The problem traces to this tweet by Stein from last week:

Several climate scientists on Twitter quickly faulted the statement. Here’s Jacquelyn Gill, a researcher at the University of Maine:

And here’s Chris Colose, a climate science Ph.D. candidate at the University of Albany:

So is there any way to defend Stein’s statement about a 9 foot sea level rise by 2050 (in 34 years), which would sweep inland in many coastal cities around the world and cause major damage and displacement?

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Perhaps the best argument in Stein’s favor would cite a recent, controversial study by James Hansen, an extremely famous climate scientist who retired several years ago from NASA, and who published the work with a long and influential list of colleagues.

That paper, which outlined a series of feedback processes which could drive a particularly catastrophic version of climate change, contemplated the idea that the rate of ice loss from Greenland and Antarctica could double, and continue to double, over time periods ranging from every five years to every 40 years.

“Doubling times of 10, 20 or 40 years yield multi-meter sea level rise in about 50, 100 or 200 years,” the paper noted. “Recent ice melt doubling times are near the lower end of the 10–40-year range, but the record is too short to confirm the nature of the response.”

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When asked if this study was indeed a source of the 9 foot figure, Stein’s press director Meleiza Figueroa commented, “James Hansen has said that we could see several meters of sea level rise as soon as the next 50 years. Considering that the effects of climate change we’ve seen in real life have consistently met, or even exceeded, what were previously considered worst case scenarios, we need to take Dr. Hansen’s alarming findings very seriously.”

Figueroa also pointed to an April article in the Insurance Journal, which paraphrased the National Oceanic and Atmospheric Administration’s Margaret Davidson, a coastal sciences advisor, as follows: “Davidson said recent data that has been collected but has yet to be made official indicates sea levels could rise by roughly 3 meters or 9 feet by 2050-2060, far higher and quicker than current projections.”

So let’s weigh all of this, starting with the Hansen study.

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What that study asserts is in effect an “if, then” statement: If the 10-year doubling time is actually correct, then yes, around 2065 you could get “multi-meter” sea level rise, because the rate of ice loss by then would have doubled five times. Thus, taking Greenland as an example, it might have gone from its current 281 billion tons of annual ice loss, which is not quite enough to raise seas by 1 millimeter, to close to 9,000 billion tons, which would raise seas by 25 about millimeters per year, or roughly an inch. (In reality, Hansen thinks Antarctica, not Greenland, will be the bigger ice loser.)

But the paper did not actually say 10 years is the right number for the doubling time — rather, it said that current empirical evidence suggests a number “near the lower end” of the range between 10 and 40 years, adding that “the record is too short to confirm the nature of the response.” In effect, the paper is testing out different time frames for doubling of ice loss in order to assess the likely response, and to provide a range of possibilities.

Indeed, in a widely watched video discussing the research, Hansen commented that

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If ice sheet mass loss has a 10 year doubling time, meter scale sea level rise would be reached in about 50 years, and multimeter sea level rise a decade later. 20 year doubling time would require about 100 years. The data records are too short, but if we wait until the real world reveals itself clearly, it may be too late to avoid sea level rise of several meters, and loss of all coastal cities.

But not every scientist agrees with Hansen, or about whether this thought experiment is anything more than just that. The study has driven both praise and also criticism, as our coverage of it has made clear. One of the most important comments on the study came from Penn State glaciologist Richard Alley, who said the work “usefully reminds us that large and rapid changes are possible.” But as Alley added, “the paper does not include enough ice-sheet physics to tell us how much how rapidly is how likely.”

Certainly, we should not discount Hansen outright. He’s too celebrated and important of a scientist for that.

Still, the consensus projection of sea level rise, for the moment, remains that of the U.N.’s Intergovernmental Panel on Climate Change (IPCC), cited by Colose above, which predicts up to about a meter (3.28 feet) by 2100 at the high end. Increasingly, though, researchers are suspecting this could be too low, and one major study earlier this year suggested Antarctica alone could contribute close to a meter by that year. But that’s still not close to 9 feet by 2050.

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So in sum, the Hansen study suggests a possibility of multi-meter sea level rise this century, but certainly does not establish a firm prediction that this is actually going to happen — and does not represent a scientific consensus position right now.

As for the article quoting NOAA’s Margaret Davidson, she appears to have subsequently clarified her words, in an email to Slate’s Eric Holthaus. There, Davidson cited concerns about rapid loss of West Antarctica and said “actually said my personal opinion was increasingly leaning towards 2-3 meters in next 50 years (that 2100 was not a useful frame for most people).”

The punch line is just about the same: An official has at least floated the possibility of super-rapid sea level rise, but this does not mean it’s a scientifically accepted or consensus projection right now.

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It also isn’t clear where Stein got the figure of 12.3 million Americans seeing their homes inundated by this much sea level rise. Ben Strauss, a sea level rise expert with Climate Central, which has often performed mapping projections to determine how much U.S. land would be flooded by different levels of rising seas, said he wasn’t sure the source of the number.

Strauss said that some of his research suggests that by 2050, we could lock in 3 meter sea level rise that would affect 13.5 million Americans, but we wouldn’t actually see all of that sea level rise by 2050.

Any presidential candidate, from Trump to Clinton to Stein, has every right to dig in and explain all of this. Moreover, that candidate could easily justify the conclusion that we have good reason to worry that sea level rise by 2100 could be considerably worse than the IPCC suggests — if we don’t get our acts together. That is the way the sea level rise story is trending these days.

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But what’s more questionable is to cite only a worst case scenario, without explaining the state of the evidence or scientific opinion overall.

Stein said “could,” which is a bit of a hedge, but it’s not really enough.