Roughly speaking, we are toast if the Earth’s surface temperatures reach something like 3 degrees C above pre-industrial levels.
We have already reached about 1.5 degrees C above pre-industrial, and we will go higher even if we stop adding more greenhouse gasses to the atmosphere, because it takes time for the Earth’s system (the oceans, atmosphere, and ice, mainly) to catch up.
It is generally thought that if we don’t keep about 80% of the known fossil fuel, including coal, oil and other oily substances, and gas, in the ground, then we will go past that 3 degree level.
As noted in a recent panel discussion with the Democratic National Convention platform committee:
These numbers vary, and if you look at the literature on this topic over the last few years, you may become justifiably mystified. One of the problems is that people have been talking about a 2.0 degree limit, but with the assumption that we are now closer to 1.0 degree. But we have been beyond that for a long time, if you measure the surface temperature fairly and accurately. Another factor is some confusion and uncertainty (two different things) about the level of surface warming that will occur with a given increase on greenhouse gasses. But even if all this is straightened out, there is still another source of uncertainty. This is the degree to which Earth systems will helpfully absorb some of this extra carbon, or be altered to release even more, because of feedback effects.
Historically, feedback effects have turned out to be positive more often than negative. Here, the word “positive” is bad, because it means that when you release greenhouse gasses, it warms stuff up, and then that causes some extra greenhouse gasses that were previously stored away somewhere to also be released (such as from warmed up Arctic soils). There is no reason to expect that in the future this trend will reverse, and in fact, there are some systems that are likely to become more of a positive (as in bad) effect than they are now. The degree to which this may occur is not clear.
There are two important things you need to know about the 80% limit and its relation to effects on the planet. First, if we meet that 80% limit, things are still going to continue to warm up and change, and things are going to get pretty bad for some people. The difference between staying just under the limit and going well beyond the limit is the difference between things getting bad and things getting so bad that we can start talking about extinction and the collapse of civilization.
The second thing you need to know is that we need to remain skeptical about this number. Among those in the know, who are not deniers of the science, there are very few if any who think this is too conservative, and a good number who see it as not enough. No matter what, we have to constantly monitor what is happening with the climate as well as our energy industry.
This is doable.