By Richard Proudlove
If you are looking for an arresting “worst case” depiction of the potential consequences of climate change, it’s tough to beat the 2004 climate disaster blockbuster “The Day After Tomorrow”.
This premise provided an ideal basis for Hollywood to flex its special effects muscles, although the script writing budget clearly took a hit as a result.
Upon its release, prominent climate scientists criticised the film as an exaggeration. However, the science underpinning the events depicted does have some merit.
A climatic tipping point is a threshold beyond which an irreversible, self-perpetuating process is triggered which accelerates or amplifies climate change. In the past, this process caused the desertification of the Sahara region and tipped The Earth into and back out of five major ice ages.
The apocalyptic events portrayed in The Day After Tomorrow are caused by a massive polar ice sheet completely melting and disrupting oceanic currents. This scenario is included in many climate models and is expected to exacerbate the effects of global warming.
The melting of polar ice due to global warming is being accelerated by “albedo flip”. Albedo is a measure of how reflective a surface is. Ice has a high albedo and reflects a significant proportion of incoming solar radiation. This has historically provided a localised cooling effect at the poles.
Because open ocean reflects less sunlight than ice, disappearing sea ice amplifies the effects of global warming at the poles and accelerates melting. This is further compounded by pools of meltwater on the surface of the ice being warmed by the sun, resulting in more melting. It is expected that, at some stage, this process will reach a “tipping point” resulting in persistent, unstoppable, rapid melting of polar ice.
The influx of cold, fresh water from melting polar ice into adjacent oceans may be weakening global oceanic currents. These currents transport heat from the equator to higher latitudes, influencing the climate in Europe and North America for example.
In The Day After Tomorrow, ocean currents grind to a halt. As a consequence, global transfer of heat by these currents stops and the Northern Hemisphere is plunged into an ice age.
Ocean currents are propelled by the sinking of unusually dense surface water to the ocean floor in the North Atlantic. Surface water becomes dense here partly because it cools and partly because the formation of sea ice increases salinity. There is concern that this effect is being weakened by meltwater from the Greenland ice sheet, which is causing North Atlantic surface water to become warmer, less salty and therefore less dense.
There is evidence that North Atlantic deep water circulation has weakened markedly since the late 1950s, although a complete shutdown is not thought to be imminent. Some models even suggest that cooling caused by the weakening of oceanic currents could offset global warming for up to a century.
Is a sequel “stuck in the mud”?
So where does this leave the scientific credentials of The Day After Tomorrow? Well, the script writers clearly did some homework since the film drew attention to the valid concern of melting polar ice due to global warming. However, they were somewhat “on thin ice” in their depiction of the consequences, which are unlikely to be as dramatic, violent or immediate as those depicted in the film.
Furthermore, other climatic tipping point events could be more pressing.
One of these is the potential for large amounts of methane to be released as Arctic permafrost thaws and the organic matter it contains begins to rot. Methane is a much more potent greenhouse gas than carbon dioxide. Emissions due to permafrost thawing this century could be equivalent to 270 years of carbon dioxide emissions at current levels. This would cause a sudden spike in the rate of global warming and has been described as “a slow-motion time bomb”.
Thus, if a sequel to The Day After Tomorrow is on the cards, might I suggest that the opening scene finds the hero knee deep in a rotting mammoth in the Siberian tundra.