Flip-flop to catastrophe 

We are now experiencing the first effects of the increase of greenhouse gas concentrations in the atmosphere. Global average temperature has risen by some 0.7 degrees centigrade. Rainfall and floods are increasing in mid latitudes, snow and ice cover are decreasing, and the strength and frequencies of dominant circulation patterns such as El Niño are being affected.

A rise in average temperature, increasing rainfall and changes in circulation patterns are accompanied by changes in the frequency and intensity of extremes. It is no surprise, therefore, that the number of weather related disasters has risen rapidly over the last few decades. Furthermore, from the 1960s to the 1990s economic damage caused by weather related disasters increased eightfold even after correcting for inflation. Much of this can be explained by the fact that there are more people with greater assets, by the growth of towns and cities in vulnerable areas and similar factors. But probably some 10 per cent and maybe up to 50 per cent of the rise in the cost of weather related damage is caused by climate change. Precise attribution is very complex as the climatic factor varies by region and by type of event. For example, stronger and more frequent El Niños are accompanied

by fewer hurricanes on the east coast of the United States of America, but cause more damage elsewhere in the world.

Given the momentum of climate change and the projections of global greenhouse gas emissions, the increasing cost of weather related damage is very likely to continue over the next few decades. Present trends suggest that it will rise by $50 to $100 billion annually over the coming ten years.

Reducing risks:

Cutting the growth of global emissions would reduce the rate of climate change, so that people and ecosystems would find it less difficult to adapt, and would suffer less. Perhaps more importantly, it would reduce the risk of destabilizing the global climate. Such destabilization a relatively rapid change over a matter of decades could come about as a result of major changes in the patterns of ocean and atmospheric circulation, or in the volume of snow and ice, or through the triggering of major positive feedback from the release of ‘natural’ sources of greenhouse gases.

The major risk to the climate quite different from the gradual rise in temperature and sea level is the possibility of fast, flip flop changes in it. Small changes in the energy balance of the Earth can trigger large changes in atmospheric and ocean circulations and these can have major consequences on temperature, rainfall, wind and storm patterns, and extreme events.

The probability of such changes occurring in the climate’s regime may be low over the next 100 years and they are difficult to predict but when they do happen they will have a major impact on life on Earth. Evidence that they have occurred in the past has been found in paleoclimatic records. Some impacts might be reversed when atmospheric concentrations of greenhouse gases decrease again, but others could be largely irreversible, as the climate system has many equilibria.

One such possible fast change could shut down Europe’s natural heat pump, through the stagnation of the Ocean Conveyor Belt, a thermohaline circulation driven by differences in the density of seawater controlled by temperature and salinity. This conveyor belt transports an enormous amount of heat northward, through the Gulf Stream, making the climate of north-western Europe on average 8 degrees centigrade warmer than the mean value at its latitude. The water cools and sinks in the North Atlantic, it then moves south and circulates around Antarctica, before going northward to the Indian, Pacific, and Atlantic Oceans basins. Density differences between seawater determine the ‘strength’ of this circulation: a change in them, as a result of climate change, could lead to a ‘weakening’ or even a stagnation of this ocean current.

The Conveyor Belt circulation pattern is susceptible to perturbations resulting from injections of excess freshwater (from precipitation or melting ice) into the North Atlantic. At present the sinking of saltwater near Greenland ‘pulls’ less saline warm water to the North Atlantic Ocean. But increasing rainfall at higher latitudes would make this surface water less salty, so it would sink less and the strength of the conveyor circulation could decrease.