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CDR 101 – Chapter 1: Where Does the Global Carbon Budget Come From?

It is well known that CO₂ emissions drive climate change, which emphasizes the need to reduce them. But even people who have worked in the green industry for many years can still be confused about how far emission reductions need to go in the future.

Here, we introduce the important concept of the global carbon budget, which is defined as the amount of CO₂, measured in GtCO₂ or PgC, that we can emit while keeping global warming below a specific target. Global warming is an important one-dimensional indicator of climate change. It refers to the change in global mean surface temperature relative to pre-industrial levels.

So how do scientists know the remaining carbon budget?

Scientists use Earth system models, which are more or less digital versions of the Earth inside a computer, to simulate future global warming under different levels of CO₂ emissions. It has been shown that there is a near-linear relationship between cumulative emissions and increases in global mean surface temperature. Hence, for a given level of global warming, there is a corresponding budget for cumulative CO₂ emissions.

We can therefore read the remaining carbon budget directly from the figure. At the current level of global warming of approximately 1.5°C, limiting warming to 2°C would allow roughly another 1,907 GtCO₂ of cumulative emissions, equivalent to about 520 GtC. At the 2025 global emissions rate of approximately 42.2 billion tonnes of CO₂ per year, this corresponds to around 45 years of emissions at today’s rate.

If we emit more than this budget, the Earth will undoubtedly warm by more than 2°C. One might be skeptical about this additional 0.5°C of warming and ask how much difference it would really make. This will be discussed in a future chapter. For now, the intuition the author would like the reader to take away is that every 0.1°C increase matters greatly.

This non-negotiable physical constraint is also embedded in Article 2.1(a) of the Paris Agreement, which calls for holding the increase in global average temperature to well below 2°C above pre-industrial levels, while pursuing efforts to limit warming to 1.5°C.

Yes, we are now inevitably going to exceed 1.5°C. We will talk more in a future chapter about how to deal with this problem.

Now, let us return to the linear relationship between global warming and cumulative emissions. We can be very critical of this scaling relationship by asking: how robust is it?

In fact, the figure below shows what this relationship should look like in reality. It acknowledges that the relationship contains a great deal of uncertainty. Not only the slope, but also the linearity itself, can be affected by the Earth system.

In reality, we also need to consider non-CO₂ emissions and aerosols, which are not included in cumulative CO₂ emissions but still affect the global carbon budget. Emissions calculations are also far from trivial. Assessing the remaining carbon budget is therefore a much more rigorous and complex procedure.

This relationship has nevertheless become a central rule of thumb in climate action. Countries participating in international climate agreements submit nationally determined contributions, or NDCs, outlining their efforts to reduce emissions and, in many cases, achieve net-zero emissions.

Under this framework, global temperatures are expected to broadly stabilize once global net CO₂ emissions reach zero. However, hard-to-abate sectors such as aviation and steel are likely to continue producing residual emissions. These emissions will need to be counterbalanced by carbon dioxide removal, or CDR, making CDR an essential component of any credible net-zero strategy.

Reference

[1] https://globalcarbonbudget.org/fossil-fuel-co2-emissions-hit-record-high-in-2025/?utm_source=chatgpt.com

[2] https://www.carbonbrief.org/guest-post-refining-the-remaining-1-5c-carbon-budget

[3] Friedlingstein, P. A Modeling Perspective of Global Carbon Cycle Science: From a Missing Sink to Net Zero. Tellus 79, (2026).










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