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Hurricane Michael, the third most intense storm on record to make landfall in the U.S., has caused widespread destruction, turning places like Mexico Beach, Florida, into a hellscape of broken homes and overturned cars. It will be a while before we learn the full extent of the damage — and the human suffering and death — caused by the storm’s 155 mph winds and the 14-foot storm surge that swamped the coastline.

Bad as the hurricane was, imagine the damage and destruction if that storm surge had been 15 feet or so higher. And if instead of receding, that wall of water never went away. That is what we could be facing in the not-so-distant future if we don’t dramatically cut fossil-fuel pollution.

If that sounds alarmist, watch this short video. In it, you’ll see a scientist named Richard Alley in a Skype discussion with students at Bard College, as well as with Eban Goodstein, director of the Graduate Programs in Sustainability at Bard. It would be just another nerdy Skype chat except Alley is talking frankly about something that few scientists have the courage to say in public: As bad as you think climate change might be in the coming decades, reality could be far worse. Within the lifetime of the students he’s talking with, Alley says, there’s some risk — small but not as small as you might hope — that the seas could rise as much as 15-to-20 feet.

Let’s pause to think about what 15-to-20 feet of sea-level rise in the next 70 or so years looks like. I’ll put it bluntly: It means not just higher storm surges from hurricanes, but the permanent drowning of virtually every major coastal city in the world. Miami, New Orleans, large parts of Boston and New York City and Silicon Valley, not to mention Shanghai, Jakarta, Ho Chi Min City, Lagos, Mumbai — all gone. And I don’t mean “sunny day flooding,” where you get your feet wet on the way to the mall. I mean these cities, and many more, become scuba diving sites.

There are not enough economists in the world to calculate the trillions of dollars worth of real estate that would be lost in a scenario like this. Nor are there enough social scientists to count the hundreds of millions of people who would be displaced. You think the world is a chaotic place now? Just wait.

Richard Alley is not a fringe character in the world of climate change. In fact, he is widely viewed as one of the greatest climate scientists of our time. If there is anyone who understands the full complexity of the risks we face from climate change, it’s Alley. And far from being alarmist, Alley is known for his careful, rigorous science. He has spent most of his adult life deconstructing past Earth climates from the information in ice cores and rocks and ocean sediments. And what he has learned about the past, he has used to better understand the future.

“Sea-Level Rise: Inconvenient, or Unmanageable?” Richard B. Alley

For a scientist of Alley’s stature to say that he can’t rule out 15 or 20 feet of sea-level rise in the coming decades is mind-blowing. And it is one of the clearest statements I’ve ever heard of just how much trouble we are in on our rapidly warming planet (and I’ve heard a lot — I wrote a book about sea-level rise).

To judge how radical this is, compare Alley’s numbers to the latest report from the Intergovernmental Panel on Climate Change, which was released on Monday. That report basically argued that if we don’t get to zero carbon emissions by 2050, we have very little chance of avoiding 1.5 Celsius of warming, the threshold that would allow us to maintain a stable climate. The report projected that with 2 Celsius of warming, which is the target of the Paris Climate Agreement, the range of sea level rise we might see by the end of the century is between about one and three feet.

So why is Alley arguing that the risk of catastrophic sea-level rise is so much higher than the report that is often cited as “the gold standard” of climate science?

For one thing, IPCC reports are notoriously conservative. They are written in collaboration with a large group of scientists and are often watered down by endless debate and consensus-building. (There are 18 lead authors and 69 contributing authors on the chapter that considers sea-level rise.) For another, they rely on published science that is often out of date — or at least, far from the cutting edge. The new IPCC report has already been criticized for low-balling risks by climatologists like Penn State’s Michael Mann, who has pointed out that the report understates the amount of warming we’ve already experienced as a result of burning fossil fuels, which means that we are much closer to the 1.5 and 2 degrees Celsius thresholds than the report implies.

Alley simply has a broader understanding of ice dynamics than many scientists, who tend to be highly specialized in their research. Alley’s analysis includes not only geology and paleoclimatology, but also a big dose of physics and engineering — which is especially helpful when it comes to understanding the possibility of rapid ice sheet collapse. (To help me visualize how quickly ice cliffs on Antarctic glaciers can disintegrate, Alley sent me a video of a 1978 landslide in Norway.) In the IPCC report, “tipping points” in the climate system, such as ice-cliff collapse, are either disregarded or buried deep in the 1,000-page document.

The Quick Clay Landslide at Rissa – 1978 (English commentary)

Alley is not the only one who has suggested that the risks of rapidly rising seas are higher than this IPCC reports acknowledges. The National Ocean and Atmospheric Administration, the top science agency in the U.S., says the seas are likely to rise by between one and eight feet by 2100. And a few years ago, James Hansen, the godfather of global warming science, argued that the world could see six feet of sea level rise in as little as 50 years and then keep rising at an exponential rate after that.

Hansen’s expertise, however, is in atmospheric chemistry and physics. Alley understands the secrets of ice.

For Alley, the engine of potential catastrophe is West Antarctica. The details are complex, but here’s a short version of what’s happening: warm water from the Southern Ocean is melting the underside of big glaciers like Thwaites and Pine Island, which, due to the unusual terrain there, have the potential to collapse quickly. (I wrote a much longer, more detailed account of the mechanics of ice sheet collapse here). If West Antarctica goes, that’s 10 feet of sea-level rise right there. Then if you add in ice loss from Greenland, a little from East Antarctica and other sources, you quickly get to 15 to 20 feet.

The big question is, how soon could it happen?

“We don’t really know,” Alley tells Rolling Stone via email. He points to the lack of constraints in physical data and models that would put a speed limit on the collapse. “The most-likely future as projected by the IPCC is well on the small-change/small-damage ‘good’ end of the possible futures, with potential for slightly better, slightly worse, and much worse, but without a balancing ‘much better,’” Alley writes.

In other words, when it comes to ice-sheet collapse, uncertainty is not our friend. The collapse might not happen fast. Then again, he can’t rule out the possibility that it will happen fast, very fast.

Alley points out that the best way to avoid this uncertainty is to keep climate warming below 1.5 Celsius or less. In existing climate models, West Antarctica remains fairly stable below that threshold. But given the world’s current burn rate of fossil fuels, and the massive industrial and political transformation required to keep temperatures below that threshold, Alley knows that’s unlikely.

“I personally am not planning to tell people that I know what [amount of warming determines if] ice shelves will or won’t break off, leaving cliffs that will or won’t crumble rapidly,” Alley writes to me. “So, for now, I have to leave large, rapid changes within my error bars, and I believe I have a duty to tell people this.”

And that’s one of the things that makes Alley such a great scientist. He not only understands the world-changing risks we face better than almost anyone. He also understands that it’s his job to warn us about them.