A new analysis suggests that human-caused climate change may be having a much bigger impact on East Coast superstorms than we thought. A global slowdown in crucial Atlantic Ocean currents — caused by global warming — appears to be supercharging both precipitation and storm surge.

Once-in-a-century extreme weather events like Superstorm Sandy and record-shattering Supersnowstorm Jonas and have become more common in recent years.

We’ve known for a while that record-setting blizzards and other intense precipitation events have been fueled in part by the extra moisture in the air from warming-driven sea surface temperatures (SSTs). I discussed the science on this at length in my Friday post on Jonas.

We also know that warming-driven East Coast SSTs helped fuel Superstorm Sandy in many ways. As Dr. Jennifer Francis of Rutgers University’s Institute of Marine and Coastal Sciences — one of the world’s leading experts on the connection between climate change and extreme weather — explained back in 2013:

“Abnormally high sea-surface temperatures all along the eastern seaboard at the time, which must have some component associated with globally warming oceans, likely helped Sandy maintain tropical characteristics longer and allowed the storm to travel farther northward than would be expected in late October. Warmer ocean waters would also increase evaporation rates, adding to the moisture and latent heat available to the storm.”

The key question has always been what component of the recent rise in eastern seaboard SSTs (and sea level rise, for that matter) can be attributed to global warming. In general, many climatologists have tended to conservatively attribute just the amount driven by the global average rise in SSTs and sea levels.

But the fact is that both East Coast SSTs and sea levels have been rising considerably faster than the global average. Recent research increasingly suggests that this apparent anomaly may not be random but rather driven in large part by global warming itself.

Stefan Rahmstorf, Co-Chair of Earth System Analysis at the Potsdam Institute for Climate Impact Research, has published a summary of that research (some of which he himself conducted) in a must-read RealClimate post, “Blizzard Jonas and the slowdown of the Gulf Stream System.”

Since higher sea surface temperatures helped drive record snowfall for Jonas, Rahmstorf asks why SSTs are so high on the East Coast, as shown in this graphic from Climate Reanalyzer:

Sea surface temperatures

Sea surface temperature (SST) anomalies for Sunday, January 24 showing very warm water off the East Coast and a cold blob of water south of Greenland. Recent climate research connects the two.

The key point is that, as 2015 studies by Rahmstorf and others have shown, global warming is weakening a crucial ocean circulation in the North Atlantic, the Gulf Stream system, more officially known as the Atlantic Meridional Overturning Circulation (AMOC). [Yes, this slowdown was the basis of the otherwise very unscientific climate thriller, “The Day After Tomorrow,” and no, there is exactly zero chance this shutdown will cause an Ice Age for us.]

As a team led by Rahmstorf concluded last year in the Nature Climate Change study, “Exceptional twentieth-century slowdown in Atlantic Ocean overturning circulation,” the “AMOC weakness after 1975 is an unprecedented event in the past millennium (p > 0.99).” And since, they argue, the AMOC slowdown is driven by Greenland ice melt, which is currently accelerating, there is every reason to believe the slowdown will also continue.

There are two reasons this slow-down matters, Rahmstorf argues:

  • The warm sea surface temperatures are not just some short-term anomaly but are part of a long-term observed warming trend, in which ocean temperatures off the U.S. east coast are warming faster than global average temperatures.
  • Climate models show a “cold blob” in the subpolar Atlantic as well as enhanced warming off the US east coast as a characteristic response pattern to a slowdown of the AMOC.

Indeed, Rahmstorf directs us to a brand new study by NOAA scientists in the Journal of Geophysical Research: Oceans titled “Enhanced warming of the Northwest Atlantic Ocean under climate change.” According to the NOAA researchers, who use a high-resolution global climate model, both the extra warm East Coast SSTs and the extra cold blob south of Greenland are features of a warming world:

Modeled sea surface temperatures in a 550 ppm world shows a “cold blob in the subpolar Atlantic and enhanced warming off the US east coast.”

If this research holds up, then global warming could well be responsible for a much larger fraction of the higher East Coast SSTs that fueled both superstorm Sandy and the latest superstorm. In that case, carbon pollution could be responsible for the vast majority if not all of the 3 to 4°F SST warming (above 1970s level) over large regions of the Atlantic off the East Coast in recent days. Such warming boosted atmospheric moisture swept into — and then dropped as snow by — Jonas by as much as 15 to 20 percent, enough to turn it from a major blizzard into the monster all-time record smasher it became.

Significantly, sea level rise for a 600-mile-long “hotspot” along the East Coast (north of Cape Hatteras) has been measured at a stunning “3-4 times higher than global average,” according to a 2012 Nature Climate Change study, “Hotspot of accelerated sea-level rise on the Atlantic coast of North America”. A number of recent studies find that the slowing of the Gulf Steam System should drive faster sea level rise on the East Coast.

Again, if this research holds up, then the devastating storm surges the East Coast is now experiencing, like the one we saw in superstorm Sandy, are more attributable to human-caused climate change than we realized. At the same time, the probability that future Sandy-level storm surges become the norm by mid-century (rather than, say by 2100) rises greatly.

A 2015 discussion paper by some of the world’s leading climatologists argues that “Shutdown or substantial slowdown of the AMOC, besides possibly contributing to extreme end-Eemian events, will cause a more general increase of severe weather.”

It is still an area of active research whether the recent AMOC slowdown “is a decline due to global warming or part of the so-called Atlantic Multidecadal Oscillation/Variability,” as a 2015 review of the literature in the journal Science concluded.

I asked Dr. Francis for her take on this, and she told me, “While I would like to see a few more studies supporting the contribution of Greenland meltwater to the AMOC slowdown (as opposed to just a natural oscillation of the AMO) before I jump on that linkage bandwagon, I do think the Cool Blob (whatever its source) is affecting weather patterns in the N. Atlantic.”

Two decades ago, Wallace Broecker, the “dean of climate scientists,” said, “The climate system is an angry beast, and we are poking at it with sticks.” He stood by that warning in a 2012 interview explaining that his metaphor meant “that by adding large amounts of CO2 to the atmosphere, we were poking our climate system without being sure how it would respond.”

“We’re in for big trouble,” Broecker warned. That is more true than ever now, which is why it’s time to stop poking.