On Oct. 29, 2012, Hurricane Sandy devastated the Northeast, killing more than 100 people, destroying entire communities, and inflicting more than $70 billion in damages.

At the time, New York Governor Andrew Cuomo pointed out that Sandy slammed his state just two years after it was deluged by Hurricane Irene, ”We have a one-hundred year flood every two years now.” In a now-famous cover story, “It’s Global Warming, Stupid,” Bloomberg Businessweek explained many of the links between Sandy’s unique level of devastation and global warming.

Over the last two years, our ever-strengthening understanding of human-caused global warming has confirmed these links. Also, the science is now clear that, absent aggressive action to slash carbon pollution, warming-driven sea level rise will make Sandy-level storm surges the norm on the East Coast in the coming decades.

Climatologists explain that global warming made Sandy more destructive in several ways:

1. Warming-driven sea level rise makes storm surges more destructive. In fact, a 2012 study found “The sea level on a stretch of the US Atlantic coast that features the cities of New York, Norfolk and Boston is rising up to four times faster than the global average.”
2. “Owing to higher SSTs [sea surface temperatures] from human activities, the increased water vapor in the atmosphere leads to 5 to 10% more rainfall and increases the risk of flooding,” as leading climatologist Kevin Trenberth explained to me in a 2011 email about Hurricane Irene. He elaborates on that point for Sandy here and for all superstorms in this article.
3. “However, because water vapor and higher ocean temperatures help fuel the storm, it is likely to be more intense and bigger as well,” Trenberth added (see another of his articles here). Relatedly, warming also extends the range of warm SSTs, which can help sustain the strength of a hurricane as it steers on a northerly track into cooler water (much as apparently happened for Irene). September 2012 had the second highest global ocean temperatures on record and the Eastern seaboard was 5°F warmer than average (with global warming responsible for about 1°F of that).
4. The unusual path of the storm — into the heavily populated east coast rather than out to see — was caused by a very strong blocking high pressure system that recent studies have linked to warming. Meteorologist and former Hurricane Hunter Jeff Masters has an excellent analysis of this, “Why did Hurricane Sandy take such an unusual track into New Jersey?“

I have put these in order from most scientific certainty to least. The first two — the impact of sea level rise and increased water vapor — are unequivocal. The third is extremely likely. The fourth is more speculative — but a 2013 paper in the journal Oceanography makes a strong case that the Sandy storm path was affected by global warming.

Why does Sandy remain so relevant today? Our continued inaction on climate change means we are facing up to 10 times as much warming this century as in the last 50 years. And that means the 4 factors described above are going to have a greater and greater impact over time.

Let’s focus on the most destructive and most analyzed of the impacts — storm surge. A 2012 study of storm surges since 1923 found “that Katrina-magnitude events have been twice as frequent in warm years compared with cold years” — so clearly more severe surges are on the way.

NOAA dug deeper into that issue in a 2013 report “Explaining Extreme Events of 2012 from a Climate Perspective.” Their news release summarized the key finding:

The record-setting impacts of Sandy were largely attributable to the massive storm surge and resulting inundation from the onshore-directed storm path coincident with high tide. However, climate-change related increases in sea level have nearly doubled today’s annual probability of a Sandy-level flood recurrence as compared to 1950. Ongoing natural and human-induced forcing of sea level ensures that Sandy-level inundation events will occur more frequently in the future from storms with less intensity and lower storm surge than Sandy.

So we have nearly doubled the chances for a Sandy-type storm surge with just the modest several-inch sea level rise we have caused to date with carbon pollution. This study points out future Sandy-level storm surges will result from weaker storms than Sandy as sea levels rise.

The NOAA study has an “intermediate high scenario” of 2 to 4 feet of sea level rise by 2100 and a “High scenario,” where sea level rises 4 to 7 feet by 2100. But while those labels might have been appropriate back in 2013, this year we’ve seen multiple studies on the growing likelihood the West Antarctic Ice Sheet faces collapse sooner than scientists projected — and similar findings that “Greenland will be far greater contributor to sea rise than expected.”

NOAA’s “intermediate high scenario” should probably be relabeled “business as usual,” and the “High scenario” renamed the “planning case,” since we typically do infrastructure design and planning not on the best case or most likely case, but rather the plausible worst-case.

Even with just 2 to 4 feet of sea level rise by 2100, the NOAA researchers find Sandy-level storm surge events recurring about once a year (or more frequently) over the vast majority of the coast from Connecticut to southern New Jersey by century’s end.

But in the higher sea level rise case we ought to plan for, even areas that had the very worst storm surges from Sandy — like Sandy Hook and The Battery — will be inundated by such storm surges every 1 or 2 years. In fact, in that scenario, the Jersey shore from Atlantic City to Cape May sees Sandy level storm surges almost every year by mid-century. That is pretty stunning when you look at what Sandy did to Atlantic City:

It’s worth underscoring that a 2012 study found, “the 600-mile stretch of coastline from North Carolina to Massachusetts is experiencing [SLR] rates that are nearly three to four times higher than the global average, a trend that may continue during the coming decades.”

So while New York City has taken modest steps to prepare itself for the next Sandy, it’s clear the nation as a whole has not learned the lessons of Sandy — and so we are condemned to repeat that disaster again and again.