Over the past decade, the idea of a global “carbon budget” has become a key concept in both climate science and policy. A carbon budget represents the total amount of carbon dioxide that can be emitted into the atmosphere before global temperatures rise above a critical threshold. Such as the 1.5°C and 2°C limits set in the Paris Agreement. The carbon budget concept has found its way into climate policy discussions, supporting a variety of objectives, from specific policy suggestions and global justice discussions to recommendations for financial investment and calls for reducing fossil fuel use.
Like many scientific concepts, the carbon budget has garnered both praise and critique within the scientific community. Some see it as an easily understandable framework for linking scientific knowledge about Earth’s physical limits to practical actions in climate policy. Whereas others argue, that this concept oversimplifies complex science, lacks clarity, and can even lead to a false sense of security in addressing climate change. The basis of this criticism is that climate change is influenced by numerous factors beyond just carbon emissions, including other greenhouse gases, feedback loops in the climate system, and regional variations in climate impacts. Reducing the challenge to a single “budget” can overlook these complexities, making it difficult to capture the full scope of climate change.
At the same time, it is important to acknowledge the fact that not all policymakers are climate scientists. Currently, we see scientists looking into various aspects of climate change and every year thousands of research articles get published in addition to 1000+ pages long reports by IPCC (Intergovernmental Panel for Climate Change) and other organizations. Yet, we do not see that much interest among policymakers to address these issues and proposed actions. The lack of proper communication between scientists and policymakers is one of the walls that keep these two communities aside. Therefore, my argument for the above critics is that we need to simplify some of the complexities to present the scientific findings to policymakers and other non-specialist communities. The carbon budget is one such approach because it translates scientific data into a concrete, actionable measure that can guide policy decisions, target setting, and emission reductions in ways that are understandable to both the public and decision-makers.
The Science Behind the Claim
The term “carbon budget” has been used to mean different but related things over time, evolving in both scientific research and climate policy. In the physical sciences, a carbon budget traditionally refers to the way carbon moves and cycles through the Earth, including between the atmosphere, oceans, and land. In policy, the term has come to signify a limit on how much greenhouse gas can be emitted over a specific time, like the allowances set in international climate agreements (e.g., the Kyoto Protocol) or national laws, such as the UK Climate Change Act.
Today, the idea of a global carbon budget combines both these perspectives. It refers to the maximum total emissions humanity can release while still keeping global warming under a certain threshold, like the 1.5°C or 2°C targets of the Paris Agreement. This idea became widely accepted around 2009, following a series of influential studies that found a near-linear relationship between cumulative carbon emissions and temperature rise (data from 1850 to 2019 in the following figure shows this near-linear relationship). This means every ton of CO₂ added to the atmosphere results in roughly the same amount of warming, regardless of when it’s emitted. This discovery simplified the challenge of setting climate targets by linking them directly to cumulative emissions rather than complex emission pathways.
The Future Through the Lens of the Carbon Budget and Its Reliability (Let’s Discuss)
If we continue to pay attention to the above figure, we can notice IPCC has generated forecasts for future warming we can expect based on different SSPs (Shared Socioeconomic Pathways), which simulates how different socioeconomic developments might influence greenhouse gas emissions, resource use, adaptation options, and vulnerability to climate impacts (Read More). Therefore, the next question is, what is the remaining carbon budget to reach a certain temperature target? A recent IPCC report presented an explanation for this question in a summarised form through the following table.
The first column in the above table shows three different global average temperature increases we can expect in the future compared to the average global temperature between 1850 and 1900. The second column shows the remaining temperature increase to reach each of these by taking the average between 2010 and 2019 as a reference. According to the given values, we have already reached 1.07°C above the average global temperature between 1850 and 1900 by the end of 2019. Then we have different likelihoods of exceeding global warming beyond the mentioned limits in the first column and the remaining carbon budgets.
As a further explanation of this table, we can use the estimated remaining carbon budget to calculate when we can expect to reach different warming scenarios. Given that 1.5°C warming is the one we are closest to, by taking the 83% likelihood scenario and approximating the emissions in 2023 as 40 GtCO2, and assuming the annual emission since 2020 is equal to this value, we get a result of 7.5 years left to reach 1.5°C warming. That suggests we should experience this warming scenario in 2027/28.
However, you might have seen in recent news that 2024 is on track to be the hottest year on record as warming temporarily hits 1.5°C (Reference). This is based on global climate model results as shown in the following figure.
So as the above figure shows, all the mentioned Global Climate Models show we have already exceeded 1.5 degrees. What we can conclude about the reliability of the “Carbon Budget” from this observation?
One thing you can see is that the temperature does not increase linearly, there are fluctuations but the overall increase has a linear trend. Therefore, even in these modelled results, we will probably see global mean temperature fluctuate around 1.5 degrees for some time. However, from the previous calculation for the 83% likelihood scenario, the “carbon budget” projected we will reach 1.5°C warming around 2027. That is just 3 years from now. When we look at the historical change in global mean temperature shown in the above figure, we can expect the fluctuating behaviour to cross 1.5°C completely around the same time. This suggests “Carbon Budget” is a possible concept to rely on for decision-making.
However, you might have overlooked one important fact. In the calculation, we used the 83% likelihood scenario, in other words, the most extreme scenario from the table and saw an agreement with the global temperature variation. This suggests that global warming and climate change have already reached an extreme stage, placing us at a critical turning point.
References:
(1) Lahn, B. (2020). A history of the global carbon budget. Wiley Interdisciplinary Reviews: Climate Change, 11(3), e636.
(2) https://wmo.int/news/media-centre/2024-track-be-hottest-year-record-warming-temporarily-hits-15degc
(3) IPCC, 2018: Summary for Policymakers. In: Global Warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty. Cambridge University Press, Cambridge, UK and New York, NY, USA, pp. 3-24,doi:10.1017/9781009157940.001.
