So, what happens if we go above the two-degree limit?
If the average global temperature rises above two degrees Celsius, we risk initiating what are called feedback loops. A positive feedback loop is a dynamic system in which a change in one direction causes further change in the same direction. This is already happening in the case of melting sea ice. Ice has a reflective white surface that bounces about 90% of incoming sunlight back into space, preventing build up of solar energy in the planetary system. But sea ice melts as the Earth warms, exposing more open seawater, which is dark in color and absorbs sunlight instead of reflecting it. This causes more global warming, which causes more ice to melt, which exposes yet more open water, which causes more warming, etc. The hotter it is, the hotter it gets. The ice-melt feedback loop is already in effect and we have lost about half of the Arctic ice cap so far. We are likely to lose it all some summer in the next thirty years or so.
Somewhere above two degrees the permafrost feedback loop begins to kick in. Methane (natural gas) is an especially bad greenhouse gas. It does not stay in the air as long as carbon dioxide, but while it is there it traps twenty times as much solar radiation. Billions of tons of methane are frozen in the permafrost soils of Siberia and northern Canada. As the planet warms, these soils begin to thaw, releasing huge quantities of methane into the air. This results in more global warming, which leads to more thawing, which leads to yet more warming, etc. The hotter it is the hotter it gets. Once we enter this feedback loop the climate will continue to heat on its own even if we stop burning fossil fuels altogether. This is what is meant by a runaway climate.
And that’s not all. The carbon cycle feedback loop can be either positive or negative. Enormous quantities of carbon are tied up in trees, grasses, algae and other plant life, but the overall amount varies. As biomass increases, more carbon is taken out of the atmosphere, but as biomass decreases, carbon is released into the air. Initially, a negative feedback loop can set in: as atmospheric carbon levels increase, plants photosynthesize at a higher rate, sucking carbon out of the air and cooling the Earth. It would be nice if we could depend on this to protect us from a runaway warming, but at the very time that we could benefit from it, we are cutting down tropical forests at an alarming rate. Also, as temperature ranges change and rainfall patterns shift, vegetation tends to die back, releasing stored up carbon back to the air and creating a positive feedback loop that heats the Earth. El Ninos, for instance, have twice in the last ten years shifted rainfall away from the Amazon, resulting in massive droughts that have killed billions of trees. The Amazon forest, the largest carbon sink on the planet, became a net producer of atmospheric carbon.
And then there’s the gas hydrate feedback loop. But it’s way too scary to get into right now. It may have had a lot to do with the greatest mass extinction of all time (the one that killed 90% of all species 250 million years ago) but it would take a long time to set in, so let’s not talk about that one just yet.
Let’s just avoid the two-degree limit, for now. At this rate we’re heading for it sometime in the 2030s.