Calculating the Global Carbon Budget: How the 1.5°C Threshold Closes Before 2030

National governments and corporate coalitions frequently assert that achieving net-zero emissions by 2050 will secure the Paris Agreement's goal of limiting global warming to 1.5°C. The geophysical evidence dictates otherwise, demonstrating that the physical allowance for that temperature threshold will be exhausted nearly two decades before those political deadlines arrive.[8]

"From a physical science perspective, limiting human-induced global warming to a specific level requires limiting cumulative CO2 emissions, reaching at least net zero CO2 emissions," states the Intergovernmental Panel on Climate Change (IPCC) in its Summary for Policymakers. The panel established that as of January 2020, the world had a remaining carbon budget of exactly 500 billion tonnes (GtCO2) to maintain a 50 percent chance of halting warming at 1.5°C.[6]

According to the European Commission's Global Carbon Budget 2023 report, global fossil CO2 emissions reached 36.8 GtCO2 in 2023. When land-use changes are included in the calculation, the total annual emission rate climbs to 40.9 GtCO2.[4][5]

Between the IPCC's January 2020 baseline and the end of 2023, the global economy emitted approximately 160 GtCO2. Subtracting that consumed volume from the initial 500 GtCO2 leaves a remaining physical allowance of roughly 340 GtCO2 entering 2024.[8]

Dividing that remaining 340 GtCO2 by the current annual emission rate of 40.9 GtCO2 yields a depletion window of just over eight years. If global emissions remain flat, the entire carbon budget for a 50 percent chance of 1.5°C will be exhausted in 2032.[2][8]

The mathematics shift significantly for the 2.0°C threshold. The IPCC calculates the remaining budget for a 67 percent chance of staying below 2.0°C at 1,150 GtCO2 from the 2020 baseline.[1][6]

Deducting the 160 GtCO2 emitted through 2023 leaves 990 GtCO2. At the current rate of 40.9 GtCO2 per year, that budget extends to 2048—aligning much more closely with the 2050 net-zero targets established by the European Union, the United States, and other major economies.[8]

These calculations are highly sensitive to how historical temperatures are measured. Research published in the National Center for Biotechnology Information demonstrates that defining the pre-industrial baseline as 1850–1900, rather than an earlier period, alters the remaining budget by shifting the starting point of observed warming.[3]

The carbon budget is also not exclusively about carbon dioxide. The IPCC's 500 GtCO2 figure assumes specific, rapid reductions in non-CO2 greenhouse gases, primarily methane and nitrous oxide. If those secondary emissions are not curtailed at the rates modeled by the panel, the CO2 budget shrinks further.[1]

Because the 1.5°C budget will likely be exhausted by the early 2030s, climate models increasingly rely on "overshoot" pathways. In these scenarios, global temperatures breach the 1.5°C threshold before being pulled back down later in the century.[7]

Reversing an overshoot requires Carbon Dioxide Removal (CDR) technologies to extract billions of tonnes of CO2 from the atmosphere annually. Chapter 3 of the IPCC's Special Report notes that deploying CDR at this scale is unproven and carries massive land, water, and energy requirements.[7]

The gap between the geophysical 2032 deadline for 1.5°C and the political 2050 net-zero deadline creates a structural deficit in climate policy. Governments are legislating for a target date that applies to 2.0°C while claiming it secures 1.5°C.[8]

To stretch the 1.5°C budget to 2050 without relying on unproven atmospheric removal, the global annual emission rate would need to drop by roughly 1.5 billion tonnes every year, starting immediately. Until that annual reduction materializes in the data, the physical budget will continue to close exactly on schedule.[8]