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Gulf Stream

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Non-Linear

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.
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To shorten to 725 words, omit paragraph 7. (“Indeed…year-round”)

Climate Change Two

14 August 2003

What They Don’t Mention About Climate Change

By Gwynne Dyer

On several days last week, it was hotter in London than in Cairo. France has just declared a national emergency because of the extreme heat. But the winemakers of Germany are ecstatic about this year’s vintage, and Asian manufacturers of compact air-conditioning systems are having a boom year for European sales.

That’s how most people see climate change: a gradual warming-up that will hurt some people and benefit others. The people of Tuvalu and quite a lot of the Netherlands will be severely inconveniences as rising sea levels cover their homes, but they will either move or learn to breathe underwater. So no rush, no panic, and don’t take any measures that might hurt economic growth.

But anyone who has been paying attention to the evidence coming out of the Greenland ice cores for the past twenty years should know that the real threat is not gradual warming. It is that the warming will trigger an abrupt and rapid cooling of the global climate, with catastrophic consequences for existing human populations. And the Europeans would be hit first and worst, for the mechanism that would cause this shift is the disappearance of the Gulf Stream.

People talk about the ‘last ice age’ as if it were over, but it’s not. The current cycle of global glaciation began around three million years ago, when the land that is now Panama rose above sea level, closing the old ocean channel between North and South America and forcing a major reorganisation of ocean currents. Since then, ice sheets have covered around 30 percent of the land surface of the planet most of the time, although this has been regularly interrupted by major melt-offs called ‘inter-glacials’ when the ice coverage drops to about 10 percent.

During the past million years these warm, wet episodes have come along approximately every 100,000 years. The present inter-glacial began about 15,000 years ago, and nobody knows for sure how long it will last. We do know, however, that the previous inter-glacial began about 130,000 BC, and lasted for 13,000 years — so we could already be in overtime on this one.

The good news is that we don’t automatically slide back into maximum glaciation. What happened in 113,000 BC was that the global climate flipped into a cool, dry, windy phase that was much less pleasant than our current balmy conditions: average temperature at least 5 degrees C (9 degrees F) lower than the present, and massive droughts all over the place. It took a further push — probably massive volcanic eruptions in Indonesia around 70,000 BC — to start the ice sheets growing again.

The bad news is that even the ‘cool, dry, windy’ phase of the global climate would wreck human civilisation. The whole enterprise of civilisation that has allowed the human population to grow from perhaps 10 million to over 6,000 million has occurred within the warm, wet climatic bubble of the past ten thousand years. At least half the lands that now support agriculture would revert to tundra or semi-desert if we flipped back to the cool, dry and windy climate, and billions would die in the chaos of war and starvation that would follow.

Now for the worse news. When the flip happens, it isn’t gradual at all. The Greenland ice cores, a quarter-million-year record of annual snowfall that also tells us about average temperature, precipitation and even wind speed, contain an alarming message. When the climate mode shifts, global temperatures crash in 10 years or less — and stay down for centuries or millennia. And the very worst news is that the sudden flip into cool-and-dry is caused by gradual global WARMING.

The key to the whole cycle seems to be the Gulf Stream, which normally delivers huge amounts of warmth to the northern North Atlantic and western Europe (which would otherwise have the climate of Labrador). But ocean currents are basically conveyor belts for moving salt around the world’s oceans. If the warm water of the Gulf Stream, made even more dense and saline by evaporation on its long journey north, does not sink to the bottom and flow back south when it reaches the Greenland-Iceland-Norway gap, then the whole conveyor belt shuts down.

What stops the salty water from sinking? Dilution by too much fresh water on the surface, coming either from increased rainfall over the North Atlantic or from glacial melting and sudden outflows of fresh water from the Greenland fjords. What might cause these events? A rise in temperature in the region — and while average global temperature has only risen about one degree in the past century, the rise in the Arctic region has been several times greater.

The evidence in the Greenland ice cores is clear: the abrupt, high-speed flips in global climate known as Dansgaard-Oeschger events have happened many dozens of times. Maybe if we have a couple of more centuries of warm-and-wet conditions, we will learn enough about the fine detail of global climate to postpone the next flip indefinitely. But if it goes over the edge now, it’s a calamity for everybody.

Europeans, whose agriculture could no longer feed even a tenth of their current population, would be hit hardest of all, though nobody would get away with less than a fifty percent loss. So why didn’t this prospect get more media attention during the recent unprecedented heat wave in Europe? Maybe all the science journalists were on vacation.

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To shorten to 725 words, omit paragraphs 9 and 10. (“The key…greater”)