Most people know that the world is gradually warming up because of the enhanced greenhouse effect. It is also generally appreciated that a long-term world climate cycle operates, with a period of about 100,000 years, of alternating ice ages and warmer interglacial periods, writes Dr William Reville.
Overall climate change seems slow and gradual. However, geological records also show that a series of abrupt world climate changes, occurring every several thousand years, is superimposed on the longer-term cycle.
William Calvin described this phenomenon in January 1998 in the Atlantic Monthly and returns to it in his latest book A Brain for All Seasons: Human Evolution and Abrupt Climate Change (University of Chicago Press, 2002). He argues that the slow world greenhouse warming could suddenly plunge Europe into a Siberian-like climate by causing the Gulf Stream to fail.
If you look at a map of the world you will see that Europe sits at the same general latitudes as Canada and Siberia. However Europe, on average, is about 10 degrees Celsius warmer than Canada and Siberia, largely due to the Gulf Stream.
The Gulf Stream is a warm current of the North Atlantic Ocean, which flows northeast from the Straits of Florida to the Grand Banks east and south of Newfoundland, and from there to the shores of west Europe, Scandinavia and the islands of the Arctic Ocean. The accompanying prevailing southwesterly winds also carry warmth and plentiful rainfall to Europe.
The warm surface water that travels northeast in the Gulf Stream eventually sinks to the bottom of the ocean up around Greenland and returns south again. The overall water movement works like a giant conveyor belt bringing warm water north and cold water south. Much of the heat in the warm Gulf Stream water is removed when cold winds blow across from Canada, evaporating and cooling it, and leaving the salt behind. The resulting cooler, saltier, surface water is denser than the water beneath and consequently sinks to the ocean bottom and starts to flow south.
This normal Gulf Stream operation is vulnerable to disruption. For example, if the surface water density doesn't increase sufficiently it will not sink and make way for more warm water to flow north, and a large influx of fresh water into the North Atlantic could have this effect by diluting the surface water, such as floods out of Greenland, Norway or Iceland, or increased rainfall in northern latitudes carried by north-flowing rivers into the Arctic and then out into the Greenland or Labrador Sea.
The gradual global greenhouse warming that is now under way could easily cause changes that would disrupt the Gulf Stream and trigger a sudden chill. This would cause enormous disruption for two reasons - its suddenness and the nature of our modern society.
Firstly, the chilling climate change would happen over a period of several years. If it happened gradually, say over a period of 500 years, we could adapt to it. But what could be done if it happened over a period of five years?
Secondly, our modern world is very complex and this makes it fragile. For example, Europe has a very efficient agriculture, with 2 per cent of the population feeding all the rest, but it depends on a high level of rainfall coming in off the Atlantic and on staying warmer than Siberia.
These conditions would vanish if the Gulf Stream stopped. We could end up with farmers able to feed only two or three times their own numbers, not 50 times. City life would collapse.
Over the longer term, the Gulf Stream fails regularly and has failed about 24 times over the last 100,000 years. When it fails, not only does the climate change in Europe, but it changes all over the Earth. Basically, things change from a warm, wet regime to a cool, dry, dusty and windy regime and the changes worsen human living conditions everywhere.
Information gleaned from gas bubbles in ice cores from Greenland indicate that past sudden coolings have happened every several thousand years. They come on suddenly, last for centuries and then suddenly warm up again. The last major cooling happened 12,000 years ago and, on that basis, another one is now overdue.
Calvin cites evidence that the Gulf Stream flow has declined by 20 per cent over the last 50 years, and also suggests what we can do to help it. For example, if increased rainfall in high northern latitudes is a problem it might be possible to seed clouds so as to ensure that the rain falls elsewhere. If floods from Greenland are a problem, steps could be taken to ensure that floods don't happen.
Floods occur when fjords get blocked by ice dams and two or three years of ice melt-water builds up behind them. When the dam breaks, the accumulated water gushes out and you can get 100 times normal runoff in one week. This could be prevented by exploding ice dams with dynamite soon after they form.
I will return in a future article to discuss William Calvin's proposals as to how these flip-flops in climate affected the evolution of the human brain over the last two million years.
William Reville is associate professor of biochemistry and director of microscopy at UCC