Is the climate changing?

In the UK, central England temperatures rose by almost 1 °C during the twentieth century. The 1990s was the warmest sing]e decade since records began in the 1660s; average sea level is rising by approximately 1 mm yr^-1; and winters across the UK have been getting warmer, with a larger proportion of the winter precipitation falling on heavy rain days. According to general circulation models (GCMs), some degree of further climate change is probably inevitable. Much of the change over the next 30-40 years will already have been determined by past and present emissions of greenhouse gases. The current Hadley Centre climate models produce a range of scenarios of future climate for the UK. Scientific judgments have been made on the basis of physical reasoning involved, consistency between different computer models, and statistical significance of the results. However, there are several reasons why we cannot be completely confident about climate-change predictions. First, there are uncertainties about future emissions of greenhouse gases and other pollutants due to the possible future changes of populations, economies, energy technologies and other social factors. Second, the response of climate to these emissions depends upon several poorly understood factors: ■ what proportion of the emitted green-house gases remains in the atmosphere to increase their concentration ■ how much additional heat is trapped by these increased concentrations ■ how this increased atmospheric heating changes climate ■ the importance of positive and negative feedback effects such as the cooling effect of aerosols or the warming effect of soot particles.