After record heat, could the Atlantic make Britain's weather even more extreme? - BBC

Somewhere in the stormy waters off Greenland, a bright yellow robotic probe, known as an Argo float, is sinking silently beneath the waves.

It is roughly the size of a person, with a tough metal body and an array of sensors packed inside.

The float is part of a global effort to solve one of the great mysteries of the ocean: how its hidden movements help shape the climate above.

There is no crew, no one steering it. Instead, it drifts with the currents, measuring temperature, the amount of salt in the water and pressure as it moves through the waves.

When it rises, it briefly breaks the surface and sends its data home by satellite. Then it does it all again. Dive, drift, measure, surface, transmit. The question those floats are helping investigate is one of the most important - and most contested - in climate science: whether one of the world's great systems of ocean currents is beginning to change.

It is called the Atlantic Meridional Overturning Circulation, or AMOC - a vast, north-south system of currents that carries warm surface water towards the Arctic and returns colder water thousands of miles south through the deep ocean.

But scientists say the AMOC is under pressure. Most agree it is likely to weaken as the planet warms. The UK government has said that, as "a key component within the Earth's climate system" the AMOC contributes to the UK's long-term climate risks.

The disagreement is over how much and how fast the current could change, what that would mean for the weather and crucially, whether the seasons we know today could begin to change.

A current under pressure

The question matters because the AMOC is part of a vast heat-moving system that influences the climate far beyond the Atlantic. But for the UK and north-west Europe, its influence is much closer to home: it helps shape the climate we live in and the weather we get.

The tropics receive far more energy from the sun than the poles. That imbalance sets both the air and the ocean in motion. Winds, storms, rainfall and currents are all, in different ways, the planet trying to even out that difference.

The UK sits in the middle of that exchange. Heat released from the Atlantic feeds into the air above it, helping fuel storms, steer winds and influence the pressure systems that reach north-west Europe. So, if the ocean changes, the weather can change too.

That includes a possibility that might seem bizarre in a warming world: changes in the Atlantic could bring more extreme swings in the UK's weather, including colder winters, even as average global temperatures continue to rise.

The AMOC includes the Gulf Stream and helps explain why Britain and north-west Europe are milder than their latitude would suggest. Its scale is hard to grasp. It carries about one petawatt of heat northwards - roughly 50 times the total energy humanity uses.

Some researchers say the warning signs are already visible: a strange patch of cooling in the North Atlantic and changes in how salty the water is.

They point to recent studies suggesting the circulation may be less stable than previously thought - raising the possibility that it could weaken sharply or even shift into a different state. In the most extreme scenario, some scientists warn it could "collapse".

Others urge caution. They say weakening is not the same as collapse, and that the evidence may point to a slower decline or reorganisation rather than a sudden shutdown.

But if the AMOC weakens significantly, it could shift storm tracks, alter rainfall and make winters more volatile.

A severe weakening, or collapse, could go further, bringing colder, drier winters to the UK and north-west Europe even as the planet as a whole continues to heat up.

And the consequences would not stop at the Atlantic. The AMOC helps shape rainfall and temperature patterns far beyond Europe.

A major shift could tug at the West African monsoon, tropical rainfall belts and rainfall over the Amazon. These are not abstract changes on a map: they could affect harvests, water supplies and the livelihoods of hundreds of millions of people.

Which is why scientists are watching the ocean so closely.

The floats are only part of the effort. Satellites, moored arrays of sensors and research ships are gathering data from the modern ocean too.

Other clues are much older. They lie in mud, shells, ice and rock - fragments of evidence from a time when the Atlantic appears to have changed with startling speed.