Article
Every Tonne of CO2 Adds to Global Warming
Every tonne of carbon dioxide (CO2) emitted drives up temperatures and does so almost linearly: if twice as much is emitted, the planet will warm roughly twice as much. This simple rule of thumb is the scientific foundation of today’s climate policy, but could sunlight reflection methods or solar radiation modification (SRM) undermine it?
Key takeaways
- Every tonne of CO2 adds to global warming, which means the planet will only stop warming when humans eliminate their CO2 emissions.
- This rule allows a global temperature target to be converted into a remaining “carbon budget”, which can help to inform emissions goals.
- SRM would cool the planet without addressing the build-up of CO2, with implications for how climate policies are crafted.
Both CO2 emissions and temperatures have been rising since the start of the industrial revolution, and scientists have definitively linked rising temperatures to fossil fuel burning.1 Since the start of the industrial revolution, human-caused CO2 emissions have increased temperatures by more than 1.3°C.
Researchers have found a near linear relationship between the total amount of human-caused CO2 emissions since the 19th century and global warming.1 This relationship holds true in observations and in climate models and is estimated to be about 0.45°C of warming per trillion tonnes of CO2 – or per 1,000 GtCO2 – released.1 Policymakers use this relationship to determine the emissions reductions required to limit warming.
How much CO2 is emitted?
Globally, 37 billion tonnes of CO2 were emitted in 2022. That is a little over 4.5 tonnes for every person on Earth. However, many countries emit much less than this per capita, and others emit much more. For example, in 2022, per capita emissions in the United States (US) were about 15 tonnes of CO2, while per capita emissions in Nigeria were about 0.6 tonnes of CO2.
Though emissions have decreased in the US in recent years, it remains by far the largest contributor to cumulative CO2 emissions since the start of industrial era. Research shows that between 1972 and 2022 US citizens emitted on average around 1,000 tonnes of CO2 over this time.2 The linear relationship between cumulative CO2 emissions and rising temperatures is based on observations and climate models, so it includes the impacts of how all the climate and carbon cycle processes interact with emissions.1
Every tonne of CO2 adds to global warming
The “first phase” of international climate policy, in the 1990s, focused on stabilising atmospheric CO2 concentrations.3 However, the temperature impacts of stabilising CO2 concentrations are uncertain, and this was not easily translated into actionable targets.
The link between cumulative emissions and temperature change – about 0.45°C of warming for every trillion tonnes of CO2 emitted1 – emerged as simple and surprisingly consistent relationship.4
CO2 emissions drive warming
There is a linear relationship between cumulative CO2 emissions and rising global temperature that holds in historic observations and model projections. Scientists’ best guess is that the planet will stop warming once CO2 emissions reach net-zero.
Faster emissions cuts
Slower emissions cuts
3
4
1
2
5 TtCO2
Global temperature change
+3.0°C
2100
Emissions continue
2.5
2.0
2070
Reaches net zero emissions
1.5
2020
1.0
2000
1950
0.5
1900
Cumulative CO2 emissions
Note: 1 Tt equals 1 trillion tonnes
Source: SRM360 analysis using the FaIR model and RCMIP data
This linear relationship has allowed policymakers to convert the goal of limiting warming into emissions cuts – a specific policy target. Policymakers can do this by establishing a remaining “carbon budget” – the total amount of CO2 that can be emitted before temperatures rise beyond the target level.
In the 2015 Paris Agreement, leaders from around the world agreed to limit warming to 2°C above pre-industrial levels, with an ambition of keeping temperatures to 1.5°C of warming.
Policy efforts in the years since the Paris Agreement was signed have decreased the rate at which annual CO2 emissions are increasing. When the treaty was signed, annual CO2 emissions rates were projected to grow by 16% in 2030. In 2023, this projected increase for 2030 was 3%.
Slowing the rate of CO2 emissions growth is insufficient to stop warming – the world would need to reach net zero CO2 emissions5 to achieve this. The best estimate from climate models indicates that reaching and maintaining net zero CO2 emissions would keep global temperatures at a constant level, but there are large uncertainties in this assessment.6
Would SRM undermine the foundation of climate policy?
“Every tonne of CO2 adds to global warming” is a simple rule that is often used to inform climate policies. The implications of this rule are clear: the rate of global warming is determined by the rate at which CO2 is emitted, and unless these emissions are eliminated, the planet will continue warming.
This rule also has policy implications. It allows the remaining carbon budgets for limiting warming below 1.5°C or 2°C to be calculated, informing assessments of whether global and national emissions commitments are sufficient to meet those goals.
SRM would break this rule by lowering temperatures through reflecting some sunlight away from the Earth without addressing the root cause of climate change – CO2 emissions. This might help manage some of the impacts of climate change, but it could also introduce new risks.
By providing a way to lower temperatures independently of addressing CO2 emissions, there are concerns that SRM might undermine efforts to cut emissions. At the extreme, this could lead to a situation where the warming effect of continued emissions is offset by an ever-larger deployment of SRM. This would lead to ever-larger ocean acidification impacts, and ever-larger side effects from SRM, e.g., stratospheric aerosol injection’s impact on ozone.
Such a scenario could be avoided if countries commit to cutting emissions even while temperatures are lowered by SRM, for example, by setting separate targets for limiting both global temperatures and CO2 emissions.
Adoption of the Paris Agreement at the 21st Conference of the Parties to the United Nations Framework Convention on Climate Change.
Open questions
- When will net zero CO2 emissions be achieved? Later this century or in the early decades of the next?
- How much will the planet warm in response to CO2 emissions and will it stop when emissions are eliminated as models predict?
- How can the climate policy framework be adapted to account for the possibility of SRM deployment?
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Endnotes
- IPCC. (2021). Summary for Policymakers. In V. Masson-Delmotte, P. Zhai, A. Pirani, S. L. Connors, C. Péan, S. Berger, et al. (Eds.), Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change (pp. 3–32). Cambridge, United Kingdom and New York, NY, USA: Cambridge University Press. https://doi.org/10.1017/9781009157896.001
- Data on annual per capita CO2 emissions in the US from 1972 to 2022 used to calculate cumulative per capita emissions were obtained from Our World In Data.
- McLaren D, Markusson N. (2020). The co-evolution of technological promises, modelling, policies and climate change targets. Nature Climate Change;10(5):392-7. https://doi.org/10.1038/s41558-020-0740-1
- Allen MR, Friedlingstein P, Girardin CAJ, et al. (2022). Net Zero: Science, Origins, and Implications. Annual Review of Environment and Resources, 55, 22. https://doi.org/10.1146/annurev-environ-112320-105050
- Net zero CO2 emissions will be achieved when there is a balance between human-caused CO2 emissions and human-caused CO2 removals from the atmosphere, i.e., carbon dioxide removal.
- Palazzo Corner S, Siegert M, Ceppi P, et al. (2023). The Zero Emissions Commitment and climate stabilization. Frontiers in Science. 1:1170744. https://doi.org/10.3389/fsci.2023.1170744
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