An Inconvenient Truth (Original)

During the past two decades, the IPCC has tended to regard the Earth's climate as something that will change gradually and smoothly, as carbon dioxide and global temperatures continue their lock-step rise. But there is a growing consensus among many climate scientists that this may be giving a false sense of security. They fear that feedback reactions may begin to kick in and suddenly tip the climate beyond a critical threshold from which it cannot easily recover.

Climate feedbacks could turn the Earth into a very different planet over a dramatically short period of time. It has happened in the past, scientists say, and it could easily happen in the future given the unprecedented scale of the environmental changes caused by man.

There are two types of feedback that can play a role in the future direction of the Earth's climate. The first is a "negative" feedback, which is largely good for us, because it works against things getting worse. The classic example of a negative feedback is the fertilising effect of carbon dioxide. As concentrations rise, then so does the amount of carbon absorbed by the higher growth rate of plants. The result is a negative feedback that tends to check rising levels of carbon dioxide.

A "positive" feedback makes things worse by adding to the existing problem. It brings about a vicious circle, in which a rise in carbon dioxide or global temperatures causes some change in the climate system which, in turn, leads to further rises in carbon dioxide or temperatures.