What is this AMOC thing?

One of the things that really got me interested in the question of how ocean currents will affect future climate is a report by the US Global Change Research Program (USGCRP*), called Abrupt Climate Change. One of my sources told me to read Ch. 4, entitled "The Potential for Abrupt Change in the Atlantic Meridional Overturning Circulation." For those of us who like to state things dramatically, this is kind of stuff that Hollywood loves -- yep, think, "The Day After Tomorrow":

Now, anyone reading this blog probably knows that this movie took a couple of scientific facts and blew them way out of proportion. As George Monbiot says in The Guardian, "it was a great movie and lousy science." Nevertheless, it did bring attention to the idea that changing ocean currents could and will affect our climate.

But first, what is the Atlantic Meridional Overturning Circulation -- abbreviated AMOC -- anyway? It's not, as many people have said, the Gulf Stream (which is mainly wind driven, as it happens). But it is a kind of giant conveyor belt of heat and salt that cycles in the Atlantic, with a northward flow of warm salty water in the upper layers of the Atlantic, traveling from the Equator to the poles, and then a southward flow of colder water in the deep Atlantic.

One important feature of this circulation is the warm water that comes up to the Arctic and cools off, releasing heat. As the water gets colder, it gets denser and then sinks. It then flows south towards the Equator. That sinking helps pull the warm water up from the Equator. It's kind of like a perpetual motion machine in the ocean, driven by imbalance between the heat of the sun coming in at the Equator compared to the relative coolness at the poles. Simple, right?

Here's a picture from Wikipedia that shows how it works:

There are many things that are interesting about this movement of the ocean's currents, one of which is that scientists still aren't completely sure about the relative importance of all the physical forces that makes it work. (!)

Yes, it's winds, and tides, and the buoyancy of warm salty water that then becomes cold dense water, with a dash of heat added at the bottom of the ocean by from the warmth of the earth's core, but essentially, we don't really know how if we change one component (for example, if the poles get warmer), how ocean currents will react.

So that brings me back to the Potential for Abrupt Climate Change and the Atlantic's circulation.
First of all, while we don't know exactly how all the forces that drive the Atlantic's circulation work, we do know that doing anything to change the circulation will have a big effect.

We know because we have pretty solid evidence for this in paleoclimate records. These changes include "changes in African and Indian monsoon rainfall, atmospheric circulation of relevance to hurricanes, and climate over North America and Western Europe," according to Ch. 4 of the USGCRP report.

Based on this, the report goes on to say that scientists don't think that it will stop, although it will slow down. Here's science speak for that "No current comprehensive climate model projects that the AMOC will abruptly weaken or collapse in the 21st century." Whew, so Hollywood DID get it wrong -- no icebergs in the East River after all!

But wait... if the Atlantic's circulation were to collapse, forget about icebergs in the East River, we're talking almost three feet of sea level rise, and to quote the report, this 80 cm of sea level rise would be in addition to "what would be expected from broad-scale warming of the global ocean and changes in land-based ice sheet due to rising CO2, changes in atmospheric circulation conditions that influence hurricane activity, a southward shift of tropical rainfall belts with resulting agricultural impacts, and disruptions to marine ecosystems." What would 80 cm of sea level rise look like in Manhattan? Found this great image on geology.com:

Looks like Wall Street gets wet feet.

But as the report writers state, this is very unlikely.

Of course, what is very unlikely? That's the kicker, I think. And you have to wade through quite a bit of this report until you find out that very unlikely is 10 percent. TEN PERCENT.

And that is why there are some two dozen monitoring efforts across the Atlantic, trying to get a handle on how the AMOC works, and why scientists are pushing to set up a comprehensive, coordinated international program to figure it all out. More about that next time.


*Since this is a blog, I get to interject all kinds of personal observations, like this one: Am I the only person out there who looks at this acronym and thinks immediately of the CRP and the Watergate Plumbers? I know it's not exactly the same, and I guess I am showing my age, but I can't help but see that acronym and think about Nixon.