Under the Microscope: Many readers will have seen the film The Day After Tomorrow in which the US and Europe are suddenly plunged into a new ice age. Is this a fantastic exaggeration dreamed up by Hollywood or is there a significant probability that such a situation could abruptly develop?
Scientists who study climate tell us that sudden climate change could happen, has happened often in the past, and will inevitably happen again in the future. It is unlikely however that any sudden cooling would be anything like as severe as depicted in the recent film. The science behind sudden climate change is described by Richard Alley in Scientific American November 2004.
A record of climate conditions in ages past is preserved in the thick ice sheet that covers Greenland. Ice cores up to 3km long are drilled from this ice sheet and analysed in the laboratory. The deeper the ice below the present surface, the longer ago it was deposited and this has allowed accurate climate records to be established for the past 110,000 years.
The ice cores reveal over 20 cycles of warming and cooling over this period. The warmings were strong and abrupt but after a few hundred to a few thousand years of the start of a warming period a slow cooling set in lasting thousands of years and culminating in a fast cooling over in as little as 100 years. Then the pattern began again with an abrupt warming.
Cold times in Greenland correlated with cold, dry and windy times in North America and Europe and with unusually warm weather in Antarctica and the South Atlantic. Colder times in the north correlate with droughts in India and Saharan Africa. The Sahara was once a green lake district but was converted to an arid desert by a sudden drying about 5,000 years ago. A 200-year dry spell helped to end the Mayan civilisation in Central America about 1,000 years ago.
Sudden flip-overs in climate conditions are caused when a key climate driver, for example temperature, is changing, reaches a threshold limit and suddenly flips the climate into a new gear. Alley uses a canoe analogy to illustrate this point. When you lean over in a canoe, the canoe tilts with you. But lean over more than a certain amount and the canoe suddenly flips upside-down.
As the Greenland ice core data shows, climate naturally cycles between colder and warmer phases as the canoe is tilted by natural forces. But, Alley explains, superimposed now on these natural tilting forces is the random dancing of the canoeist and this is likely to cause the next upending of the canoe much sooner than would otherwise happen. The canoeist is dancing to the tune of industrial emission of greenhouse gases that are artificially warming the world, replacement of forest with cropland that alters the fraction of incoming sunlight reflected by earth, and so on.
Scientists explain the extreme cold episodes in Greenland's cycling climate by the behaviour of North Atlantic currents that have a huge effect on the long-term weather patterns of this region. Europe and eastern North America have a temperate climate today because Atlantic surface water warmed by southern sunshine flows northwards across the Equator.
This water cools as it migrates northwards, with much heat removed by the prevailing winds blowing eastwards from over Canada that are then warmed and blow onwards over Europe to produce temperate moist conditions. Seasonal monsoons power growing seasons in Africa and the Far East.
The cooling north-moving Atlantic surface water becomes denser and eventually sinks east and west of Greenland and then migrates southwards along the ocean floor. This sinking water powers a conveyor belt system - warm water from the south flows northwards to replace the sunken water and the overall circulation warms the north and cools the south.
If the conveyor belt stopped, warming of North America and Europe would cease and climate there would become like Siberia's. Also, monsoons would fail in Africa and the Far East. If the conveyor belt slowed down the effect would be intermediate between present conditions and those just described for a stationary conveyor belt.
There is scientific consensus that the world is warming and that this trend is being exacerbated by industrial emissions of various gases that are strengthening the natural greenhouse effect. The danger is that our rapidly warming world will accelerate the melting of Arctic glaciers, greatly increasing the rate at which fresh water enters the very northern seas. This could so greatly dilute the salty seawater flowing up from the south that it would freeze before it had an opportunity to sink, thereby stopping the great conveyor belt.
And this is one way in which, paradoxically, our warming world could overturn the canoe and tip us into a very cold situation. Once the conveyor belt stopped, the cooling would start right away and significant effects would be felt in Europe in the first winter after the currents failed. The full severity of the cooling would be established within a few years.
Unfortunately climate scientists are unable to predict when sudden climate change will occur or what form it will take. All that we can do is to slow down and, as soon as possible, stop practices that are artificially warming the world. This will reduce the possibility that the catastrophic sudden climate change will occur any time soon.
• William Reville is associate professor of biochemistry and director of microscopy at UCC