Energy Crisis: The real threat of global warming

Over the past century, carbon dioxide (CO2) in the atmosphere may have increased from around 3/100ths of 1 per cent to about 4/100ths of 1 per cent, and average global temperature may have increased by about 0.6°C.

I say "may have", because both figures are derived from complex statistical treatment of thousands to millions of individual measurements which are subject to both high levels of natural variation and a variety of errors.

Predictions of ongoing future warming are based on computer models of global climate. While impressively complex, such models are still only crude, greatly simplified approximations of the actual climate system. They include numerous assumptions, estimates and uncertain measurements. They have also been elaborately adjusted until they produce results the modellers deem appropriate. This is then called "optimising".