Predicting the Climate of The Day After Tomorrow

Intro to the film
Disaster film The Day After Tomorrow, released in May 2004, proved to be hugely popular – it was at the top of the box office list in many countries, and took  £69.8 million in the first 10 days after it was released.
In the film, changes in the circulation of the Atlantic, brought on by man-made climate change, ultimately lead to the rapid start of an ice-age. But is this a scientific likelihood, Hollywood exaggeration or complete and utter rubbish?

What is the thermohaline circulation?

Simplified Model of the Thermohaline circulation in the oceans. Below the well-mixed, warm surface layer there is a thermocline; a zone in which temperature falls rapidly with depth. Below that is the cold, deep ocean. The Sun’s radiation heats the surface ocean in the Tropics. At the Poles, the surface ocean loses radiation to the atmosphere, cools, and sinks. This partly drives the circulation of the oceans.  	

Evaporation
Within a liquid some molecules have more energy than others, and may have sufficient energy to escape from the surface of the liquid as vapour. This process is called evaporation. If the liquid is warmer, more molecules will have enough energy to escape and so the liquid evaporates more rapidly. Evaporation is also assisted by windy conditions which help to remove the vapour particles from above the liquid so that more can escape. If the liquid is a solution, the solvent (e.g. water) can evaporate leaving the solute (e.g. salt) behind.

The circulation of the oceans is driven by the heating of surface waters in the tropics, and cooling at the poles. Cold water is more dense than warm, so it sinks, and water flows in to take its place. In its simplest form, the circulation of the oceans therefore consists of water sinking at the Poles and rising in the Tropics, with surface and deep currents flowing to complete the circulation. 
The Gulf Stream is partly caused by this, and partly by the winds blowing across the ocean surface. It is a warm, surface current which flows from the coast of North America, keeping North-Western Europe a few degrees warmer than it would otherwise be. As the current flows across the Atlantic, water evaporates. The process of evaporation leaves the Gulf Stream water comparatively salty. 
Meanwhile, in the Arctic, the surface water radiates heat to the atmosphere and forms sea-ice – again leaving the surface water colder and saltier than the surrounding water. When the Gulf Stream meets the Arctic waters in the Greenland Sea, the combination is so much more dense than the water around it that it rapidly sinks and flows equatorwards. 
The full, large scale, three-dimensional circulation pattern of the ocean is called the thermohaline circulation, because it is driven by differences both in temperature and salt concentrations. As yet, relatively little is known about it. 

The global oceanic circulation, highly simplified. The red part of the conveyor represents the transport of warm surface water, and the blue part the transport of cold water below the thermocline. It would take any individual water molecule about a thousand years to do a complete circuit!  Source: www.CLIVAR.org

How could it ‘switch off’?

The Greenhouse Effect
Some gases in the Earth’s atmosphere (e.g. Carbon Dioxide, water vapour) act as an insulating blanket for the Earth – they keep the Earth’s surface and the lower atmosphere warmer than it would otherwise be. With no greenhouse gases, the surface of the Earth would be about 33˚C colder than it currently is (so Britian would be about the temperature of the inside of a freezer).  Most scientists now think that the gradual increase in greenhouse gases over the past 200 years is having a noticeable effect on the Earth’s climate.
Floating ice shelves fringe much of the Antarctic ice sheet (only a few small ice shelves exist in the Arctic). Huge icebergs regularly break off as part of the normal life-cycle of an ice shelf. However, around 8000 km2 of ice has been lost since the 1950's, and this may well be linked to the changing climate - in the same period meteorological stations in Antarctica measured an increase in the air temperature of about 2°C. The final stages of the loss of Larsen Ice Shelf A in 1995 were particularly spectacular; in fifty days an area of ice shelf the size of Surrey broke up into thousands of football pitch-sized icebergs and floated away. If the warming continues more ice shelves may be threatened. 
As ice-shelves melt, they release a huge amount of fresh water into the oceans. This may be enough to slow down the thermohaline circulation. There is some evidence that at the end of the last ice-age, when the glaciers covering North America melted, the thermohaline circulation may have switched off altogether for a while. However, because the Gulf Stream is driven by both the thermohaline circulation and by the wind, it can never be switched off entirely.

  Iceberg: a broken-off piece of ice shelf. (source British Antarctic Survey website)

What is a climate model?
In The Day After Tomorrow, scientists use computer models to try and predict how the atmosphere will respond to the slow-down of the Gulf Stream. This is the same approach that scientists all over the world are taking – together with using satellites and ocean buoys to try and understand how the oceans work.  A climate model basically consists of all the sets of equations which represent how the atmosphere and oceans behave. The conservation of energy, salt, water and momentum are taken into account as well as things like how clouds form. These equations have to be evaluated at many places on the Earth’s surface, in the depths of the oceans and the heights of the atmosphere. If the models are only run a few days into the future, scientists can use the data produced as a weather forecast. They have to be run tens of years into the future to produce a climate forecast.

An example of climateprediction.net results. This model shows a very cold North Atlantic. 

Are scientists trying to model The Day After Tomorrow
In the film, the scientists struggle to get enough super-computer time to predict how the weather will develop. The climateprediction.net project, based at Oxford University is taking an alternative approach. Rather than use super-computers, the project is asking people all over the world to run a climate model on their home, school or work PCs.  A new experiment, inspired by the Day After Tomorrow, investigates both how the atmosphere reacts to both a doubling of the Carbon Dioxide concentration in the atmosphere, and to a slow-down of the thermohaline circulation. While the model runs on your computer, you can watch the climate patterns develop. The results should show how the atmosphere is likely to respond if the thermohaline circulation does slow down – will the effects counteract global warming? Will the atmosphere change in a way which will tend to increase the thermohaline circulation again? Or are we all doomed to a Day After Tomorrow type ice-age?