Semarang, Indonesia, 2017. Photo: Ulet Ifansasti/Getty Images

Climate change will bring (and has already brought) a wide variety of menacing disruptions to human existence. Some of these are well-known and already operative, like the wildfires that have been racing along California’s freeways or the perpetual droughts that have been immiserating Mediterranean farmers. Others are more obscure, like the higher rates of interpersonal and geopolitical violence a warming climate is expected to bring (social science suggests that high temperatures make human blood figuratively boil).

But none of the challenges posed by our warming climate has loomed larger in the popular imagination than sea-level rise. With global populations and wealth heavily concentrated in low-lying coastal cities, humanity has been preoccupied by the prospect of the oceans reclaiming the high points of our civilization. And for good reason: The best available models suggest that 37 million people currently live in places that will be below high tide by 2050 — in an optimistic low-carbon-emissions scenario.

Or rather, that’s what such models suggested before this week. On Tuesday, a new study revealed that those alarming statistics — which had gotten so many of us all worked up about our favorite cities’ impending doom — were wildly inaccurate.

The actual impacts of sea-level rise are going to be much, much worse.

Previous estimates of the impact that rising tides would have on coastal cities relied on (essentially) a three-dimensional map of Earth derived from satellite readings. But those readings were fundamentally unreliable because they often measured the planet’s upper surfaces — such as treetops and tall buildings — rather than its ground level. These mistakes led scientists to overestimate the elevation of many regions of Earth, particularly those with lots of vegetation and/or skyscrapers.

In a new study published by the journal Nature Communications, scientists affiliated with the organization Climate Central and Princeton University detail this methodological problem, then use artificial intelligence to determine — and correct for — the previous literature’s error rate. Their research yields some eye-popping (or stomach turning) updates to our conventional understanding of what the next century has in store for our coastlines. MIT Technology Review helpfully breaks down the corrections in this chart:

Photo: MIT Technology Review

The New York Times captures the study’s results in more vivid and harrowing fashion by illustrating its implications for some of the globe’s most populous low-lying cities. Mumbai — the financial capital of what will soon be the world’s most populous country — is now at risk of being entirely erased by mid-century.

Photo: The New York Times

In its optimistic scenario, the Princeton study projects that lands currently occupied by 150 million people will lie below high tide in 2050. At mid-century, that number is all but certain to be higher because of both population growth and internal migration. Between now and 2050, the percentage of the global population living in urban areas is expected to increase from 55 to 68 percent. And climate change could accelerate migration from rural areas to coastal cities as warming devastates many of the world’s agricultural regions. In other words, many coastal cities in the developing world are likely to see influxes of climate refugees, just as rising tides begin displacing their existing populations.

The new study does include one piece of slightly encouraging news. While previous models suggested that 28 million humans currently live in places that already lie below high tide, the actual number is closer to 110 million — which means seawalls and other barriers have proven sufficient to keep many cities dry even as sea levels have risen perilously around them. Still, the scale of barrier construction necessary to save low-lying cities from collapse is now, ostensibly, far greater than previously understood when the task already looked harrowingly expensive, particularly for developing countries.

If the Princeton researchers’ projections are correct, averting mass death and suffering in the coming decades will require not only rapidly reducing carbon emissions and ramping up construction of seawalls and other fortifications but also facilitating mass migrations away from low-lying cities and islands and toward higher ground.