Why is the Gulf Stream slowing down and what does it mean for the future of the UK’s climate?

What exactly is the Atlantic Meridional Overturning Circulation (AMOC)?

The simplest idea of it is just these very large-scale ocean currents in the top thousand metres of the Atlantic. That's kind of a system of currents that transports warm, salty water northwards throughout the Atlantic and then at high latitude, in the subpolar North Atlantic, the water gets cold, becomes dense and it sinks down to below a thousand metres and it flows back southwards. It's a loop of warm, salty water travelling north in the upper kilometre of the Atlantic.

It’s matched by a transport of deep, dense water southwards throughout Atlantic Ocean. It's part of a global conveyor belt system because the Southern Ocean links the Atlantic to all the other major ocean basins. You can think of it as a conveyor belt in the sense of those kind of loops of rubber that you have on the supermarket checkout.

What influence does this conveyor belt system have?

It's all about heat, it's like a giant radiator system, if you like. The North Atlantic Ocean is transporting a huge amount of heat northwards, from the tropics and subtropics into the subpolar ocean, the area between the UK, Iceland, Greenland and Canada. The waters around the UK would be much colder if you didn't have this heat through the Atlantic in this overturning circulation. You can think of it as sort of a battery for the atmosphere. It is incredibly important for the weather and the climate that we receive.

One nice statistic that is quite useful to reflect on is that the the heat held in the top one metre of the ocean contains as much heat energy as the whole of the atmosphere. It has this capacity for storing heat. It's this movement of the heat around the globe that keeps our climate nice for us to live in. If all the heat was just concentrated in the tropics, they'd be too hot to live in and everywhere else would be too cold to live in. That's the importance to us, the way it modulates our climate and our weather.

How has the AMOC been changing?

How it's going to change in the future is the critical question really, because it's so important for our climate and weather that we have in the UK that we really need to know how it's going to change. The bad news is that we don't have very high confidence in those predictions, but all climate models show a slowing of the overturning circulation over the next few decades. With the low emissions scenario it takes longer to get to a very low point but in the high emissions scenarios it might only be another 50 years before it's reached a very low point.

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Why is the shifting of ocean heat important for the climate?

In the model projections our climate is going to change over time with our carbon emissions. It's quite difficult to separate out the effects of a slowing of the overturning circulation from all the other effects - the ocean, the atmosphere. They interact with each other. There are all kinds of processes going on. So actually being able to pick out what a slowdown in the overturning circulation causes is quite difficult.

But what we think is that it has the effect of cooling parts of the ocean west of the UK because less heat is being carried northward through the Atlantic Ocean. So that distribution of heat from the tropics to the subpolar areas slows down a bit. That part of the ocean gets a bit colder and doesn't warm as fast as the rest of the world.

That has a direct impact on our weather. In the UK weather is getting wetter, it's getting warmer and there's more extreme weather. But with the slowdown in the AMOC, some of that is counteracted a little bit. And that's because if there's not as much warming in the Atlantic Ocean then that has an impact on things like where the jet stream sits.

If it moves north of the of the UK, which is something that could happen as the AMOC slows down, then that makes all that makes our weather drier, because all of the movements of the storms that bring our rain might run north of the UK. But it's unclear specifically what a weakening of the AMOC does.

In terms of the International Panel on Climate Change (IPCC)projections for what's going to happen to the UK it is basically going to get warmer, get wetter and we’ll have more extremes in our weather.

How does this all fit into the wider picture of climate change?

What's critical for the overturning circulation to take place is the fact that the Atlantic is a salty ocean. As the upper 1000m of the water moves through the tropics and the subtropics it loses freshwater. There's lots of evaporation, so it becomes saltier and saltier as it's travelling through.

And that level of salinity is really important because when the water is cold and salty it becomes very dense. So that's what makes it sink to the bottom of the ocean basins. If the water isn't as salty, it'll still cool, but it'll stay closer to the surface.

It won't sink so deep. So, in the future, what the models all show with this decline. Is that it happens because the ocean isn't as salty. And that's partly because of changes in the hydrological cycle and the precipitation and evaporation in different parts of the ocean. And it's also because of changing precipitation or fresh water that's coming from the Greenland ice sheet

So if those things add fresh water to the high latitude oceans, it becomes less salty so you get less of this water sinking to a deeper level. So that can have the effect of slowing down the overturning circulation. That's why salinity is really important.

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How confident can we say that human-made climate change is responsible?

We have high confidence that the projected decline that's going to happen into the future is driven by human carbon emissions because there's been a lot of work done with a model that allows you to test the effect on the climate in the ocean made by changes in the carbon emissions. You can find any emissions and it'll tell you how I would have looked if there hadn't been an emitted.

We have a real challenge on our hands because we only have observations of direct observations of the strength of the overturning circulation from 2004 onwards, which isn't very long. So if you're looking at trends over decades, we don't really have direct observations of the strength of the overturning to help us understand how well we're doing in the models. But we have used proxies for estimating the strength of the past overturning circulation.

So things like evidence from the sea floor can show us whether we think the overturning has slowed down over time. And previously, there have been studies which present evidence that the overturning has been declining over the past 100 years.

But then there's also evidence to show from different ways of reconstructing the strength of the understanding that there hasn't been a decline over the past hundred years. So there is there's a challenge for us is different sets of evidence. And it's not clear at the moment, or rather we don't have high confidence that we know how strong the overturning has been or how much the variability has been in the past one hundred years. So that remains a challenge for us to keep improving our understanding.

It goes back to improving how well the models that we use actually represent the real ocean, because the interpretation of these proxy records is dependent on model behaviour. And if the behaviour of the models doesn't represent reality, then it makes us a little less confident in the ability or the interpretation of those proxy records.

What can we do to mediate the effect?

Well, reducing emissions is the way to mediate the effect. One thing that's become very clear in the IPCC report is that there is evidence for a decline in the strength of the AMOC. What's important to us at the moment is for us to be able to understand what the process is, the physical things that are happening in the ocean. We need to understand what they are and how they change over time to make sure that we get it right in the models.