We’re Aiming At 200 Feet Or More Of Sea Level Rise: Here’s What That Looks Like

First published on ClimateProgress.org, a project of the Center for American Progress Action Fund, which was recently named one of Time magazine’s Top 25 blogs of 2010.

The bad news: If we burn all of the planet’s fossil fuels, we’ll melt all of the world’s land ice.

The good news: You’ll be long gone so … party on!

Homo sapiens sapiens, the species with the ironic name, is not known for long-term thinking. So if the very real danger of Sandy-level storm surges coming every year or two in a half century — along with Dust-Bowlification of a third of the Earth’s habitable and arable landmass — isn’t enough to stop us from using the atmosphere as an open sewer for carbon pollution, then the prospect we are going to melt all of the Earth’s land ice and raise sea levels more than 200 feet over the next few millennia or so ain’t gonna do the trick.

Still, here’s what that would look like for the United States (via National Geographic):

On the bright side, think of all the great scuba diving there will be off of our new coastline!

A new analysis in Science Advances, “Combustion of available fossil fuel resources sufficient to eliminate the Antarctic Ice Sheet,” finds just that if temperatures rise 11°C (20°F). The earth would lose all of the Antarctic ice sheet — which is where 90 percent of the world’s land-locked ice is, enough to raise sea level by itself more than 160 feet. When you add in the Greenland ice sheet — whose point of no return is probably closer to 3°C warming — plus all of the other land-based glaciers and thermal expansion, you get sea level rise of over 200 feet.

You can read the news release of the study here.

My main issue with this modeling study is that it is doubly conservative. First, it lowballs how fast rapid sea level rise can start, given the latest observational studies from Antarctica, as leading scientists have pointed out. Second — and equally important — it lowballs how fast temperatures might rise.

On the first point, climatologist Michael Mann told Mashable, “I’m not convinced that the ice sheet model fully accounts for the complex dynamics relevant to predicting ice sheet collapse on timescales of decades to centuries, which I consider to be most relevant.”

Mann, the director of Penn State’s Earth System Science Center, added, “There is considerable empirical evidence that has been provided over the past couple years that we are likely already now committed to at least 3 to 4 meters of sea level rise from West Antarctic Ice loss, due to warming already in the pipeline and the destabilization of the ice shelves, which support the inland ice, due to southern ocean warming that has already taken place.”

James Hansen and 16 leading climate experts — including some of the world’s foremost authorities on Antarctic melt and sea level rise — warned in July we face “sea level rise of several meters in 50, 100 or 200 years,” which means as early as this century but in any case, sooner than expected. They also warn that even with the less than 1°C of warming we already have, ice sheet melt appears to be putting sea level rise on an exponential growth path that would bring 10 feet of sea level rise sooner, rather than later — even if we stabilize at 2°C total warming.

The science has made clear for a long time that we don’t need to burn anywhere close to all of the world’s fossil fuels to have beyond-catastrophic sea level rise. A March 2012 National Science Foundation news release on paleoclimate research warned, “Global Sea Level Likely to Rise as Much as 70 Feet in Future Generations.” The lead author pointed out, “The natural state of the Earth with present carbon dioxide levels is one with sea levels about 70 feet higher than now.” A 2009 paper in Science concluded that when CO2 levels were this high 15 million years ago, it was 5° to 10°F warmer and seas were 75 to 120 feet higher.

Second, it is very far from clear that we need to burn all of the world’s fossil fuels to get 11°C warming. The study uses an “assumed climate sensitivity” of “about 3.1°C.” That number is in the mid-range of current estimates from the U.N. Intergovernmental Panel on Climate Change for how much warming you get with a doubling of CO2 levels — say, from preindustrial levels of 280 parts per million to 560 ppm. But, as I’ve discussed, such estimates assume the climate is sensitive to only fast feedbacks like sea ice retreat and increased water vapor and that there are no slow, decade scale feedbacks.

Yet we know there are many major slow feedbacks that are not included in current climate models, such as the release of carbon dioxide and methane from the thawing permafrost. And we know that all of those feedbacks become stronger and faster the hotter the temperature gets. That’s why various paleoclimate studies find that the Earth has warmed up much more in response to pulses of CO2 and CH4 than current models suggest.

We have been headed toward 900 to 1000 ppm of CO2 over the next century on our current emissions path. According to a 2011 study in the journal Science, the last time the Earth saw such levels of CO2, it was 29°F (16°C) hotter. The paleoclimate data suggests CO2 “may have at least twice the effect on global temperatures than currently projected by computer models.”

The good news is that, if Paris succeeds and the climate science deniers don’t, then we may well avoid that level of CO2 — assuming we can also avoid the worst carbon cycle feedbacks, which appear to really start kicking in around 2°C.

The bad news is that if, as still appears likely, we blow past 2°C warming, then we are likely to see quite rapid and irreversible sea level rise this century and beyond.

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