What is a carbon budget?

The carbon budget is the amount of greenhouse gases that humanity can emit into the atmosphere by the end of this century and still limit the global temperature increase compared to the pre-industrial levels (1850-1900). According to the IPCC, the atmosphere can absorb, calculated from the beginning of 2020, no more than 400 gigatonnes (Gt) of CO₂ if we are to stay below the 1.5°C thresholds. 

Annual emissions of CO₂ – from burning fossil fuels, industrial processes and land-use change – are estimated to be 42.2 Gt per year, the equivalent of 1,337 tonnes per second. At current emissions rates, the budget for staying below the 2°C thresholds would be exhausted in about 25 years. The concept of the carbon budget is based on an almost linear relationship between cumulative emissions and temperature rise. Nevertheless, this does not mean that the Earth would be 1.5⁰C warmer precisely when the remaining carbon budget for staying below the 1.5⁰C threshold is exhausted.