What is our upper limit for greenhouse gas emissions?

We’ve previously shared an estimate of global greenhouse gas emissions for 2021 of 54.59 Gt. But that figure is just a snapshot in time. How does it compare to other years and what is our best guess of how many gigatonnes of CO2e can we release into the atmosphere before we exceed the Paris Agreement’s target of 1.5 degrees Celsius of warming?

The chart below from Our World in Data shows global greenhouse gas emissions from 1850 to 2021. It doesn’t make for great reading - although emissions dipped in 2020 due to widespread pandemic lockdowns, generally the numbers have been getting higher and higher.

If you add up the total amount for each year you get a figure of about 3,400 Gt of CO2e. That’s the world’s greenhouse gas emissions in their entirety from 1850-2021. The International Panel on Climate Change estimated global CO2 (excluding other greenhouse gases) emissions from 1850-2019 were 2,340 Gt.

The IPCC also attempted to estimate how much more CO2 we could release into the atmosphere before we exceeded global temperature rises of 1.5, 1.7 and 2 degrees Celsius.

A study titled “Indicators of Global Climate Change 2022: annual update of large-scale indicators of the state of the climate system and human influence” built on these estimates by adding some additional information.

The Paris Agreement set an ambition of keeping the global temperature rise to no more than 2 degrees Celsius, but ideally no more than 1.5 degrees, above the pre-industrial average. To have a 50% chance of meeting the 1.5 degree target we can only emit another 250 Gt of CO2. Put another way, we’ve already used up 90% of our carbon budget.

The Global Carbon Project has also calculated our remaining carbon budget:

“The remaining carbon budget for a 50% likelihood to limit global warming to 1.5°C, 1.7°C and 2°C has respectively reduced to 380 GtCO2 (9 years at 2022 emissions levels), 730 GtCO2 (18 years) and 1230 GtCO2 (30 years).

Reaching zero CO2 emissions by 2050 entails a total anthropogenic CO2 emissions linear decrease by about 1.4 GtCO2 each year, comparable to the observed fall in 2020 emissions resulting from COVID-19 measures, highlighting the scale of the action needed. This would result in additional emissions of 560 GtCO2 from year 2023, leading to a 50% likelihood to limit warming to around 1.6°C.”

In the words of Professor Piers Forster, the director of the Priestley Centre for Climate Futures at the University of Leeds and lead author of the study cited above:

“This is the critical decade for climate change. Decisions made now will have an impact on how much temperatures will rise and the degree and severity of impacts we will see as a result. We need to change policy and approaches in light of the latest evidence about the state of the climate system. Time is no longer on our side.”