Hollywood 10-day ice age highly unlikely

Hollywood’s latest disaster movie, The Day After Tomorrow, is about to be released. It is a fictional account of the havoc wreaked by out-of-control climate as North America is beset by the chilling beginnings of a new Ice Age in the course of 10 days. The movie features numerous catastrophic weather events including hurricanes, tornadoes, floods, and tidal waves striking New York.

“It’s a good yarn,” said Dr Tony Haymet, Chief of CSIRO Marine Research. “Like many of the catastrophe films, it’s loosely based on scientific fact. Enjoy the film for what it is, entertainment, but don’t go home afterwards and begin preparing for imminent disaster. Climate just doesn’t change that quickly.”

CSIRO Marine Research and the Antarctic Climate and Ecosystems CRC are actively involved in international ocean observation programs that are monitoring ocean currents. Changes in ocean currents can cause climate change so the observations are critical for testing computer models used to project what the future climate will be like.

Europe’s climate, for example, is much warmer than other regions at the same latitude because an ocean current pattern called the overturning circulation, or ocean conveyor, carries heat from low to high latitudes.

Input of fresh water to the sea can disrupt the ocean conveyor, decrease the ocean heat transport, and therefore cause cooling. At the end of the last Ice Age, a large flood of fresh water released by melting ice sheets caused temperatures in the North Atlantic region to plummet by 5°C in a few decades.

Most computer models show a weakening, but not a complete shut down, of the warm currents by the year 2100, according to the Intergovernmental Panel on Climate Change. Cooling due to a slow-down of the ocean conveyor offsets some, but not all, of the surface warming in Europe due to greenhouse gases.

The computer models also suggest that beyond 2100, the ocean conveyor could completely shut down, possibly irreversibly, in either hemisphere, if the greenhouse-warming is large enough and applied for long enough.

Australia is expected to warm as a result of the enhanced greenhouse effect, even if the ocean conveyor slows down. CSIRO projects that, over most of Australia, annual average temperatures will be 0.4°C to 2°C greater than 1990 by 2030.

This will lead to a 10-50 per cent increase in days over 35°C and a 20-80 per cent decrease in frosty days. There will be more extreme rainfall, stronger tropical cyclones, more fires, less snow and, in some parts of Australia, more droughts.

“The likelihood of a collapse of the ocean conveyor in the near-term is very low, but there is still much we don’t understand about the vulnerability of the climate system,” said CSIRO’s Dr Steve Rintoul. “A key issue is to determine if, by increasing the concentration of greenhouse gases in the atmosphere, we are pushing the climate system closer to a threshold where the odds of an abrupt change in climate are increased.

“A rapid shift in climate is possible – we know climate has changed abruptly in the geological past. But the time scale of climate change in the film has been dramatically compressed in order to create a good plot, and the impacts have been exaggerated. It is not possible for an Ice Age to occur in a few days,” Dr Rintoul said.

“Global warming is expected to increase rainfall at high latitudes. More rainfall will decrease the salinity of the ocean and may disrupt the ocean conveyor. Surprisingly, this means that global warming may lead to regional cooling.

“However, it is not clear whether the warming and increase in rainfall will be sufficient to push the climate system across the threshold to a colder climate.”

A slow down of the ocean conveyor is unlikely to cause an Ice Age in the near future. The present orbit of the Earth and high levels of carbon dioxide in the atmosphere mean that a new Ice Age, with large ice sheets on the continents of the northern hemisphere, is unlikely to occur for many thousands of years.

For further information:
Dr Tony Haymet Chief CSIRO Marine Research, 03 6232 5214