdam.de/%7Eanders/> [via The
Conversation<http://theconversation.com/the-inevitability-of-sea-level-rise-16871>
]*

Small numbers can imply big things. Global sea level rose by a little less
than 0.2 metres during the 20th century – mainly in response to the 0.8 °C
of warming humans have caused through greenhouse gas emissions. That might
not look like something to worry about. But there is no doubt that for the
next century, sea level will continue to rise substantially. The
multi-billion-dollar question is: by how much?

The upper limit of two metres that is currently available in the scientific
literature would be extremely difficult and costly to adapt to for many
coastal regions. But the sea level will not stop rising at the end of the
21st century. Historical climate records show that sea levels have been
higher whenever Earth’s climate was warmer – and not by a couple of
centimetres, but by several metres. This inevitability is due to the
inertia in the ocean and ice
masses<http://www.pnas.org/content/early/2013/07/10/1219414110>on the
planet. There are two major reasons for the perpetual response of
sea level to human perturbations.

One is due to the long lifetime and warming effect of carbon dioxide in the
atmosphere. Once emitted carbon dioxide causes warming in the atmosphere
over many centuries which can only be reduced significantly by actively
taking the greenhouse gas out again. This is because both the amount of
heat and carbon dioxide the ocean can absorb is reduced, and so the
temperature stays up for centuries or even
millennia<http://dx.doi.org/10.1073%2Fpnas.0812721106>.
Of course, not cutting emissions would exacerbate the problem even further.

The other reason is that both the ocean and the ice masses are very big and
a warming of the surrounding atmosphere will only penetrate slowly, but
inevitably, into them. As a consequence their sea level contribution
continues even if the warming does not increase. Sea level rise over the
last century has been dominated by ocean warming and loss of glaciers. Our
recent study<http://www.pnas.org/content/early/2013/07/10/1219414110.abstract>indicates
that the future sea level rise will be dominated by ice loss from
the two major ice sheets on Greenland and Antarctica – slumbering giants
that we’re about to wake.

[image: levermann2013]<http://www.realclimate.org/images//levermann2013.jpg>

*Graph: Sea level rise contributions over 2000 years from: ocean warming
(a), mountain glaciers (b), Greenland (c) and Antarctic (d) ice sheets. The
total sea level commitment (e) is about 2.3m per degree of warming above
pre-industrial.
*

It is easier to understand a future world that has adjusted to a new
equilibrium of higher temperatures than it is to understand the dynamic
(perhaps rapid) transition from today’s world to a warmer one. That is why
we used physical models for the ocean, the mountain glaciers and the big
ice sheets to compute how the systems would be different if the world was
warmer.

What we found was that for each degree of global warming above
pre-industrial levels the ocean warming will contribute about 0.4 metres to
global mean sea-level rise while Antarctica will contribute about 1.2
metres. The mountain glaciers have a limited amount of water stored and
thus their contribution levels off with higher temperatures. This is
over-compensated for by the ice loss from Greenland, so that in total sea
level rises quasi-linearly by about 2.3 metres for each degree of global
warming (see figure).

How fast this will come about, we do not know. All we can say is that it
will take no longer than 2,000 years. Thus the 2.3 metres per degree of
warming are not for this century. They need to be considered as our sea
level commitment – the sea level rise that cannot be avoided after we have
elevated global temperatures to a certain level.

Ben Strauss of Climate Central <http://www.climatecentral.org/> has
considered <http://www.pnas.org/cgi/doi/10.1073/pnas.1312464110> the
different possible future pathways that society might take and computed
which US cities are at risk <http://sealevel.climatecentral.org/> in the
long-term. He poses the question as to what year, if we continue with
greenhouse emissions at current rates, we will have caused an inevitable
sea level rise that puts certain cities at risk.

According to his analysis, within the next few years Miami in Florida will
be committed to eventually lie below sea level, while our future actions
can still decide on whether we want to one day give up cities such as
Virginia Beach, Sacramento, Boston, Jacksonville or New York City.

This is a decision society has to take for future generations. We will need
to adapt to climate change in any case, but some things we will not be able
to adapt to. Society needs to decide whether we want to give up, for
example, the Tower of London, or to put the breaks on climate change so
that we don’t have to.

*Weblink:* The New York Times has a good current
article<http://www.nytimes.com/2013/08/13/science/timing-a-rise-in-sea-level.html?_r=1&>on
this issue.

*Anders Levermann <http://www.pik-potsdam.de/%7Eanders/> is department head
at the Potsdam Institute for Climate Impact Research, Germany*

*-------------------------------------------*

*Vision2020 Post: Ted Moffett
*
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