Here are two interesting facts. One is that the winter temperatures in the Arctic this year were the highest ever recorded. On two days in February, it was actually warmer at the North Pole than it was in Zurich, Switzerland. At one location in Greenland, the temperature rose to 36 degrees Celcius, higher than the usual average for that time of year.

The other interesting fact, revealed last month in Nature, one of the world’s leading scientific journals, is that the Gulf Stream is slowing down. In fact, it is now moving more slowly than at any time in the past 1 600 years (which is as far back as studies have gone). This could be bad news for Western Europe.

The difficulty comes in figuring out what these facts mean – if they mean anything at all. And this is the point in the discussion at which you start to hear the climate scientists use the word “nonlinear” all the time.

Most people think of global warming as a smooth, gradual process. It might end up doing a lot of damage, but it will sort of creep up on you, not smack you in the face. But that is not how climate change works.

The change can be abrupt and quite extreme – and once it has happened, it becomes the new normal, perhaps for a very long time. Like many complex systems, the climate is nonlinear: it stays the same for a long time, and then some “tipping point” is reached.

The warming in the Arctic is not nonlinear. It’s a trend that has accelerated greatly: the amount of sea ice coverage at the point of maximum freeze-up, in late March, was far lower in 2015-18 than ever before.

The focus of concern for the moment, however, is on what the warming is doing to the Greenland ice cap. This ice is on land and, when it melts, it raises the sea level. More importantly, it is putting a large volume of fresh water into the northern North Atlantic Ocean.

That may be part of what is slowing the Gulf Stream down. It’s a surface current of warm water from the tropics that travels at an average speed of 6km per hour, contains as much water as there is in all the world’s rivers and moves it up to the seas between Iceland and Norway. Then the water cools off, drops to the bottom, and returns southwards as a cold current.

The Gulf Stream helps keep northwestern Europe warm.

But the Gulf Stream has stopped entirely in the distant past, sometimes for centuries.

To be more precise, it stops going so far north; it overturns, dives to the bottom and heads back south long before it reaches the latitude of European countries such as Ireland and Britain – and then the average temperature in those countries drops by up to 10 degrees Celsius.

There is reason to suspect that a global warming trend was melting a lot of cool fresh water into the northern seas and blocking the Gulf Stream from getting so far north. So is that about to happen again? Nobody knows, but according to the latest studies, the Gulf Stream has already slowed by 15% in the past 50-150 years.

When it shut down in the past, it was abrupt and fast – nonlinear, in other words.

The 15% slowdown is not necessarily an indicator that the whole northern branch of the current is on the brink of shutting down.

But then again, it might be.

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