Human emissions of greenhouse gases are causing global warming. There is a strong relationship between the total amount of CO2 emitted to the atmosphere before achieving net zero emissions, and the total amount of warming achieved. A carbon budget is an amount of CO2 that can be emitted, while still limiting the warming to a specific number of degrees. To make sure a carbon budget does not overrun, net zero emissions must be achieved before the budget runs out.
The Paris Agreement state that the global temperature increase should be kept well below 2°C above pre-industrial levels and that efforts should be pursued to limit the temperature increase to 1.5°C above pre-industrial levels. The SR15 report shows clearly that the risks with 1.5°C of warming are lower than with 2.0°C. (There are still substantial risks even at 1.5°C of warming.)
This tool allows users to calculate carbon budgets at the global scale, as well as for individual countries