Confession time: I’ve been conflating the Atlantic Meridional Overturning Circulation (AMOC) and the Gulf Stream in my head. The two aren’t unrelated, but they’re not the same thing. As I currently understand it, the Gulf Stream is a surface current driven by trade winds, which in turn are driven by Earth’s rotation. The AMOC is a set of currents deeper in the ocean, driven by changes in temperature and salinity at the poles, and it’s just the Atlantic “loop” of a global set of currents. While the AMOC does follow a similar path to the Gulf Stream, and may contribute to its speed from below, they’re different phenomena driven by different things.

This distinction is good to make, because as Rebecca Watson noted, a number of news outlets have been wrongly claiming that recent AMOC research is about the Gulf Stream. The good news is that the Gulf Stream is not expected to collapse any time soon, because the factors driving it – Earth’s rotation and the shape of the continents – aren’t affected by global warming very much.

The bad news is that the research does indicate that the AMOC is in danger of collapse. Worse, while that collapse had initially been forecast for some time in the 22nd century or even later, it now seems that we can expect it this century, with a low but non-zero chance of it happening as early as two years from now.

Now. What does this mean?

Well, it doesn’t mean that the current will collapse in 2025. While it is technically possible, that’s the scary end of this study’s margin of error, not The Prediction. That’s important to state, because as with Al Gore’s predictions of an ice-free Arctic, deniers will absolutely pretend that the prediction was for a collapse no later than 2025. That is not what’s going on here. It seems increasingly likely that, on our current trajectory, the AMOC will collapse this century, but beyond that, it’s hard to tell. Climatologist and oceanographer Stefan Rahmstorf gave his thoughts on this over at RealClimate.org, and you should really read the whole thing if you want to understand what’s going on. In particular, though, I wanted to draw attention to his reasoning for why the standard climate models used by the IPPC are likely underestimating the risk of an AMOC collapse:

Standard climate models probably underestimate the risk. There are two reasons for that. They largely ignore Greenland ice loss and the resulting freshwater input to the northern Atlantic which contributes to weakening the AMOC. And their AMOC is likely too stable. There is a diagnostic for AMOC stability, namely the overturning freshwater transport, which I introduced in a paper in 1996 based on Stommel’s 1961 model. Basically, if the AMOC exports freshwater out of the Atlantic, then an AMOC weakening would lead to a fresher (less salty) Atlantic, which would weaken the AMOC further. Data suggest that the real AMOC exports freshwater, in most models it imports freshwater. This is still the case and was also discussed at the IUGG conference.

The post overall provides a great breakdown of the available data, why scientists say the AMOC has been weakening, and how the “blob” of cold water off Greenland is a part of that process. It also, nearer the beginning, gives a great overview of the importance of this subject:

The AMOC is a big deal for climate. The Atlantic meridional overturning circulation (AMOC) is a large-scale overturning motion of the entire Atlantic, from the Southern Ocean to the high north. It moves around 15 million cubic meters of water per second (i.e. 15 Sverdrup). The AMOC water passes through the Gulf Stream along a part of its much longer journey, but contributes only the smaller part of its total flow of around 90 Sverdrup. The AMOC is driven by density differences and is a deep reaching vertical overturning of the Atlantic; the Gulf Stream is a near-surface current near the US Atlantic coast and mostly driven by winds. The AMOC however moves the bulk of the heat into the northern Atlantic so is highly relevant for climate, because the southward return flow is very cold and deep (heat transport is the flow multiplied by the temperature difference between northward and southward flow). The wind-driven part of the Gulf Stream contributes much less to the net northward heat transport, because that water returns to the south at the surface in the eastern Atlantic at a temperature not much colder than the northward flow, so it leaves little heat behind in the north. So for climate impact, the AMOC is the big deal, not the Gulf Stream.

If the AMOC collapses, western Europe gets colder. It wouldn’t happen overnight (since people have been referencing The Day After Tomorrow), but it would happen pretty quickly. As always, the damage of this isn’t just about the absolute temperature – plenty of people live with winters of the kind we’re likely to see from this – but about what we’re used to, and what our infrastructure can withstand. The flat in which I currently live is a for example, doesn’t appear to have much if any insulation. If it does, it’s undercut by the drafts coming in around the loosely-set windows, and the fact that every single room has an un-closeable vent to the outside, probably to prevent carbon monoxide buildup. If we were hit by a Newfoundland winter, I think there is approximately zero chance that our ancient gas boiler could keep the apartment at a livable temperature.

It goes well beyond that, however. It’s not clear to what degree an AMOC shutdown would affect the overturning circulation in the Pacific, but even if we’re lucky and the collapse was limited to the Atlantic, there would be global weather implications. I would freeze, but the heat that I’d be missing would be released elsewhere, possibly causing a big spike in temperature further south. It would also, without question, affect rain patterns all over the planet, which would increase the odds of those multiple simultaneous crop failures we were talking about earlier this month.

This research is more of a warning than a forecast. It’s not that we have a collapse coming and there’s X amount of time to prepare, but rather that we’re entering a period during which that collapse could happen with very little warning.

That means that we don’t know how much time we have to prepare, but the sooner we get to work on that, the better, because I expect it would take a decade of unprecedented effort and investment to make us “ready” for any effects of an AMOC shutdown. I’ve discussed some this before, but “preparation”, in this case, means a few different things. The biggest one, in my opinion, is changing food production. We should move as much of it indoors as possible, and work on things like factory-produced bacterial and algal foodstock. We should also have stored food, of course, but it’s not clear that weather patterns would ever recover, at least within our lifetimes, from this. It also means investment in infrastructure, both to handle the extreme weather that would be sure to follow the shutdown, and to handle any sudden changes in regional sea level due to the current’s disruption.

As always, this news adds to the urgency, but it doesn’t really change what you or I need to be doing. Join unions and left-wing political organizations, organize those unions and organizations if they don’t exist, build a store of food for you and your neighbors if you’re able, encourage organizing and direct action in those around you, and do what you can to elevate and empower those around you. We’re in a bad place, and on our current path, it’s only going to get worse. The way out is to reject the individualistic and anti-community dogmas of capitalism, practice solidarity, and work together.

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