The cumulative emissions concept is beautiful! Given a goal of not exceeding 2°C, and how much we’ve already emitted, we can calculate the actual amount of CO2 – the ‘carbon budget’ – that we can emit in the future.
Starting today, we can emit no more than about 800 billion tonnes CO2 to have a likely chance to limit total human-induced warming to less than 2°C above the period 1861–1880.
A remaining carbon budget of 800 billion tonnes CO2 sounds big, but with current emissions levels of 40 billion tonnes CO2 per year, this remaining carbon budget will be entirely gone in just 20 years.
The remaining carbon budget is also more than four times larger than the estimated total fossil carbon reserves, with resources much larger. See the figure to the right, and learn.
The implications are clear: time is short and we cannot use all known fossil fuel reserves.
What about the remaining carbon budget for a reasonable chance to stay below 1.5°C? This is, after all, the aspirational ambition of Paris Agreement.
Give or take, the remaining carbon budget for 1.5°C is about 150 billion tonnes CO2. Given the lack of data, one should give this number appropriate uncertainty.
By 2020, we would have emitted enough carbon dioxide to push us over 1.5°C. Because of the inertia in the climate system, 1.5°C may come 10 years later. Whatever, that is just around the corner!
Of course, the real world is not as simple and beautiful as the concept of a carbon budget might imply. There are complications, uncertainties, non-CO2 emissions, and many other quirks. Have a look at these blog posts, where it is explained more thoroughly.
Whether the remaining budget is 700, 800, or 900 billion tonnes CO2 is beside the point. The core message of the ‘carbon budget’ is that emissions need to go to zero at an unprecedented rate.
We have plenty of fossil reserves, but only a limited carbon budget.