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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 C higher than the usual average for that time of year.

The other interesting fact, revealed last month in two articles 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 that at any time in the past 1,600 years (which is as far back as studies have gone). This could be very bad news for Western Europe.

The difficulty comes in figuring out what these facts mean – if they mean anything at all, and are not just random variations of an unusually extreme kind. And this is the point in the discussion at which you start to hear the climate scientists use the word ‘non-linear’ 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. Unfortunately, that is not how climate change has proceeded in many past cases of warming or cooling.

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 non-linear: it stays the same for a long time, and then suddenly some ‘tipping point’ is reached, and the whole thing flips into a different configuration.

Now, the warming in the Arctic is not non-linear. It’s a trend that has been continuous for decades, although it has accelerated greatly in recent years: the amount of sea-ice coverage at the point of maximum freeze-up, in late March, has been far lower in 2015-18 than ever before.

Indeed, we’re almost certain to see an ice-free Arctic Ocean at the end of the summer melt season at some point in the next decade. Some of the ice will reform in the following winter, but less and less of it as the years pass. Without ice cover the water will be warmed directly by sunlight, so one day the whole ocean will be mostly ice-free year-round.

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 for the near term may be the fact that 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 six km per hour, contains as much water as there is in all the world’s rivers, and moves it all the way up to the seas between Iceland and Norway. Then the water cools off, drops to the bottom, and returns southwards as a deep-water cold current.

The Gulf Stream helps keep north-western Europe warm: England is at the same latitude as Labrador, but the average temperature is more than 10 degrees C higher. Norway, with 5 million people, is about the same latitude as southern Greenland (pop. 50,000). But the Gulf Stream has stopped entirely a number of times 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 like Ireland, Britain and Norway. And when it has done that in the past, the average temperature in those countries dropped by up to 10 degrees C.

There is reason to suspect that what was happening in these incidents was 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: non-linear, 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.
To shorten to 725 words, omit paragraph 7. (“Indeed…year-round”)