How difficult has it been to discriminate human influence on climate from natural influences?
It’s been very difficult. Climatologists only have comprehensive global temperature and rain fall measurements going back a century. Using so-called “proxys” such as corals, tree rings, ice cores and lake sediments, scientists have been able to reconstruct longer records–back about 1,000 years. Over this period, the last 100 years appears to stand out. (see graph) Since 1900, the Earth’s average global temperature has warmed by about 1 degree F–hardly noticeable to most of us, but a rapid increase compared to most natural changes. Interestingly the shape of this curve is not a straight line. Much of the warming occurred early in the century, then it leveled out for three decades around mid century. This, it’s argued, shows that human greenhouse gas build-up is only one factor influencing the climate. Other factors–for example, the changing output of the sun, other pollutants like sulphate aerosols which have a cooling effect and, volcanoes–also contribute to the aggregate effect.Wh
