If the world is to stay below 2C of warming, which scientists regard as the limit of safety, then emissions must be held to no more than 450 parts per million (ppm) of carbon dioxide in the atmosphere; the level is currently around 390ppm.
We see this point being made all the time. But I\’m not actually convinced that it\’s the scientists saying it.
The point is that how much the temperature rises depends upon the value of climate sensitivity.
That is, how much does the temperature go up with a doubling of CO2? Pre-industrial levels were, can\’t remember, 260 ppm? 280 ppm?
So, 520 ppm or 560 ppm is a doubling.
And climate sensitivity is estimated to be somewhere from 1.5-4.5 oC I think? With 3 oC being the most likely value (William, help out here).
And here\’s the bit I don\’t understand. That 2 0C \”limit of safety\”: that\’s usually taken to mean, is often used by campaigners to mean that, passing that means we get \”runaway climate change\”. The feedbacks all swing into action and thus once we\’ve passed the point it all just gets catastrophically worse.
But the whole point about this climate sensitivity number is that it\’s already including all of these feedbacks. That\’s why it\’s so imprecise, because we don\’t actually know how all the feedbacks interact. Whatever the number is, say it\’s 3 oC, then that\’s how far the temperature goes then stops: because that\’s how all of the various feedbacks interact and play out from a 520 ppm CO2 level. Over quite some period of time too: not a few years, not even a few decades.
So I don\’t understand the 2 oC point.
OK, rather, I do understand it if it is being used to mean that that\’s the sort of limit before changes that we really won\’t like very much. But I don\’t understand it as it\’s often used, as some trigger point after which we get a cascade of ever more horrible things. Because our climate sensitivity number is already including the totality of the feedbacks.
Anyone care to straighten me out on this?