In 2015, the world gathered in Paris and decided to limit global warming to below 2°C, and aim for 1.5°C. We know how much of the warming we have experienced so far is due to our fossil emissions. This leads to the question: How much more can we emit before the planet warms more than 1.5°C? What is our remaining carbon budget?
A carbon budget is a fairly simple concept. You calculate how much you can emit before the global temperature goes beyond whichever temperature target you have set for yourself. Then you subtract what we've emitted so far since the industrial revolution. What you have left is your budget.
The carbon budget is inextricably connected to what we call cumulative emissions. Usually we talk of annual emissions. Cumulative emissions are total emissions since we started emitting CO2 on a large scale - usually defined as sometime between mid-1700s to mid-1800s. Climate scientists find these cumulative emissions interesting as they have a rather direct link to the temperature increase.
Basically, they're annual emissions stacked on top of each other. Remember Tetris?
Sort of similar.
So, we have a good idea how much we've emitted so far and how much the temperature has changed. But estimates on how much we can emit in the future differ quite a lot.
In a recent commentary in Nature Geoscience (open access), CICERO-researcher Glen Peters argues that trying to determine the unique carbon budget for 1.5°C is a futile exercise, as uncertainties out of our control will dominate. According to Peters, the main problem is that there is no one budget telling you how much you can emit before we pass for example 1.5°C warming. There are many of them!
To say something about the future, researchers run models. These models generate pathways showing an estimate on how global emissions have to evolve over the course of the century for us to keep global warming below 1.5°C.
The problem is that there are many models and many pathways. This complicates things. Have a look at the pathways below.
All scenarios assume zero emissions around mid-century, and they all assume a lot of negative emissions from then till 2100. The steeper the curve, the more negative emissions they assume
So where do you read off the remaining budget? Is it when you pass the point you eventually have to come back to (1) ? Is it at the peak of one of these paths? (2)
Or is it at the end of the line? (3)
Go ahead. Pick your magic number.
According to Glen Peters, policy makers should worry less about how much we can emit, and still stay below 1.5°C warming. Instead, they should be concerned about how to get annual global emissions down from where they are today ...