Sea level rise could be limited to 5-6 ft this century if we keep temp increases below 2°C – transition needed in next decade or two to save east coast cities

CREDIT: South Florida and “Miami Island” in 2100 after 5 feet of sea level rise (via Climate Central).

From Joe Romm/Climate Progress, 31 Mar 2016 — We still could limit sea level rise to two feet this century if we keep total warming below 2°C, according to new analysis. Otherwise, we should be anticipating five to six feet of sea level rise by 2100 — which would generate hundreds of millions of refugees. That isn’t even the worst-case scenario.  This latest research from the journal Nature underscores that what the nation and the world do in the next decade or two will determine whether or not cities like Miami, Boston, New York, or New Orleans have any plausible chance to survive by 2100.

The study, “Contribution of Antarctica to past and future sea-level rise,” analyzes “new processes in the 3-dimensional ice sheet model.” It makes use of mechanisms involving the impact of warming oceans on the unstable Antarctic ice sheet “that were previously known but never incorporated in a model like this before.” It then tests its findings “against past episodes of high sea-levels and ice retreat.”

The researchers dramatically raise the likely contribution to sea level rise we will see from the disintegration of the Antarctic ice sheet, which, as we reported two years ago, has already begun. The “authors find that Antarctica has the potential to contribute greater than 1 meter (39 inches) of sea-level rise by the year 2100, and greater than 15 meters (49 feet) by 2500 if atmospheric emissions continue unabated.”  The study confirms what leading climate scientists have warned about for many years now: Only very aggressive climate action can save the world’s coastal cities from inundation by century’s end.

Climate Central has created a number of interactive “slider” maps to show the choices we face. They add the new Antarctic melt numbers to earlier projections for the other main contributors to sea level rise.

Is the choice really that tough for humanity? Remember that what Climate Central labels “extreme pollution cuts” would be super-cheap to achieve according to every major independent economic analysis.

One key point is that the deep pollution cuts case means vastly lower sea level in 2100. It also means that the rate of sea level rise is much more manageable from an adaptation perspective. If we stabilize below 2°C, then it’s possible Antarctica will contribute very little to the rate of sea level rise, which might be only two to three inches a decade by 2100. But in the business-as-usual case, Antarctic ice loss by itself could be raising seas a staggering inch per year within the century!

Significantly, these worrisome conclusions are not really new. Back in December 2009, we headlined our coverage of a new study, “Sea levels may rise 3 times faster than IPCC estimated, could hit 6 feet by 2100.”

I asked one of the authors of that study Global sea level linked to global temperature,” Stefan Rahmstorf, for a comment. Rahmstorf, who leads Earth System Analysis at the Potsdam Institute for Climate Impact Research, said:

The new results based on detailed modeling of the Antarctic ice sheet support our own estimates published in 2009 which were based on a much simpler method. Back then we also concluded that up to 6 feet of sea-level rise could result by the year 2100 if we keep increasing our greenhouse gas emissions. The new study shows once again how urgent it is to reduce greenhouse gas emissions drastically in order to prevent a catastrophic sea-level rise.

So why is the new study such a big deal? Well, the 2009 study proposed “a simple relationship linking global sea-level variations on time scales of decades to centuries to global mean temperature” and verified that model with historical data. But that study did not offer a detailed physical mechanism for the ice loss required to see that much sea level rise.

As a result, their findings were ignored by the Intergovernmental Panel on Climate Change in its “consensus” projections for sea level rise in its 2013 report assessing the scientific literature. In the unconstrained CO2 emissions case, the IPCC basically projected a sea level rise of 0.5-1.0 meters (20 to 40 inches). [For more details on what the IPCC projected, see this RealClimate post by Rahmstorf.]  The IPCC’s findings were instantly obsolete. Indeed, a 2014 study made use of expert opinion (from 2012) to conclude that the worst-case scenario was closer to six feet of rise by 2100. The experts estimated Greenland would probably contribute under 0.2 meters (20 centimeters or 8 inches). Same for Antarctica.

Yet that 2014 study was also instantly obsolete because we had already learned from two studies earlier in the year that glaciers in the Amundsen Sea region of the Antarctic ice sheet had begun the process of irreversible collapse.

But it has not been until this new Nature study that somebody actually put together a model using advanced ice sheet dynamics and then tested that model against paleoclimate data for accuracy.

So now we know that when you model the kind of dynamic disintegration of Antarctica that has happened in the past and that is clearly happening now, you get a contribution from Antarctica that is vastly higher than the experts thought just four years ago. If you add that to the expert assessment for Greenland along with the other more easy-to-calculate contributions (such as thermal expansion of the oceans as the planet warms), then you get a sea level rise double what the IPCC had said. And you get pretty much exactly what a simpler historically accurate model had found seven years ago.

To be clear, though, the five to six feet of sea level rise is not the worst-case scenario. For instance, it doesn’t include a more dynamic modeling of what will happen to Greenland’s ice sheet.

For a “worse-case scenario,” NASA scientist Eric Rignot directs us to the study he coauthored with James Hansen and others. That study posits 10 feet of sea level rise by 2100 is possible. But, again, it doesn’t provide a physical mechanism for how that much ice-melt could occur that fast, so some scientists tend to view it as unrealistic.

The bottom line is that we now know that if we don’t quickly redouble our current efforts to slash carbon pollution, we are risking rates of sea level rise that are catastrophic and beyond adaptation.

Cross-posted from Joe Romm’s Climate Progress