As the world warms, ice caps will melt.

What are the main feedback likely to be?

One of the easiest to estimate is the "ice-albedo" feedback. As the world warms, ice caps will melt. As this happens parts of the Earth's surface once covered with ice will be replaced by water or land. Ice is very efficient at reflecting solar radiation into space, whereas water and land absorb far more. 

So the Earth's surface will trap more heat, increasing warming--a positive feedback. Less clear-cut is the impact of the extra water vapour likely to enter the atmosphere because of greater evaporation in a warmer world. This added water vapour itself contributes to the greenhouse effect, another positive feedback. 

But it may also increase cloud cover. The dominant effect of some kinds of cloud is to shroud the Earth--a negative feedback--but other clouds, such as cirrus, may trap heat at low levels--another positive feedback. 

Disputes about how water vapour and clouds will influence global warming are at the heart of many of the disputes between mainstream scientists and the handful of greenhouse sceptics. Overall, the mainstream view is that feedbacks will amplify warming by perhaps 2.5 times. But some sceptics believe the feedback effect could be neutral or even predominantly negative.