The Carbon Budget Has Almost Run Out — Here Is What That Actually Means

At current emission rates, the remaining carbon budget for 1.5°C will be exhausted in 5-7 years. Here is what that means in practice and whether it's actually too late.

The concept of a 'carbon budget' — the cumulative amount of CO2 that can be emitted globally while keeping warming within a specific temperature target — is one of climate science's most important and most misunderstood quantitative frameworks. The IPCC's Sixth Assessment Report (2021) estimated the remaining carbon budget for a 50 percent probability of staying below 1.5°C of warming at approximately 500 gigatonnes of CO2 as of January 2020. At approximately 40 gigatonnes of CO2 emitted globally per year, this budget has been consuming at a rate that projects exhaustion in the mid-2030s.

The updated estimates from 2023-2024 are significantly more alarming: revised calculations accounting for additional warming from short-lived climate pollutants (methane, black carbon), updated assessments of the carbon cycle's sensitivity, and the additional emissions since 2020 have reduced the remaining 1.5°C budget to approximately 150-250 gigatonnes — approximately 4-6 years of current global emissions. This means that without dramatic immediate emissions reductions, the 1.5°C target that Paris Agreement signatories committed to is effectively unachievable.

What 'unachievable 1.5°C' means and what it doesn't mean: it doesn't mean that climate action is futile or that we have crossed a catastrophic threshold. The difference between 1.5°C, 2°C, and 2.5°C of warming is the difference between severe and manageable impacts in specific systems (coral reefs, Arctic sea ice, extreme weather frequency), not the difference between acceptable and apocalyptic. Every fraction of a degree matters, and every fraction of a degree below the baseline scenario that action avoids represents billions of people and trillions in avoided damage.

What it does mean: the policy framing that positions the climate goal as preventing 1.5°C of warming needs to transition toward accepting that some warming above 1.5°C is now highly probable and that adaptation — preparing for and managing the consequences of that warming — must be as central to climate strategy as mitigation. This is a difficult political transition because adaptation concedes that some harm will occur, which is easier to acknowledge honestly than to explain to electorates.