An Update on Carbon Budgets
We’re not doing what we need to be doing
I wrote about carbon budgets awhile back. That post had the distinction of being my least-read and least-clapped-for post. OK, I get it … not the most scintillating topic in the wide world of climate change. But I submit it may be the most important topic, because it describes how scientists track two crucial issues: (1) how much more CO2 and other GHGs can we afford to release into the atmosphere? and (2) how much hotter can we expect the planet to get once our carbon budget is exhausted?
Under the auspices of the IPCC, climate scientists have found that tracking carbon budgets is a good way to capture the relationships between fossil fuel use, CO2 emissions, and global temperatures. With a quantification of those interactions in hand, scientists can predict how hot it is going to get, given how much CO2 is in the atmosphere, which in turn depends on how much additional oil, gas, and coal we will burn before we stop.
The latest update of carbon budgets was completed this year (source). As can be seen in the table below, remaining carbon budgets for avoiding specific average temperature increases (here, 1.5°C, 1.7°C, and 2.0°C) have shrunk considerably between 2020 and 2023, thanks to our continued failure to curb emissions following the slight dip we experienced in 2020 (source). As of 2023, these updates show that achieving a 2-in-3 (67%) chance of holding global warming to 1.5C will require limiting our additional CO2 emissions, starting in 2024, to 150 gigatons in total.
Given that the world emitted about 37 gigatons of CO2 in 2022 (source), we can start envisioning some timelines. In a separate analysis of the Forster updates, Hausfather calculates how many years it will take to reduce CO2 emissions to zero, assuming we start in 2024 and decrease emissions linearly every year until we reach zero, all the while emitting no more CO2 than the budget allows. Here are some results:
What this chart shows is that a 66% chance to keep global warming below 1.5°C will require reducing CO2 emissions to zero by 2030. From our starting point of 37 gigatons in 2022, that translates into a reduction of nearly 15% per year. As noted in my previous post on carbon budgets, the greatest year-to-year reduction in CO2 emissions we’ve seen so far is the 4.9% decline in 2020, the year of the COVID lockdown. What climate scientists are telling us here is that we will need to triple that rate of reduction, every year, over seven consecutive years, to achieve a 66% chance to keep warming below 1.5°C. There is currently no realistic scenario in which that level of reduction is feasible, given both past history and in-place national commitments.
If we are willing to accept a 50% chance to keep warming below 1.5°C, our zero-emissions deadline extends to 2035. For a 66% chance to keep warming below a more damaging 2.0°C, our deadline is 2069. And for an even (50%) chance to keep warming below 2.0°C, the zero-emissions deadline is 2079.
It’s worth noting that a 50% chance to keep warming below a certain threshold is also a 50% chance to end up with warming exceeding that threshold. So ideally, we should seek to pursue our zero-emissions goal with a probability greater than 50%. But as the table from Forster et al. shows, if we want an 83% chance of keeping warming below 1.5°C, we can only burn 100 additional gigatons of CO2, a threshold will will surpass in 3–4 years. We could do it, but all indications so far say we won’t.
It’s time to start thinking seriously about Plan B.
