Since the beginning of the Industrial Revolution in the late 1700s, we have been conducting an unintentional experiment in atmospheric engineering. In 1990, over 400 of the world's leading climate researchers from 25 countries published a 350-page international report on this experiment. it said that available evidence powerfully suggests that Earth's climate is warming up. By the year 2050, we could start to experience major changes in climate, with unimaginable human and economic consequences. The evidence on which the scientists base this conclusion comes mainly from 'global climate models' - models of Earth's atmosphere and oceans which are run and re-run on powerful supercomputers. We know that carbon dioxide (CO2) in the atmosphere has risen inexorably since the Industrial Revolution. The models show what may happen if CO2 continues to rise. There are only a handful of centres in the US and Europe which operate global climate models, and a handful of scientists who interpret them. They are the first to admit that the models cannot forecast the future with certainty. But if they are correct, or anywhere close to correct, then it is time to take action if we are to avoid major changes to our climate.