So obviously, what counts as not dangerous versus dangerous, is not a hard scientific question, it’s a political question, and this was a political decision to take this 2C number, mainly made by European climate negotiators well over 10 years ago, and it’s just sort of stuck since then. All the countries involved in climate negotiations have basically signed on saying “Yes, this is what we want to avoid, 2ºC temperature rise.”
The bad news on this 2C number is two-fold. First of all, all the latest science done in the last 10 to 15 years has pointed to the conclusion that those impacts we thought were going to happen around 2ºC are in fact going to happen much earlier than that, the climate is more sensitive to these added greenhouse gases than we thought.
So, if those were the impacts we were worried about, then the real threshold of safety ought to be something like 1.5ºC. James Hansen is the climate scientist most famously known for raising these warnings, but it’s a growing scientific consensus that 2º is, in fact, dangerously high, which is bad, because we are almost certainly going to blow past 2ºC.
There’s some reason to believe, a recent study said that even if we stopped our carbon emissions tomorrow, we’re still going to get more than 3º this century just from momentum from the previous emissions. But stopping at 2º now would take a level of global coordination and ambition that is nowhere in evidence.
So, a lot of climate scientists don’t really want to tell you this because they don’t want to depress you, but I am just a blogger, so I am happy to depress you: 2ºC is probably off the table. So, then the question becomes “Well, what would it look like if temperature goes higher than that? What would, say, 4ºC look like?”
Oddly, there hadn’t really been a lot of concerted scientific attention to that question because climate scientists honestly thought we wouldn’t do that to ourselves, but we are doing it to ourselves. So, in 2009, several climate change research groups in England drew together a group of scientists, commissioned some papers and had them really take a hard look for the first time. What would 4ºC look like? There are a lot of papers, a lot of equations, a lot of talk and complexity I have hopefully paraphrased it here for you, to make it easier to grasp.
4ºC temperature rise would look ugly. Among other things, that would be the hottest the Earth has been in 30 million years. Sea levels would rise at least 3 to 6 feet, and this excludes some really tail end possibilities, but 3 to 6 feet at least. And persistent drought would cover about 40% of the currently occupied land on Earth, which would wreak havoc on agriculture in East Asia, Africa, South America, Western US. Well this combined will produce hundreds of millions of people who have been driven from their homes either by their cities being swamped by sea-level rise or by hunger or by all the attended ills that come along with those things.
And, to boot, probably somewhere around half of the known species on Earth would go extinct. This question of pinning down the exact number of species is very difficult, this is very much an approximation, but some substantial chunk of life on Earth would be wiped out.
The final bit of bad news — that’s not true, there’s more bad news to come — a middle bit of bad news is that, according to a recent paper by the International Energy Agency, we are currently on track — if we keep doing what we are now doing, if we go on with business as usual, as it’s called — we are now on track for 6ºC temperature rise this century; something, 5 to 7, these are obviously estimations.
So, if 4º is hell on Earth, I’ll let your imaginations fill in the blanks on 6º but, one danger that comes up when we contemplate going this high with our temperature is the possibility that climate change will become irreversible. I think when people typically think about climate change, they think, “Oh, temperature is going to rise X amount, circumstances will change, some places will get warmer, some places will get wetter, we’ll adjust, we’ll move our farms around, people will migrate from one city to another, we’ll get resettled and we’ll go on with life”.
The really dangerous possibility is that what are called — the Earth has several of what are called positive feedback systems, so, for instance, in Siberia there is this permanent ice, the permafrost and it contains a bunch of methane in it. As it melts, it releases that methane, the methane causes more warming, which melts more ice, which releases more methane, it’s a self-sustaining process; or sea ice melts, ice is white, it reflects energy, when it melts becomes dark blue and absorbs more energy, which heats the oceans, which melts more ice, which creates more dark surfaces.
You see, there’s a number of these systems that are self-perpetuating, and the danger, the great danger of climate change, that towers above all these other more specific dangers, is that these positive feedback systems will take on a momentum of their own that becomes unstoppable, and human beings will lose any ability to control it at all, even if we’d stop all our climate emissions on a dime.
Will that happen at 2º? Probably not though there is a real chance of it and there is a lot of debate about that. Will it happen at 4º? Well, it looks a lot more likely at 4º. Will it happen at 6º? Almost certainly. So, if we continue on our present course, climate change will probably take on a life of its own, spiral out of control and, according to a recent paper, by 2300, we could see temperature rise of up to 12ºC.