Forecasting by computer is based on a mathematical model of the atmosphere - a description of the behaviour of the atmosphere in terms of mathematical equations. The equations used in the models to estimate possible changes in the global climate assume concentrations of the various gases in the atmosphere that may differ significantly from those known to exist at present, and it is on this basis that conditions at some future time are calculated.
Unlike those used to produce the daily forecast, which must paint the weather picture in considerable detail, climate models are concerned only with very broad trends in the behaviour of the atmosphere - trends which can be expressed in average figures over a very large area. Let's have a look at what the latest versions tell us.
The best models available predict that if nothing is done to reduce greenhouse gas emissions, the average temperature of the world will rise by about 2 Celsius between now and the end of the next century, a rate of increase unprecedented in the past 10,000 years. What the local effects at any particular spot may be, however, are uncertain: some places may experience warming significantly in excess of the average, while others may well remain unchanged - or may even become colder at certain times of year. The greatest warming, by and large, is predicted to take place in cold, northern parts of the world in winter; and inland regions are also projected to warm faster than coastal zones, because of the moderating effect of the world's oceans.
The Earth's average sea level is predicted to rise by about 50 cm by 2100. The main cause of this expected rise will be the thermal expansion of the upper layers of the ocean. The faster melting of the Greenland and Antarctic ice sheets is not calculated to have any significant effect, since it is expected to be balanced by increased snowfall in both regions.
In general, wetter winters and drier summers are predicted as the norm. It is also likely, we are told, that the frequency and intensity of extreme weather events, such as storms and hurricanes, may change - but nobody knows quite how. Neither do we know for sure whether or not greenhouse warming could trigger significant changes in ocean circulation - such as, for example, a weakening of the Gulf Stream that keeps western Europe warm in wintertime. But the most dramatic change sometimes mooted for a greenhouse world, the collapse of the West Antarctic ice sheet, which would lead to a catastrophic rise in sea level, is now considered unlikely to occur in the next 100 years.