Not so positive feedback

Global warming: the vicious circle

"A warmer world is increasing evaporation from the oceans causing atmospheric concentrations of water vapour, a powerful greenhouse agent, to have increased by 4 per cent over the sea since 1970. Water vapour in the atmosphere exacerbates the greenhouse effect. This is the largest positive feedback identified in the report, which details for the first time the IPCC's concern over the uncertainties - and dangers - of feedback cycles that may quickly accelerate climate change."

and this is a very interesting part - a counterfactual argument that humans are the cause:

The IPCC also finally nails the canard of the climate sceptics who argue that global warming is a myth or the result of natural climate variability; natural factors alone cannot account for the observed warming, the IPCC says. "These changes took place at a time when non-anthropogenic forcing factors (i.e. the sum of solar and volcanic forcing) would be expected to have produced cooling, not warming.

"There is increased confidence that natural internal variability cannot account for the observed changes, due in part to improved studies demonstrating that the warming occurred in both oceans and atmosphere, together with observed ice mass losses."