Carbon Budget Concept

The relationship between cumulative CO2 emissions and global warming is nearly linear. This means there is a fixed amount of CO2 humanity can emit before crossing any given temperature threshold. That fixed amount is the carbon budget, and it is shrinking every year.

The carbon budget for a temperature target (say, 1.5°C above pre-industrial levels) is the total cumulative CO2 emissions from today forward that would keep warming below that target with a given probability. The IPCC estimates the remaining budget for 1.5°C (at 50% probability) was roughly 500 gigatonnes of CO2 as of 2020. At current emissions of about 40 Gt CO2/year, that budget runs out in about 12 years. For 2°C, the budget is roughly 1,150 Gt CO2, or about 29 years at current rates.

The linearity is the key insight: it does not matter much when emissions occur, only how much total. A decade of high emissions followed by rapid cuts produces roughly the same warming as moderate emissions spread over a longer period, if cumulative totals are equal.

Worked Example

The remaining carbon budget for 1.5°C is 500 Gt CO2. Current annual emissions are 40 Gt CO2/year.

• Calculate time at current rate. 500 / 40 = 12.5 years.

If emissions decline linearly to zero, how many years does the budget last?

With linear decline, average emissions are 40/2 = 20 Gt/year. Time = 500/20 = 25 years. A steady decline doubles the timeline compared to flat emissions, but the total emitted is still 500 Gt.

The carbon budget converts a diffuse climate problem into a concrete accounting constraint: every tonne emitted today is a tonne subtracted from the finite total.

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