Slowing Gulf Stream current to boost warming for 20 years

Matt McGrath BBC 19 Jul 18;

The prospect of the Gulf Stream slowing down and even stopping altogether has worried many experts in recent years.

Some believed that this would cause a rapid cooling around the world with resulting global chaos.

But a new study finds the Gulf Stream go-slow will have a significant impact on planetary temperatures, but not in a chilled out way.

The Gulf Stream is an ocean current that keeps the UK warmer than it would be given its latitude alone.

Researchers say a slower current will carry less heat down to the deep oceans meaning more will enter the atmosphere.

Worries over the fate of the Atlantic Meridional Overturning Circulation (Amoc), of which the Gulf Stream is part, were graphically illustrated in the 2004 film, The Day After Tomorrow.

It focused on a sudden collapse of the Amoc caused by global warming leading to a disastrous freezing and the dawning of a new ice age.

So much for Hollywood - the reality according to the corresponding author of this new study is very different.

"The headlines have said that the Gulf Stream is collapsing and the Ice Age is coming sooner than scientists think," Prof Ka-Kit Tung from the University of Washington told BBC News.

"The answer from our work is no to both of them."

Instead Prof Tung and his colleagues have reconstructed what's happened with the flow of the Amoc over the past 70 years. They found a natural pattern with declines, flat periods and increases over the decades.

What is the Gulf Stream?

It's a powerful ocean current that is part of the Amoc and it flows from the Gulf of Mexico, around Florida and up along the east coast of the US, before crossing the Atlantic towards Ireland, the UK and Europe.

Why is it important?

For decades we've believed that the Gulf Stream and the Amoc were like a giant hot water bottle, keeping Europe and the Eastern US warm in the winter.

It's believed that the impact was as much as 5C, keeping London and Western Europe far less cold than say parts of Canada which are at the same latitude.

But this new study indicates that the Amoc plays a far more important role as a massive global heat distribution system than it does in keeping Europeans toasty.

It works like this - The warm waters from tropical regions are carried up to the North Atlantic where the current sinks them deep into the oceans, with cooler waters then returning south in their stead.

When the Amoc current moves faster, more of the heat that is trapped in our atmosphere from the burning of fossil fuels is taken and stored up to 1,500m below the surface of the ocean. When it slows down, less heat is sequestered in the seas and so our land surface temperatures increase.

What's the evidence for this new Amoc theory?

When the Amoc was at a minimum between 1975 and 1998, more heat entered the atmosphere and global temperatures gradually went up. When the current started to accelerate from the mid-1990s to the early 2000s, and sink more heat it coincided with a so-called slowdown in the pace of global warming.

Now the authors say that the big decline is Amoc flow since 2004 means less heat going into the waters and more into the air, leading to higher global temperatures. And that is likely to continue.

"It is difficult to predict changes," said Prof Tung.

"But under the scenario that greenhouse gas forcing hasn't changed much, it would be comparable to the warming in that period of 1975-1998, if slightly lower, but it's comparable."

Will the slowdown in the Atlantic current continue?

That's unlikely according to this study.

"We think that the decline of Amoc is reaching the minimum and if history repeats, we will think this one will last about two decades."

"Where we have direct measurements, such as off the coast of Florida, the measurements there have flattened since 2011. In the northern Atlantic it is still declining."

So what will this mean for the UK?

While the waters of the North Atlantic will definitely cool as a result of changes in the flow, the experts says it's likely that the UK will see continued impacts of climate change over the next 20 years according to this study.

"The air temperatures globally will be warming and there's no barrier for that so there won't be much cooling in the UK, you will probably still see the normal global warming," said Prof Tung.

The study has been published in the journal Nature.