If you look at the proposals and pledges bandied about in the climate policy world, they mostly focus on getting to a particular emission target in a given year. This would be reasonable if the greenhouse gas problem were like air pollution from particulates like soot, where a certain amount of the stuff is spewed into the air, people breathe it and get exposed to higher risks of lung disease, and then the pollutant is dispersed or precipitated away.
But it’s not. CO2 and other greenhouse gases accumulate in the atmosphere, and it is this accumulation that determines how much climate change we are imposing on ourselves. Graphically the problem looks like this.
But GHG’s accumulate, so it isn’t the endpoint that matters, but how much has been cumulatively emitted along the way. In other words, it’s the area under the curves that tell you how much climate change you’re going to have, and the blue shading shows the difference between the two. No, the two paths, identical in terms of emission goals by some target date, are not the same.
To carry the logic one step further, suppose you have agreed to be on one of these paths and then, part of the way along it, you overstep the amount you were suppose to emit. According to the mindset focused on annual emission goals, you should try to get back to the path as soon as possible. But if the commitment is to a given accumulation of GHG, emitting too much in one year means you have to deduct that overage from your original target for the next year. Of course, in the real world, we have been dawdling year after year for decades, so our plans have to be much more stringent than if we had taken action instead.
This is why the IPCC, in its latest assessment report, called for a fixed carbon budget, 1000 billion tonnes—which we are more than halfway to filling. Annual emissions have no meaning in themselves; they matter only insofar as they add to the overall accumulation.
If you want to get real about climate change, drop the yearly emission target language and start talking about carbon budgets.