The worst-case scenario for sea level rise is 6 feet (1.8 meters) by 2100, according to a new study. Unfortunately, this study is already out of date because it is based on expert opinion from back in 2012.

This year, however, we’ve seen multiple bombshell studies on the growing prospect for West Antarctic Ice Sheet collapse — and similar findings that “Greenland will be far greater contributor to sea rise than expected.”

Even so, the plausible worst-case is important to understand because it is what should drive planning and “adaptation.” Also, avoiding the worst-case is typically a driving force behind prevention measures (people quit smoking because it might kill them or cause cancer) — in this case, slashing carbon pollution.

In fact, as the study points out, given a sufficient level of risk of high climate impacts, “no cost of mitigation is too high to justify.” Significantly, as the news release for the study notes, the major Fifth Assessment report (AR5) of the U.N.’s Intergovernmental Panel on Climate Change (IPCC) “was not able to come up with an upper limit for sea level rise within this century.”

There are five core contributors to warming-driven sea level rise, according to the study:

• Thermal expansion
• Glacier ice loss
• Greenland ice loss
• Antarctic Ice loss
• Changes in land water storage

Thermal expansion is simply (ocean) water expanding as it warms up. Mountain glaciers that melt also contribute to sea level rise. Also, large amounts of groundwater are “pumped for both drinking water and agricultural use in many parts of the world and more groundwater is pumped than seeps back down into the ground, so this water also ends up in the oceans,” contributing to sea level rise.

These three factors are relatively straightforward to estimate. The study uses the uncertainty distributions from the AR5 to determine the probability of different outcomes.

But figuring out ice sheet loss in Greenland and Antarctica is more complicated, requiring knowledge of complex ice sheet dynamics that are not yet fully understood and modeled. So the authors “replaced the AR5 projection uncertainties for both ice sheets with probability distribution function calculated from the collective view of thirteen ice sheet experts” determined in a January 2013 study.

Here were the results:

As you can see, the experts estimated Greenland would probably contribute under 0.2 meters (20 centimeters or 8 inches). Same for Antarctica. Notice also that the “fat tail” of the distribution — the slow trail off on the right hand side of the figures representing the highest (i.e. worst case) sea level rise — comes almost entirely from Antarctica and not Greenland.

Here is their resulting cumulative sea level rise projection for the business-as-usual RCP8.5 emissions scenario (where the world keeps doing very little to restrict carbon pollution or carbon cycle feedbacks like the melting permafrost are high):

This is how the study comes to the conclusion that “seas will likely rise around 80 cm” [31 inches] by 2100, and that “the worst case [only a 5% chance] is an increase of 180 cm [6 feet].”

Of course, the expert opinions are now more than two years old. And so the authors of the new study note:

We acknowledge that this may have changed since its publication. For example, it is quite possible that the recent series of studies of the Amundsen Sea Sector and West Antarctic ice sheet collapse will alter expert opinion.

In fact, expert opinion already has changed. NASA’s Eric Rignot, the authors of one of the two studies from May on WAIS collapse, told me at the time that if we stay near our current emissions path, then “I think that the minimum will be the upper end of the IPCC projections (90 cm [12 inches]) by 2100 and the maximum is hard to figure out but will likely exceed 1.2 – 1.4 meters.” Note that the May studies were not worst-case analyses!

One week after the WAIS studies, a similarly stunning Greenland study came out with a similar conclusion, that its ice sheet was far less stable than thought. Indeed that study noted that “older models predicted that once higher ground was reached in a few years, the ocean-induced melting would halt. Greenland’s frozen mass would stop shrinking, and its effect on higher sea waters would be curtailed.” But as the lead author explained, “That turns out to be incorrect. The glaciers of Greenland are likely to retreat faster and farther inland than anticipated — and for much longer.” And that means Greenland “glacier melt will contribute much more to rising seas around the globe.”

So it seems likely that many experts would revise their predictions of minimum, likely, and worst-case sea level rise contribution for both Greenland and Antarctica, shifting the peak of the above curve to the right and fattening the tail significantly. Why is that important? The study explains:

Assessing how to deal with the impact and costs of sea level rise poses serious structural issues in economic cost-benefit analysis (Weitzman 2009), but widely used assumption is of a quadratic function for climate change impacts (Nordhaus 2008, Weitzman 2010). The annual damage costs for the European Union with sea level rise of 1.4 m by 2100 are projected to be six times greater than for the rise of 0.6 m with A1B climate scenario (Brown et al 2011). However, Weitzman (2009), in his ‘Dismal theorem’ shows that if the rate of decay of the probability tail of the climate impact function is polynomial while the cost of damage rises exponentially, then the cost-benefit function does not converge and no cost of mitigation is too high to justify.

The references are here. I wrote about Weitzman’s work in a 2009 post, “Harvard economist: Climate cost-benefit analyses are ‘unusually misleading,’ warns colleagues ‘we may be deluding ourselves and others’.”

BOTTOM LINE: Right now, the latest science makes clear the tail of the climate impacts graph is not merely fat, but obese, simply from the risks of sea level rise and Dust-Bowlification alone. That strongly suggests “no cost of mitigation is too high to justify.”