The temperature of the Earth’s surface has varied dramatically during the Earth’s past. Some 600 Million years ago, the Earth was so cold that the oceans were frozen and the Earth must have looked like a gigantic snowball when seen from outer space. Therefore, such a state is called the Snowball Earth. Snowball Earth is the most dramatic example of alternative stable states in the climate system which can lead to tipping points. It is also (arguably) the most plausible representation of a climate tipping point because reconstructions of past climates and complex climate models show it.
Alternative stable states usually require the existence of a very strong positive feedback. The Snowball Earth bistability is caused by the ice-albedo feedback. On the current Earth, 30% of the incoming sunlight is reflected to space, mainly by clouds, the air, and ice and snow on the ground. In a snowball state, the Earth is much brighter due to the large ice cover, and much more sunlight is reflected. This tends to keep the Earth cool.
As a consequence, if the temperature of the Earth was decreased sufficiently (for example due to low CO2 or a decrease in the Sun’s power), at some point ice growth and cooling would amplify each other so much that the Earth would glaciate rapidly and completely. In complex models, this usually happens if the sea ice edge (which is currently in high latitudes) enters the tropics.
The App Snowball 2 shows what the solutions look like for the temperature of the Earth, using a simple global mean energy balance model.
In the App, use the red handle to alter the energy balance of the Earth (corresponding to changing CO2 or solar insolation). This changes the global mean temperature (shown as a green dot), which can be in either of two stable states: The Snowball state (lower branch), and a warm state that is similar to our present climate (upper branch). When the temperature is varied beyond the point where a stable state disappears, the planet will make a rapid transition towards the remaining equilibrium. The transitions between these states (the freezing and melting of the Earth) happen at the tipping points.
Once such a tipping point has been crossed, the previous climate cannot easily be restored. For example, once in a Snowball state the Earth’s reflectivity will be so large that an insanely high concentration of atmospheric CO2 is needed to melt it again. Thereafter, the lack of ice and the high CO2 concentration would immediately kick the Earth into an extremely warm state.
For a long time scientists did not believe that this Snowball Earth really existed because they did not see how it could ever escape this state. A Snowball Earth was only a theoretical possibility coming out of simple climate models. In the 1990ies, when evidence was found that the Earth has indeed been in a Snowball state 600 million years ago, scientists started to investigate the reasons that caused the Earth to freeze and melt again. The details are still not fully known but it is clear that volcanic outgassing of CO2 over millions of years was very important to finally escape the Snowball Earth state.