Global Carbon Budgets

How much must global net CO2 emissions be reduced to meet the Paris Agreement?

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 IPCC the IPCC AR6 WGI report state that the budget to limit warming to 1.5°C above pre-industrial (1850-1900) levels with a 66% probability is 400 GtCO2. (1 GtCO2 = 1 billion tonnes (metric tons) CO2.) To make sure that the carbon budget is not overrun, net zero emissions must be achieved before the budget runs out.

The present fossil emissions are calculated as an average of 2019, 2020, and 2021, which is 36 GtCO2. Adding net land-use change emissions of 4 GtCO2 results in total global CO2 emissions of 40 GtCO2, which is used as the starting point of the diagrams below (GCP 2021).

Not a lot has happened with regard to emission reductions since the Paris Agreement was signed in 2015. How much harder will it be to meet the Paris Agreement if we wait even more? What is needed to fulfill the agreement?

Same emission reduction amount every year

One way of modelling emissions reductions is to reduce the emissions with the same amount every year. Every year of delay in action means that the emissions must reach net zero one year earlier.

How much (in GtCO2) must global emissions be reduced per year to reach net zero emissions before the carbon budget runs out?

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Same percentage of emission reductions every year

Another possible way of modelling emissions is to reduce the emissions by the same percentage amount every year, e.g. a 10% reduction of emissions every year. In the calculations below, all years with emissions above 1/100th of the emissions the start year are counted.

How much (in percent) must global emissions be reduced per year to reach net zero emissions before the carbon budget runs out?

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2030 goal vs net zero goal

The Emission Gap Report 2019 state that global GHG emissions would need to be 55% lower in 2030 than in 2018, to limit global warming to below 1.5ºC. The SR15 report state that net zero CO2 emissions must be reached in 2050 to be able to limit global warming to below 1.5ºC.
Since the total warming is depending on the total amount of CO2 emitted, there is a relationship between the 2030 goal and the net zero goal. If emissions are reduced more to 2030, the net zero point can happen later in time. Conversely, if emission are not reduced enough in 2030, the net zero point must happen a lot sooner.

What is the relationship between the 2030 and the net zero goals?

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Flat or increasing emissions

The 2019 emissions reduced the budget by about 40 GtCO2 per year (Global Carbon Budget 2021), which means that with todays emission levels, the 1.5°C warming with a 66% probability budget of 400 GtCO2 runs out in 10 years (400/40). Currently, CO2 emissions from burning fossil fuels are increasing with about 1% per year, and increased 2.7% in 2018. (Burning fossil fuels is by far the biggest contributor to CO2 emissions.)

How long does the budget last if global emissions continue increasing or stay flat?

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