There is an easy-to-follow video published by Climate Interactive titled How Could Paris Climate Talks Ratchet Up To Success? It runs quickly through a series a of what-if scenarios to calculate what the effects of various emissions promises could have on global temperatures to 2100. It starts with the Intended Nationally Determined Contributions (INDCs) pledged for the Paris COP, which in many cases only have targets up to 2030, and  extends them using various assumption out to 2100.

The numbers are based on the C-ROADS model, described here.

The presenter, Andrew Jones, shows how we can go from business-as-usual emissions to get to a fifty-fifty chance of staying below two degrees Celsius.  There are a lot of “ifs” along the way, I counted seven. You could easily count more. I assume that the expected warming for various scenarios is the median value, 50/50, derived from climate models.

If countries achieve the targets in their INDCs and if, in the cases in which the INDCs stop at 2030 (mostly developed countries), the emissions do not rise again to the end of the century, then the expected warming drops from 4.5 to 3.5 degrees C.

If developed countries continue the reductions achieved from now until 2030 at the same rate until 2100, then the expected warming drops from 3.5 to 3.2 degrees.

If China includes other greenhouse gases in its pledge and also reduces its CO2 emissions by up to 2% after 2030, then the expected warming drops from 3.2 to 3.0 degrees.

If developing countries reach an emissions peak in 2035—which will require technology transfer and financing from rich countries—then the expected warming drops from 3.0 to 2.6 degrees.

If developed countries were able to peak their emissions in 2020-2025 and all developing nations do the same by 2030-2035 and if all countries were able to reduce their emissions by 3.5-4.0% per year, then the expected warming drops from 2.6 to 2.0 degrees.

So what are our chances?

It’s really anyone’s guess what the chances of all of these “ifs” coming true might be.  Just for the sake of argument, let’s assume that they all have the same probability of happening, provided the previous “ifs” became true. Let’s assume also an optimistic view that the probabilities of every “if” are all 90% and a pessimistic view that each step has a two-thirds (66.67%) chance.

In the optimistic case that would be 0.9 to the power of seven, around 48%. That’s the chance of us getting to the level of cumulative emissions where uncertainties over climate sensitivities and carbon cycle feed backs give us a 50% chance of staying below 2 degrees. In other words, the chance of everything going right and the physics of the climate system cooperating to allow us to avoid dangerous global warming is about 24% .

In the pessimistic case,  the unforgiving probability calculation gives us a 6% of achieving the necessary cumulative emissions and a 3% chance of staying under 2 degrees.

(If every “if” were a coin toss, 50/50, the overall chances of success would be less than 1%.)

It’s not particularly encouraging to acknowledge that, even thinking optimistically, we only stand a one-in-four chance of staying under two degrees of warming. Viewed pessimistically, the target has most probably already slipped away. But this is all largely guesswork.

Everybody will quibble with my characterizations of “optimistic” and “pessimistic”.  Some people will think that my pessimistic probabilities are actually optimistic and that my optimistic numbers are unrealistically high. Some might argue that  the “ifs” are not independent and that early success will raise the chances of subsequent stages becoming true as we learn from success and become encouraged to do even more.

There’s always a chance that some technological breakthough will make all of this much easier. There’s also the chance that there could be setbacks—more likely geopolitical than technological—that will make success impossible

Certainly, there’s no reason to make the “spherical cow” assumption that every “if” has the same probability, except for computational expediency. It wouldn’t be difficult to go through the list of “ifs”, assign each one a unique value and then multiply them together.

Some commentators are fond of saying that we can reach the two degree emissions target provided that we can muster enough political will. That’s true, but what the video shows is that the political will has to be maintained over the next 85 years and throughout the world economy.

If the Paris talks are a success, then we can take a day or two to celebrate having taken an important stride in the right direction. But as the Climate Interactive video shows, it will be just the first step on a long, hard journey. There will be much more left to do.

Update, I’m reminded by reader Seth D. Michaels on Twitter that this is not a succeed/fail situation, in which everything is lost if we don’t quite make the target. Neither are the steps along the way fail/pass steps, that’s just a simplifying assumption I made to make the calculations easy. Any progress on reducing emissions is better than none.