What is the most agreed-on figure for our future carbon budget?

My answer:

Asking “what is our future carbon budget?” is a bit like asking how many calories a day you can eat. There’s really no limit on how much you can eat if you don’t care how overweight and unhealthy you become. So, to set a carbon budget, you need to say how much global warming you will accept.

That said, here’s a picture of how we’re burning through our carbon budget:

It says that our civilization has burnt 60% of the carbon we’re allowed to while still having a 50-50 chance of keeping global warming below 2 °C.

This chart appears in the International Energy Agency report World Energy Outlook Special Report 2015, which is free and definitely worth reading.

The orange bars show CO₂ emissions per year, in gigatonnes. The blue curve shows the fraction of the total carbon budget we have left to burn, based on data from the Intergovernmental Panel for Climate Change. The projection of future carbon emissions is based on the Intended Nationally Determined Contributions (INDC) that governments are currently submitting to the United Nations. So, based on what governments had offered to do by June 2015, we may burn through this carbon budget in 2040.

Our civilization’s total carbon budget for staying below 2 °C was about 1 trillion tonnes. We have now burnt almost 60% of that. You can watch the amount rise as we speak:

• Trillionth Tonne.

Quoting the International Energy Agency report:

The transition away from fossil fuels is gradual in the INDC Scenario, with the share of fossil fuels in the world’s primary energy mix declining from more than 80% today to around three-quarters in 2030 […] The projected path for energy-related emissions in the INDC Scenario means that, based on IPCC estimates, the world’s remaining carbon budget consistent with a 50% chance of keeping a temperature increase of below 2 °C would be exhausted around 2040, adding a grace period of only around eight months, compared to the date at which the budget would be exhausted in the absence of INDCs (Figure 2.3). This date is already within the lifetime of many existing energy sector assets: fossil-fuelled power plants often operate for 30-40 years or more, while existing fossil-fuel resources could, if all developed, sustain production levels far beyond 2040. If energy sector investors believed that not only new investments but also existing fossil-fuel operations would be halted at that critical point, this would have a profound effect on investment even today.

Since we seem likely to go above 2 °C warming over pre-industrial levels, it would be nice to make a similar chart for a carbon budget based on 3 ° C warming. The Trillionth Tonne website projects that with current trends we’ll burn 1.5 trillion tonnes, for a warming of 3 °C in a cautious scenario, by 2056.

But: we would never burn the 1.5 trillionth tonne if emissions dropped by 1.2% per year from now on. And we’d not even burn the trillionth tonne if they dropped by 2.6% per year.