Estimates of low frequency natural variability in near-surface air temperature

Estimates of the spectrum of natural variability are critical to the problem of detecting an anthropogenic signal in global climate observations. Without such information it is impossible to say that current climate change is different or unique from changes that have happened in the past and, therefore, potentially due to man-induced causes. We have estimated the spectrum of natural variability from a globally distributed set of palaeo-temperature proxies and compared it with comparable estimates from two long control integrations of coupled general circulation models - the type used to predict anthropogenic change due to greenhouse gases. None of the three estimates of the natural variability spectrum agree with each other on the low-frequency, near-global time/space scales. Until this dichotomy is resolved, it will be hard to say, with confidence, that an anthropogenic climate signal has or has not been detected.