Positive feedback loops
Positive feedback loops
Imagine a cold room, with an electric radiator. The radiator has a thermostat that controls its temperature. The colder the room gets, the hotter the radiator gets. As the room gets warmer, the thermostat lowers the heat from the radiator. Now imagine that the thermostat is reversed.
The hotter the room gets, the more the heat is turned up by this 'reverse' thermostat! This leads to the radiator heating up the room even more... which in turn cranks up the thermostat further... ... and so on. This type of self-reinforcing chain of events is called a positive feedback loop.
In the Earth's atmosphere, there are several mechanisms that work similarly to thermostats. For instance, the warmer the Earth's surface becomes, the more heat is radiated into space. Like the thermostat in a radiator, this keeps the temperature of the Earth in balance. Unfortunately, there are other mechanisms that work in the opposite way, as a reverse thermostat. One such mechanism involves snow.
Snow is white, so it reflects sunlight back into space. The more snow there is on Earth, the more solar radiation is reflected. If the Earth gets warmer, more and more snow and ice will melt. As the darker areas of ground and water are exposed, less sunlight will be reflected. This means that the warmer the Earth gets, the more it absorbs the radiation from the sun, instead of sending it away.
It becomes a positive feedback loop, just like the radiator with the reversed thermostat. You can observe another mechanism if you drink a cold soda. 'Carbonated water' is water that has carbon dioxide dissolved in it. When soda is cold, it can hold a lot of carbon dioxide. But as the soda gets warmer - in your stomach for instance - the solubility decreases. So the carbon dioxide bubbles away as a gas.
Oops. There is carbon dioxide dissolved in the sea, too. Like the soda in your stomach, a warmer ocean releases more carbon dioxide. Carbon dioxide released from the sea contributes to warming the Earth, making the water release even more carbon dioxide, and so on... A third mechanism has to do with this gas - methane.
It's a greenhouse gas that has approximately 20 times more effect than the same amount of carbon dioxide. In some places on Earth, such as in northern Russia and Canada, there are large amounts of methane frozen into the ground. It's so cold that the ground is frozen all year round, and the methane has been trapped there for tens of thousands of years. If the temperature increases so much that the ground thaws, some of that methane will be released into the atmosphere. The more methane released, the warmer the temperature gets.
And the warmer the temperature, the more methane is released. A less straight-forward example is water vapor. As the atmosphere gets warmer, it can hold more water. Water vapor is a greenhouse gas, so you'd think that larger amounts should lead to increased temperatures - yet another positive feedback loop... On the other hand, more water vapor in the air means that more clouds will form, and clouds reflect sunlight, just like snow.
So, more water vapor in the atmosphere might have a cooling effect instead. We're not sure which effect is stronger - or if they maybe cancel each other out. These positive feedback loops - melting snow making the ground darker and less reflective... carbon dioxide released from warmer oceans - And methane released from frozen soil... ... function as reverse thermostats.
The warmer it gets, the stronger these warming mechanisms get. While the effect of each one might be small... ... added together, they could potentially lead to the climate "tipping over". In the worst case-scenario, this kind of runaway global warming might continue for hundreds of years. Most scientists think this scenario is unlikely, since the increased radiation into space - the functioning thermostat - will stop the warming eventually.
But by then, large areas of the planet might have become uninhabitable.