Storms like last weekend’s blizzard and widespread snowfall can happen, in theory, any winter, but large snowfall storms in the US Northeast have been significantly more common in recent years than in previous recorded history. Over the last few years we’ve seen these large snowfalls happen farther south than usual, as was the case with the 2016 Blizzard. Climate scientists are pretty sure that this blizzard was either outright caused or significantly enhanced (you really can’t tell the difference) by human caused global warming. How can a blizzard, a big cold thing, be caused by warming? Because climate is not a simple thing.

Just trust me, this was an effect of global warming. Or, if you like, read on, and I’ll give you the gory details.

There are two factors that needed to come together to make a storm into a large southern-offset blizzardy mess like this one. First, there needed to be cold air tracking farther south than usual, and this happened as a result of trade wind and jet stream meanderings which have become more common with climate change, and made more likely this year, probably, because of El Niño. Second, there needed to be more moisture in the air coming off the Atlantic Ocean. This happened last weekend, and during other recent storms over the last few years, because the Atlantic is much much warmer than it usually is in the immediate region of the coast. Warmer water provides more moisture to the atmosphere via evaporation, and that relationship is not linear. More sea surface warmth equals more more moisture.

The Atlantic hasn’t been just a bit warmer. This region of the Atlantic has been anomalously very warm for several years and has been getting more warmer annually.

There are two reasons for this extra warmth. One is pretty straight forward. Sea surface temperatures globally are warmer because of human caused greenhouse gas warming of the surface of the planet. This has been enhanced over recent months because of El Niño, but it is a larger and longer term phenomenon with El Niño warming riding on top of that overall increase. Any randomly chosen patch of the world’s ocean is likely to be warmer today than it was ten or twenty years ago.

The second reason is a little more complex. Weather (and it’s big brother, climate) happen because of the uneven distribution of the Sun’s energy on the surface of the earth. Extra heat accumulates near the equator (which is pointing, relatively, more directly at the Sun), and this heat is redistributed through the movement of air and sea currents towards the poles. However, since the oceans and continents are not evenly or symmetrically distributed, or otherwise laid out to make this redistribution of heat efficient, this gets pretty complex. For example, the Pacific is huge while the Atlantic is narrower and restricted as one goes north. Notice also that the Indian Ocean is not connected directly to northern regions, only to the south, so extra heat builds up there and has to make its way towards both poles via long and convoluted currents.

One result of this complexity is what we call the Atlantic Meridional Overturning Circulation (AMOC). This is sometimes referred to as the Atlantic Conveyer and people will sometimes use the term “Gulf Stream” to refer to part of that, but really, it is all more complex than that and not so easily labeled.

Warm water that started near the Equator (including both in the Atlantic and the Indian Ocean, via South Africa) moves north in the Atlantic, on the surface. Up in the North Atlantic, this warm water becomes relatively even warmer (since the air is cooler in the north) and passes as well into areas where the air may be relatively dry. This causes heat to leave the water carried by the current, and evaporation to take place. Evaporation not only cools the water, but makes it extra salty. Saltier water is denser, so the cooling, hyper-saline waters at the northern reaches of the currents sink to the bottom of the ocean, pulling even more of the north-flowing surface current with it. This is like the electric motor that turns a conveyor belt. The lower part of the “belt” is the saltier, colder water now flowing back south, in the opposite direction, towards equatorial regions where it can later re-emerge and warm up again.

That is the simple version. If you just put water in a big place it will rotate because energy supplied by winds (or other currents) will be deflected by the Earth’s rotation, so you get, in the simple case, a counter-clockwise rotation (in the Norther Hemisphere). To the side of such a rotating masses of water, one tends to get counter-gyres (running clockwise). Trade winds push surface waters along, contributing to currents. Between the movement of the currents themselves, differential heat across the sea surface and at some depth, and air the currents, the surface of the ocean tends to not be very flat, though it looks rather flat from any given normal human vantage point. At present, the North Atlantic is mounded up in such as way that the sea surface is lower along the North American east coast than it would be were none of these things were happening.

All this results in a big blob shaped area in the North Atlantic where the surface waters are relatively cold, into which warmer currents mostly from the south (including the Gulf Stream) flow, cooling, sinking, being part of the conveyor.

What happens if you turn this conveyor off? For one thing, heat that is normally contributed to the atmosphere at northern latitudes as part of the process is no longer available to the various trade winds that pass over them. So, downwind regions (i.e., northern Europe) may experience cooling. Under certain conditions, this could cause a shift in climate in the direction of an Ice Age. We are currently experiencing such warming planet wide that this is not a possibility, though there is a famous movie in which this (rather unrealistically) happens.

Another effect can be a change in the mounding of water around the North Atlantic, with an effective regional sea level rise (measurable in inches, probably) along the Northern Hemisphere east coast.

