Under the Microscope/Dr William Reville: This week I saw The Day After Tomorrow, the climate disaster film that portrays the sudden onset of a new Ice Age in the northern hemisphere, triggered by the failure of the Gulf Stream.
The film is very entertaining and the special effects are brilliant. The suddenness of the onset of the Ice Age is grossly exaggerated, but otherwise the scientific bones of the scenario are sound. This article is basically a repeat, prompted by topically.
Most people know that the world is gradually warming 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.
Overall, climate change seems slow and gradual. However, geological records also show abrupt world climate changes, occurring every several thousand years, superimposed on the longer-term cycle. William Calvin described this phenomenon in January 1998 in the Atlantic Monthly magazine. He argues that greenhouse warming could plunge Europe into a Siberian-like climate by causing the Gulf Stream to fail.
Although Europe sits at the same general latitudes as Canada and Siberia it is, on average, about 10 degrees Celsius warmer. This is largely because the Gulf Stream keeps the North Atlantic much warmer than it would be if this ocean current didn't operate. The Gulf Stream is a warm current, 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 a lot of the water, taking the heat from it and leaving the salt behind. The resulting cooler, saltier surface water is denser than the water beneath and sinks to the ocean bottom and starts to flow south.
This normal Gulf Stream operation is vulnerable to disruption. For example, if surface water density doesn't increase sufficiently it will not sink and make way for more warm water to flow north. A large influx of fresh water into the North Atlantic could have this effect by diluting the surface water. For example, 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 could stop the re-circulation of the Gulf Stream.
The gradual global greenhouse warming now under way could easily cause changes, e.g. increased rainfall in northern latitudes 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. First, 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 without warning over a period of five years?
Second, 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 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 2 or 3 times their own numbers, not 50 times. City life would collapse.
The Gulf Stream 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 overdue now.
Calvin cites evidence that the Gulf Stream flow has declined by 20 per cent over the last 50 years. If this ocean stream is getting into difficulties, can we do anything to help it? Calvin has some suggestions to make in this regard. 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, preventative steps could be taken.
Floods occur when ice dams block fjords 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 run-off in one week. Exploding ice-dams with dynamite soon after they form could prevent this. It is not unlikely that measures such as these will have to be taken in the years ahead.
• William Reville is associate professor of biochemistry and director of microscopy at UCC