THE sky is darkened by birds fleeing New York city in anticipation of coming doom. The Statue of Liberty is engulfed by a massive wave, then coated in ice. Under an ominously orange sky, tornados rip into Los Angeles, shredding the Hollywood sign. Such dreamlike, symbolic and at times biblical images of man-made climate disaster fill Roland Emmerich鈥檚 new film, The Day After Tomorrow.
Some scientists think the film will do harm by presenting a grossly distorted picture of climate change. In fact, its Hollywood excesses may help alert the world to the real dangers of increasing greenhouse gas emissions 鈥� less spectacular, but still putting millions of lives at risk.
It is certainly true that the film鈥檚 timescale is compressed. The climate change following a shutdown of ocean circulation in the Atlantic, which in reality would take a decade, unfolds in a week. Some scenes are unconnected with global warming 鈥� the wave that hits Manhattan in the film is similar to the 20-metre tsunami that washed over the Shetlands 8000 years ago, but it could hardly be caused by man-made climate change. And some events in the film defy the laws of thermodynamics, such as the shock-frosting of people in the eye of the 鈥渟uperstorm鈥�.
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That scene reminds me of Virginia Woolf鈥檚 description of the Great Frost of 1608: 鈥淏irds froze in mid-air and fell like stones to the ground. At Norwich a young countrywoman started to cross the road in her usual robust health and was seen by the onlookers to turn visibly to powder and be blown in a puff of dust over the roofs as the icy blast struck her at the street corner.鈥� There is nothing new in using exaggeration as a literary device. Like readers of Orlando, audiences will understand that this film is fiction. But it will raise interest and give scientists opportunities to present a more realistic view. That is already happening: I have never had as much media interest in my work as during the past fortnight.
The film company invited many climatologists to pre-screenings. After one such preview in Berlin, I met the young scriptwriter, Jeffrey Nachmanoff, who impressed me with his knowledge about climate change. He was well aware of the compromises made in following the rules of blockbuster movies rather than the laws of physics, but the film has to recoup its $125-million production cost 鈥� much more than my institute has spent in its 12-year existence.
Before the special effects take over, the background against which disaster unfolds is painted in quite realistic colours. A climatologist speaks to a UN climate conference in Delhi, warning that the North Atlantic Current is a fragile system, and could be disrupted within 100 years. I have given similar talks for years. A Pentagon report warned of the same risk earlier this year. They did not predict that it would happen in the next two decades, as was widely reported, but discussed what the consequences would be if it did.
So how serious is this concern? It is now well established that abrupt temperature changes of up to 10 掳C within a decade have occurred many times, probably caused by changes in circulation in the Atlantic. The North Atlantic Current, which warms Europe, is driven by surface waters cooling and sinking between Greenland and Norway. Climate change is expected to inhibit this process as extra fresh water from rain, swollen rivers and melting ice reduces the density of surface water in the crucial sinking zone. Observations show that river run-off into the Arctic has indeed been increasing, that sea ice and the Greenland ice sheet are melting, and that salinity in high latitudes has declined in recent decades.
All of this will probably weaken the current, and some simulations indicate that it could even stop. The likelihood is hard to establish 鈥� estimates of the freshwater input are highly uncertain, and the stability of ocean circulation differs between models. Satellite data indicates that another ocean current called the sub-polar gyre, which is linked to this circulation, has weakened. On the other hand, although the crucial flow of cold deep water from the Nordic seas back into the Atlantic had also been weakening, it has strengthened again in the past five years. Most experts would probably agree that the risk of a major change in ocean circulation in the next couple of decades is small, but it could become significant later this century.
If the North Atlantic Current stopped, western Europe and eastern North America would get colder, although it would certainly not cause an ice age. Palaeoclimatic data and models suggest it would also shift the tropical rainfall belts, causing floods and droughts around the world. Then there is the disruption of marine ecosystems, a sea-level rise of up to a metre along some Atlantic coasts, a reduction in carbon dioxide uptake by the ocean, and quite possibly some effects nobody has yet thought of.
Unlike in the film, disaster is not going to strike within days. Although global warming is already on its way, we still have time to act to prevent the most serious consequences. The Kyoto protocol and EU emissions-trading are prudent first steps.