Introduction

Climate change has been heralded as the most serious environmental problem facing humankind, deserving urgent action to curb emissions of the greenhouse gases causing the problem. The scientific community is in general agreement that greenhouse gases are accumulating in the atmosphere, that temperatures are rising and it is most likely due to human activity. The Intergovernmental Panel on Climate Change (IPCC) (see Section 1.2.1) in its third assessment report (2001) suggested that there is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities. But many uncertainties still exist about the causes and effects of climate change. They are acknowledged by the scientific community and used by sceptics to argue for a greater understanding of the science before concrete action is taken. One Uncertainty is the extent to which changes in temperature might be due to the natural variability of the climate system. A second is the ability of models to simulate the natural variability of the climate system over long time scales (decades and centuries). A third uncertainty exist over how accurate a picture we have of the global mean temperatures for the past millennium since it is reconstructed using proxy information.