The world's climate is in constant flux: on time-scales from days to millennia, global and regional temperature, wind and rainfall patterns are changing. Over periods of decades and centuries, the most significant factor affecting climate appears to be changes in the output of the sun.
Man's emissions of 'greenhouse gases' (GHGs) also play a role in altering climate. However, estimates suggest that only 30 to 40 per cent of the warming seen over the past century was caused by GHGs.
Predictions made by the Intergovernmental Panel on Climate Change (IPCC) assume that most of the warming of the past century was caused by man's emissions and therefore overestimate the likely effect of future emissions.
Better estimates suggest that if CO2 concentrations double, global-mean temperatures would rise by about 1.3 degrees centigrade with an upper limit of 2 degrees centigrade.
Estimates by some of the world's most respected climate scientists suggest that even if a warming of 2 degrees centigrade does occur the impact on humankind will not be catastrophic; indeed agricultural productivity is likely to increase in many parts of the world