Weakening AMOC

AMOC schematic from Could the Atlantic Overturning Circulation ‘shut down’? by Dr. Richard Wood and Dr. Laura Jackson of the UK Met Office

Once in a while I mention that this island where I live now (Britain) is projected by climate change models to be one of the most habitable places on Earth for the longest. Please remember that’s only in terms of climate. It doesn’t speak to anything else about how livable this place will be.

You may wonder why Britain gets such a prediction. It isn’t so much about air temperature as it is about the ocean. Remember, this is an island. It would have a Scandinavian climate if not for having the good fortune to be where the Gulf Stream brings warmth across the Atlantic.

In the 1980s my father, who was a tanker captain, mentioned the Gulf Stream was changing. Its flow was less powerful and it looked different. Back then only people with a special interest in it talked about this. I should have asked more questions about it.

Now it even turns up in mainstream media. Last summer the Guardian ran an article headlined Gulf Stream could collapse as early as 2025, study suggests. Although the headline is more sensational than most, it is far from the only article about the topic in major media.

Estimates of collapse are generally sometime before the end of this century. There is plenty of disagreement among scientists about the timeline, and indeed about whether collapse will occur. However, weakening of the current is widely acknowledged. So far computer models have turned out not to go as far and as fast as reality is moving.

The Gulf Stream is part of the Atlantic Meridional Overturning Circulation (AMOC). In simplified terms, AMOC is a loop. Heated water from the tropics and subtropics moves up and across the north Atlantic, moderating the climate of much of Europe. As the water cools, it sinks and flows back toward where it started, completing the loop.

Of course reality isn’t that simple. AMOC is driven by an interplay involving temperature gradients and salinity. Geological records indicate that in the past, a large volume of fresh water quickly draining into the ocean disrupted the current. Rapid melting of glaciers over Greenland could have a similar effect.

AMOC is much more complicated than just a simple loop. Trying to understand it strains my brain, so I avoided trying to discuss it here… until now.

If you have 35 minutes to indulge some curiosity about AMOC, please click here to see Professor Stefan Rahmstorf explaining how it works and what is expected to happen if/when it falls apart. He makes it clear and understandable.

Britain, especially south of the narrow east-west “waistline” of the island, comes through differently from most other places. Although we’ve been getting hotter, the projections show our heating should stay more moderate than for the European continent next door. Eventually the island is expected to cool—especially Scotland, which is due to become quite cold. But that should take several decades.

Most other places are expected to go through larger swings in climate. As I mentioned, it won’t all be hotter. Remember, the Gulf Stream brings heat to some of Europe, not only to the UK. At present, with the world warming and the Gulf Stream flowing, much of continental Europe is subject to heat waves beyond what it is set up to readily handle.

In Professor Rahmstorf’s video, notice the formation of a blob of cold water in the north Atlantic as AMOC weakens. Like much of the rest of climate change, this is beginning to happen a little ahead of when computer models predicted. Eventually, collapse of the Gulf Stream will reverse this trend and portions of Europe that have been warmed by it will cool.

The why of what is happening to this place I adopted helps me also see more about what lies ahead for any spot in the world. When I found a tidy explanation, I wanted to share it with you.

Now you know what I’m talking about when I mention projections about habitability here. It isn’t why I moved here—I’m here to be with my wife. But it’s a happy coincidence that she was born in such a special place.

This week, with summer here, it really is a “green and pleasant land” (William Blake in Jerusalem).