Another effect is, of course, that the hot water moving north into the North Atlantic where it might otherwise cool gets stuck, almost like it is backed up, and becomes warmer and warmer.

All of these effects can happen with a mere slowdown in the AMOC, not only if it stops completely, and we seem to have seen these effects.

Stefan Rahmstorf, a scientist who studies these things, has an excellent writeup about a slowing AMOC and its effects, here at RealClimate.

The graphic at the top of this post is from his post. This shows sea surface temperatures across the world’s ocean as relative change caused by doubling the planet’s normal CO2 level. This is a model indicating that in the North Atlantic, there would be cooling in the far north, and extreme heating along the Northern Hemisphere’s east coast. So that is what the physics says is likely to happen in a warming world.

Here is a portion of the Climate Reanalyzer daily summary showing today’s actual sea surface temperature anomalies (how far above or below a long term average the actual sea surface temperature is measured to be).

Find the purple spots in the North Atlantic. That is the head of the AMOC, more or less, and here we have record low relative sea surface temperatures. Along the east coast are several blobs of red, showing near record or record high sea surface temperatures. There are stripes and blobs of very warm water all along the coast, made relatively warmer first by the simple fact that the sea surface is warmed by global warming, then made even more extra warm because of the recent slowing down of the AMOC. (Click through to see the whole globe, the scale, and to play with the data.)

Why is the AMOC slowing down?

First, note, that this is not a short term oddity of weather. Rahmstorf asserts that this is a long term condition.

(1) 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 US east coast are warming faster than global average temperatures.

(2) 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.

Stefan and other scientists have effectively argued that this slowdown is caused in large part by the addition of fresh water from melting glaciers in Greenland. The fresh water interferes with the process by which waters at the head of the AMOC becoming hyper-saline, and thus slows down the conveyor belt. There are probably also increases in freshwater flow from major rivers into the North Atlantic, also resulting from climate change, that contribute to this.

Let me clarify something here in case there is some confusion. The cooling of the regions of the North Atlantic having to do with AMOC did not provide wintery conditions to cause this blizzard. That is something happening much father away. We may be seeing cooling effects in part of Europe because of this (I’m not discussing that here) but the Blizzard of 2016 (which we hopefully don’t bother to call “2016A” assuming there will not be another) was not hyped up because of that cooling, but rather, from the backed up surface warmth much nearer New England and the rest of the US East coast.

The slowing down of the AMOC has been going on for decades, and seems likely to continue. It is not that clear what would happen if the AMOC simply shut down, or even if it could. Will the action simply move to a new latitude, or will some sort of conveyor system continue but with a very different configuration? Will additional slowdown of the AMOC cause important sea level rise in the US East? One thing that seems very likely is this. With increased surface warmth, and no reasonable expectation that warming will slow or reverse in the near future, Greenland will continue to contribute abundant fresh water to the region, and quite possibly, increased rainfall in major river basins will add even more freshening. The AMOC is not likely to stop slowing down, or to regain its strength.

The slowing and other changes in the AMOC may be a qualitative and long term outcome of anthropogenic global warming. It seems likely that enhanced sea surface warmth off the US East Coast will be with us for the long term. A blizzard like the one we had over the weekend is much more manageable in regions that normally have frequent heavy snow storms, like Massachusetts and Upstate New York. If they happen now and then father to the south, that is a bit of a disaster, but if it is only now and then, it is not likely that we could or would do much about it.

But if annual or nearly annual middle-Atlantic blizzards are now part of the “new normal” of our disrupted climate, then infrastructural changes may be required. Roads and parking lots, and even sidewalks, are constructed with the prospect of frequent snowfalls in mind in northern states. Maybe that is what we should be doing in the formerly less snowy regions along the Atlantic. Snow plows … lots of them … will be needed. Complex and annoying (and costly) parking rules to make room for snow clearing are common in snowy states. Should “snow emergency” procedures and parking rules be set up for the mid-Atlantic?

People will have to learn, either the easy way or the hard way, that during a blizzard warning, one does not simply venture out onto the highways. Minnesotans and northern New Englanders and everyone in between keep blizzard kits in their cars. These are life saving items for when you do get stuck for 30 hours on a highway in the middle of nowhere. People who commute to Washington DC may consider this inexpensive investment. And so on.

Finally, will there be another Snopocalypse this winter, somewhere in the US? I think not. With El Nino, things are warming up, and even in the usually blizzardly places, like New England or around the Great Lakes, I suspect we’ll have more slush and rain than deep snow. But you never know. On the other hand, global warming and El Niño enhanced storminess and raininess could cause more flooding, both inland and in coastal regions. But climate science denying Senator Jim Inhofe may have to wait until next winter to get a new snowball.

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