As human beings we have a deep-seated desire to explain natural and social phenomena. We have an in-built search mechanism which looks for causation in order to explain the world around us. For example, the US stock market declined on the day after Obama's election. Why? Most people connected the two events, but the two may have been totally unrelated. Another example can be found in today's Metro where it is claimed that crime falls and birth rates go up in Mexico City whenever Javier Hernandez is playing for Manchester United. These may simply be crude correlations and there may be no causation whatsoever. It requires rigorous statistical testing to demonstrate correlation never mind causation!
The Berkeley Earth Project , an independent study of global warming, has found that the earth has become a degree warmer over the past half century. However, the statistical uncertainty surrounding pre-1920 estimates makes it very hard to say much about long-term trends - click here for graph . This is one of my concerns with the global warming debate - we simply don't have trustworthy long-run data which looks at temperature changes over the last millennium (or two). My second concern with the global warming debate is that it is very hard to prove any sort of casual link between global warming and human activity. The scientists may be able to show correlation between global warming and our production of carbon dioxides etc., but correlation is not causation. My third concern with the debate is that those who are sceptical or agnostic are stereotyped as flat-earthers or intellectually-challenged crackpots. This only stifles debate and the progress of science itself.