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When James Hansen speaks, climate hawks listen. Hansen was legendary during his long career as NASA’s chief climatologist for being ahead of the curve on seeing the threat of catastrophic climate change. Now he teaches at Columbia University, and he has more bad news to deliver. According to a study conducted by Hansen and 16 coauthors, being published this week in the European Geophysical Union’s open-access journal Atmospheric Chemistry and Physics, the effects of even moderate warming on sea-level rise are worse than previously believed.

Hansen and his colleagues combined analysis of the historical record with modeling and current observation and found that the rate of oceanic ice melting in Greenland and Antarctica may exceed our expectations. As InsideClimate News explains, the scientists “analyzed how an influx of cold freshwater from the planet’s melting ice sheets will disrupt the ocean’s circulation … They concluded the influx of freshwater from melting ice sheets in modern times would essentially shut down the ocean’s circulation, causing cool water to stay in the Earth’s polar regions and equatorial water to warm up even faster.”

“The cooling mechanism is cut off, so it’s melting ice shelves,” Hansen explained in an interview with Grist. “It’s a really dangerous situation where you get melting that causes more melting.”

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Hansen also says that in the past, temperature increases of 1 degree Celsius have resulted in higher sea levels than we have projected to accompany that temperature rise in the future.

The bottom line, as Slate’s Eric Holthaus writes, is that “glaciers in Greenland and Antarctica will melt 10 times faster than previous consensus estimates, resulting in sea-level rise of at least 10 feet in as little as 50 years.” A sea-level rise of 10 feet would inundate parts of major cities from New York to Shanghai.

These findings, if accurate, have major policy implications. You’re probably familiar with the the target of keeping global average temperatures from increasing more than 2 degrees Celsius above pre-industrial levels. It’s a widely cited goal for the climate movement, based on a combination of the best we can realistically hope to achieve and estimates of the threshold at which dangerous feedback loops will begin to make global warming and its effects escalate exponentially. So far, we’ve already warmed about 0.8C, and even if we turned off the greenhouse gas spigot tomorrow, we’ve got a couple of decades of warming ahead of us from our recent emissions. Meanwhile, as global emissions keep rising, some scientists argue that 2C is at this point already an unrealistic goal because the global community simply isn’t reducing emissions enough to get there.

But Hansen’s study suggests that even 2C is too unambitious, and that the point at which warming becomes exponential instead of linear is more like 1.5C. Other scientists have reached similar conclusions recently, like those behind a U.N. report issued in May that looked at the effect of sea-level rise on Pacific island nations. And so Hansen frames his findings as a plea to world leaders to pursue that new target.

“I wanted to publish now, so the information is available well before the Paris meetings,” says Hansen, referring to the next round of U.N. climate talks this December. Hansen also said he wanted to release it long enough before Paris so that he could respond to the inevitable critiques before the climate negotiations begin. (Criticism is already emerging, some of it from other respected climate scientists. Hansen’s new paper hasn’t been peer-reviewed yet; the journal it’s being published in encourages peer review to happen in public.)

Hansen, despite his reputation for doomsaying, remains hopeful about the prospects for fending off the worst of climate change. The biggest emitting nations are not pledging to cut emissions enough to even keep warming below 2C, but Hansen says a gradually rising global carbon fee could change that. It could force emissions to drop several percentage points per year and hold us down to 1.5C in warming. To get this outcome from the messy global climate treaty process would be fantastic, but it is highly unlikely. Hansen sort of admits this, but holds out hope nonetheless.

“I don’t think it’s impossible that you could get key players to agree to the concept of an international carbon fee,” he says. “It’s not going to happen with 190 countries sitting around a table. It’s going to happen with key players negotiating directly either at Paris or in the years ahead.” Specifically, Hansen imagines that the world’s two biggest economies and biggest carbon emitters, the U.S. and China, would negotiate a carbon fee bilaterally and then use their global buying power to force all of their trading partners to join.

Conceding that a few recalcitrant nations like Russia would never cooperate, Hansen argues, “You can make [a carbon fee] almost global. It would have to be agreed to by Obama and China. The U.S. and China would put duties on products from countries that don’t have an equivalent carbon fee. That would be a big incentive for other countries to have their own carbon fee.”

Hansen is certainly right that the policy levers exist to avert climate catastrophe, if the world chooses to use them. His far more questionable claim is that it’s politically possible. Hansen seems to assume that if it would be rational for leaders to do something — because it is in their nation’s interest or consistent with their stated principles — they will do it.

“China does not dispute the science, and they stand to suffer enormously where they have several hundred million people who would be displaced from sea-level rise,” says Hansen. “So a carbon fee is going to happen.”

But what about the Republicans who control the U.S. Congress and have pledged never to back a carbon tax?

“A revenue-neutral carbon fee is something conservatives could support,” says Hansen. “Behind the scenes, most of them are beginning to realize this is not a hoax, it’s a real issue that needs to be dealt with and [they] will come around on that reasonably soon. Most serious conservatives will want to deal with it in a way that’s consistent with conservative principles.”

Alas, congressional Republicans are neither serious nor classically conservative; they are irresponsible demagogues who won office by pandering to their base’s basest instincts. It would make sense for conservatives to support an individual mandate to buy health insurance — an idea hatched at a conservative think tank and test-run by a Republican governor in Massachusetts. But only one Republican in the entire House of Representatives, and zero in the Senate, voted for the Affordable Care Act. Mitt Romney, the father of the ACA’s progenitor in Massachusetts, campaigned for president promising to repeal it.

And just look at what happened to Rep. Bob Inglis, the South Carolina Republican who lost his seat in 2010 to Tea Party challenger Trey Gowdy, in part because he was the rare red-state Republican to accept climate science. Do you think Gowdy will back a carbon tax? Even if it’s revenue-neutral, it only makes sense if you acknowledge that carbon pollution is a negative externality. Currently, the number of congressional Republicans who do so can be counted on one hand.

Still, Hansen keeps hope alive. “It’s going to happen,” he says of a global carbon fee. “It’s just a question of how soon. But the danger is that it won’t move soon enough.”