4 Carbon budgets: how much CO₂ can we still emit?

In 2015, all (then) 196 parties to the convention came together for the United Nations Climate Change Conference in Paris and adopted by consensus the Paris Agreement, aimed at limiting global warming to less than two degrees Celsius compared to pre-industrial times, and pursue efforts to limit the rise to 1.5 degrees Celsius. The Paris Agreement entered into force on 4 November 2016.

The Intergovernmental Panel on Climate Change (IPCC)⁵ provides the research-based carbon budgets associated with various climate targets and regularly updates these budgets.

The global carbon budget indicates how much CO₂ the world as a whole can still emit to keep global warming below a certain temperature level with a certain probability.

Climate scientist Professor Stefan Rahmstorf from the Potsdam Institute for Climate Impact Research has a quite useful explanation of the concept of a carbon budget:

“First of all – what the heck is an “emissions budget” for CO2? Behind this concept is the fact that the amount of global warming that is reached before temperatures stabilise depends (to good approximation) on the cumulative emissions of CO2, i.e. the grand total that humanity has emitted. That is because any additional amount of CO2 in the atmosphere will remain there for a very long time (to the extent that our emissions this century will like prevent the next Ice Age due to begin 50 000 years from now). That is quite different from many atmospheric pollutants that we are used to, for example smog. When you put filters on dirty power stations, the smog will disappear. When you do this ten years later, you just have to stand the smog for a further ten years before it goes away. Not so with CO2 and global warming. If you keep emitting CO2 for another ten years, CO2 levels in the atmosphere will increase further for another ten years, and then stay higher for centuries to come. Limiting global warming to a given level (like 1.5 °C) will require more and more rapid (and thus costly) emissions reductions with every year of delay, and simply become unattainable at some point.”

— Stefan Rahmstorf⁶

According to the IPCC’s reports, if the climate targets of the Paris Agreement are to be met, the world is left with a very limited carbon budget. The following table is an extract from the IPCC’s latest report on carbon budgets:

Table 4.1: IPCC global carbon budgets.
The table shows estimates of remaining carbon budgets (Gt of CO₂) under different probabilities that global warming will not exceed the temperature levels provided in the left column. Estimated remaining carbon budgets are calculated from the beginning of 2020 and extend until global net zero CO₂ emissions are reached. They refer to CO₂ emissions, while accounting for the global warming effect of non-CO₂ emissions.⁷

Global warming	Estimated remaining carbon budgets from the beginning of 2020 (GtCO2)
Warming relative to 1850–1900 (°C)	Probability of staying below temperature level
	17%	33%	50%	67%	83%
1.5	900	650	500	400	300
1.7	1450	1050	850	700	550
2.0	2300	1700	1350	1150	900

You can read this table in the following way:

1. Pick your preferred global warming limit in the very left column.
2. In the third row, pick your preferred probability level of staying below the warming limit.⁸
3. The associated number at the intersection of both choices provides the remaining carbon budget in gigatonnes of CO₂.

In particular, the IPCC recommends the below 1.5 degree temperature limit. So, if we assume this target and assume a probability of about 67% that we won’t surpass this limit, the world still had about 400 gigatonnes of CO₂ that could be “blown into the air” at the beginning of 2020. If we subtract 2020 emissions from this, this budget was about 365 gigatonnes at the beginning of 2021.