Many of us in Australia have a rather comfortable life. If it gets a bit hot, we can set the thermostat to cooler and let the air conditioner do its job. If it is a cold winter’s day in Canberra, on go the heaters and the house warms up to its automatically controlled temperature. We can change the comfort level as we choose.

The Earth also has a thermostat, and until recently the Earth was in charge of its own temperature setting. But much to the surprise of many scientists, and the disbelief of almost everyone else, over the past two centuries some of that control has been taken over by humans. At first we had no idea that was what we were doing, but gradually as we learned more about the way the Earth’s thermostat works, it has become increasingly clear that we have the power to set the Earth’s temperature.