Positive feedbacks
One example of a positive feedback is how warmer global temperatures could lead to naturally stored methane (a powerful greenhouse gas) being released into the atmosphere, where it would then increase global warming even further. Methane is already emitted from, for example, the world's rice paddies and waste dumps. But if higher temperatures were to melt the earth's permafrost (which traps naturally stored methane below ground) much more methane would be released into the atmosphere relatively quickly, adding considerably to the greenhouse effect, and speeding global warming.
Another example of a positive feedback is how, in warmer global conditions, large areas of snow and ice-cover melt away for longer periods (or even permanently). This reduces the reflective white surface area of the earth and exposes more dark, non-reflective land surface which absorbs more heat. This heat absorption, in turn, raises temperatures further.
Negative feedbacks
In contrast, higher average temperatures also increase the growth rate of plants and trees. This means that they naturally increase their intake of carbon dioxide and so act as temporary "sinks" (or stores) for carbon which would otherwise have been released into the atmosphere sooner, as part of the carbon cycle. The role of trees and vegetation as carbon "sinks" is one of the reasons why deforestation (reducing the number of trees) is considered so damaging.
The problem of uncertainty
The balance between positive and negative feedbacks in relation to global warming is a major cause of uncertainty in climate predictions. But uncertainty about the specific effects of global warming does not imply uncertainty about global warming itself.
