Science has a special term for what is popularly known as a vicious circle: “positive feedback.” Positive feedback describes how a phenomenon speeds up or slows down once it gets under way. In the case of global warming, at least three such patterns have been identified. The first is the melting of the Arctic. Ice reflects more of the sun’s rays than any other surface. Water, on the other hand, absorbs more rays than any other. When ice floes melt, an area that previously reflected the sun’s heat back into the atmosphere begins instead to accumulate it. The more the ice melts, the more the water heats up and causes remaining ice to melt. This explains in part why the temperature rise recorded for the Arctic region over the course of a century is approximately twice the global average: 3.6-5.4°F (2-3°C).
Another “positive feedback” cycle affects the permafrost, the permanently frozen ground of the Earth’s far north. When permafrost melts, it releases methane, a greenhouse gas 21 times more powerful than CO2. This effect could be accelerated if the methane trapped at the bottom of the oceans came to be released in the same way.
A third example relates to the behavior of the world’s forests and oceans. Normally, they store billions of tons of CO2 annually, in the soil, in the living trees, in the water, and in marine micro-organisms. They are known for this reason as “carbon sinks.” But global warming interferes with the way they function, so that they could end up absorbing much smaller quantities of greenhouse gas, or even releasing it, which would be catastrophic.
The precise mechanisms of these three effects are still the subject of debate, but one thing is sure: beyond a certain limit, the situation will run away with itself. This is why experts have set themselves the target of keeping global warming to within a fixed limit. This is calculated in terms of the quantity of CO2 in the atmosphere and is set at 400 parts per million. The figure stood at 280 before the industrial age, and at 370 in 2000, and it could exceed 450 by the year 2020. A number of organizations have fixed this threshold (or even a threshold of 350 ppm) as a target figure not to be exceeded (or as yet to be achieved) and as a challenge for international campaigns.