The Problem with '12 Years'

It’s been a long year.

It’s almost exactly a year since the Intergovernmental Panel on Climate Change came out with a major report describing what 1.5°C of warming will look like (I know, it feels like an eternity ago). Today we’re at about 1 degree of warming compared to pre-industrial times, so we’re already two-thirds of the way to this threshold.

This report catapulted the idea of carbon budgets into the zeitgeist. A carbon budget tells us how much carbon dioxide we can continue to emit and still have a reasonable chance of remaining below some particular threshold of warming.

Think of a carbon budget as the amount of money that remains in our carbon bank account, and our annual carbon emissions as our annual spending. As the IPCC famously warned us a year ago, at our current rate of emissions, we’ll burn through our budget for 1.5°C of warming in about 12 years.

Actually, the report didn’t say “12 years”. They leave it to you to do the math. The IPCC came up with carbon budgets for a reasonable shot at staying below 1.5°C of warming. If you divide these budgets by our annual emissions, you’ll find that at our current rate we’ll blow through them in a little over a decade. The report concludes that unless “global CO₂ emissions start to decline well before 2030”, we will surpass the 1.5°C threshold.

So that’s where ‘12 years’ comes from. Some people believe that this a simple and effective slogan that captures the urgency of our climate crisis, and has helped galvanize people into action. Others feel that it’s overly simplistic at best, and misleading at worst, because it singles out a specific number as an all-or-nothing target, whereas in reality the effects of climate change are a continuum. As climate scientist Kate Marvel put it, “Climate change isn’t a cliff we fall off, but a slope we slide down”.

But that’s not what I want to discuss here, there are already many good pieces on this debate. Instead, I want to know where carbon budgets come from in the first place. What’s the justification for these numbers, and how accurate are they?

So in this post, I’m going to take you through how you can look at the data for yourself and come up with a rough estimate of the carbon budget.

Let’s start ...