A new peer-reviewed paper in Nature Communications sent shock waves through those paying attention when it concluded that the Atlantic Meridional Overturning Circulation – or AMOC – is most likely at the beginning of collapse, a process that could take decades before it finally, suddenly, happens at the Tipping Point. The Gulf Stream that runs from the coast of Florida and up to North Carolina and then across the Atlantic, is part of the AMOC. A future collapse of this vital ocean current, triggered by global warming, would have serious consequences including dramatic, rapid changes in weather patterns and climate, especially for northern Europe, but also for the Atlantic Coast of the U.S. and beyond.
Between The Lines’ Melinda Tuhus spoke with Robert Marsh, a professor of oceanography and climate at the University of Southampton in the UK. Although he wasn’t involved in the Nature study, he’s an authority on AMOC and helped develop one of the computer models referenced in the study.
Here, Marsh explains what the AMOC is, the impact it has on moderating climate and what could happen if it collapses. But he cautions that “nature is far more complex than simple models and statistics can properly represent and so we should be quite uncertain of the latest prediction.” He noted that around 2010, the AMOC stalled for a year, which had a definite impact on weather in adjacent countries.
ROBERT MARSH: AMOC is really an acronym for the Atlantic Meridional Overturning Circulation, so it’s in the Atlantic Ocean. Meridional means north and south. Overturning talks about movements from above, down and then back below and it’s a circulation as part of the ocean current system, all water moves and joins back together again in an enormous, indefinite loop.
The AMOC is about half of the Gulf Stream I should say. So, the Gulf Stream is a very large movement of water along the east seaboard of the U.S. about half of which we can think of as AMOC. There’s another half of the Gulf Stream that goes more ‘round and round in the sub-tropics. We would consider that to be a different kind of ocean circulation.
It’s a bit confusing, because we sometimes hear, alarmingly, that the Gulf Stream is going to stop. And that’s not really plausible because about half of the Gulf Stream is quite independent of the AMOC. AMOC is the movement of water not just at the surface – which is where we see the movement of the Gulf Stream. The Gulf Stream is sitting in the upper maybe one kilometer of the ocean, and it’s really strong at the surface so that is sort of the well-known surface expression of the Gulf Stream. We can see that in satellite images as a warm ribbon of water. AMOC involves also these very deep flows; the ocean is four or five kilometers deep. So well beneath the Gulf Stream is a return flow, approximately the same water that went north with the Gulf Stream – about half of it – is coming south again several decades later.
MELINDA TUHUS: This is a global warming related concern that this thing could collapse. That’s a piece of it, right?
ROBERT MARSH: That’s correct, yes, the AMOC is part of that suite of operating systems on planet Earth that have been stable for the last several thousand years, and we’re really pushing the limits of what the AMOC can take, basically.
MELINDA TUHUS: So, one thing I wondered is, there seems to be such a huge time difference in experts’ predictions – some are saying as early as 2025 or 2035 and others are saying hundreds of years out. So, what is the difference based on? Is it based on how much warmer the planet gets or on something else?
ROBERT MARSH: So, we have over the last 20 or 30 years, as a scientific community gained ever more understanding of how climate is changing and also how the ocean feels that change and participates in that change. So, the reason for the great uncertainty in when this might happen – or even if it might happen – is I think reflecting that ongoing, but reducing uncertainty. So, one of the reasons the uncertainty is reducing is because literally time is going by and we are gathering very accurate information about climate change.
MELINDA TUHUS: If it really collapses, what would the impacts be?
ROBERT MARSH: We don’t really know quite how serious and far-reaching might be the consequences of a collapsed AMOC. The impacts I think will be most keenly felt in north Europe, just based on the fact that from all we know about the AMOC, that is the location which is most directly affected by the normal operation of AMOC. So AMOC has a profound effect on the average weather, or the climate, or northern Europe. So, that in particular affects Norway and the UK and other countries around northern Europe, subject to a relatively benign maritime climate. So what would happen in the absence of a normal AMOC is those countries would be subject to a more extreme climate, and that could include a more severe winter as well as more difficult summertime conditions, such as droughts and floods. Then that has consequences for weather elsewhere. If we’re getting very different weather over here then likewise you can expect the weather in North America may be a bit different, too.
There are also consequences for the Atlantic hurricane season, which are, when the AMOC fails, we get more heat from the tropics and that builds up to the extent that greater hurricanes and more hurricanes can form. Clearly, we need to stop the amount of warming that is building up over time, but we need to slow down the rate of that warming, because to some extent it’s the speed of change which is too much for the AMOC, potentially. So, if we could just slow things down a bit, we may still get to a warmer world, but if we could do it at a slower rate it would give the AMOC an opportunity to keep up with that.
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