Most governments have agreed that it would be a good idea to limit global warming to 2oC. In other words, to make sure that the global mean temperature (because obviously the temperature varies depending on where you are in the world) increases by no more than 2oC.
Is 2oC a lot? It doesn’t sound like very much at all, not when you put it in an everyday context. After all, 0oC is the point at which water freezes, and 100oC is the temperature at which it boils. 2oC is nothing as a step between those extremes. And the difference between the temperature in the day and at night is usually much more than 2oC too; again, it doesn’t seem such a big deal.
But what if you look at it in a global context? If 2oC is really a very small temperature difference, 6oC isn’t very much more than that, is it?
But 4–7oC is how much colder the last ice age was than the global mean temperature in the pre-industrial period. Let’s just get that straight, because it sounds crazy: the global temperature difference between the time before we started burning fossil fuels and the glacial maximum of the ice age that ended about 10,000 years ago (when the world was the most covered in ice) is 4–7oC. To give a bit more context to that, at that glacial maximum, so much of the world’s water was ice that the sea level was 120m lower than today, because there was so much water trappped in the ice covering the land. It seems it doesn’t need to be a lot cooler to be a problem!
And warmer is equally unappealing! In the last interglacial (the warmer period between ice ages), temperatures are estimated to have been about 2–3°C warmer than now – imagine the North Pole being warm enough for alligators to live there, and you’ll get the idea. And what about the sea level then? It was woughly 4–6m higher than now.
So in terms of global mean temperature, 2oC is potentailly a very serious change. It’s hard to believe it could not make a big difference.
We also know from well understood aspects of science that a few degrees of warming at the global scale translates into much greater warming at local scales. Climate models show this behaviour as well and illustrate just how dramatic the consequences could be at any individual location. How to interpret them requires an understanding of the sources of uncertainty and confidence and how to explore them
This knowledge leads us to two key questions about climate change.