In order to meet the Paris Agreement goal of limiting global warming to well below 2°C above preindustrial levels by the end of the century with a CO₂ concentration of 400 ppm in 2100, 1300 Gt of CO₂ would need to be captured from the atmosphere.

Summary

The IPCC's Representative Concentration Pathways (RCPs) outline different climate scenarios based on projected greenhouse gas concentrations. RCP 2.6 requires immediate emission reductions and massive carbon removal (1300 Gt CO₂ by 2100) to limit warming to 0.9 - 2.4°C and is representative of the Paris Climate Agreement. RCP 4.5 involves moderate reductions needing 866 Gt removal, while RCP 8.5 ("business as usual") would require removing 3522 Gt CO₂ to keep atmospheric CO₂ concentrations close to 450 ppm. Current atmospheric CO₂ levels (420 ppm in 2024) already exceed preindustrial levels, making ambitious pathways like RCP 2.6 increasingly challenging. Calculations show the staggering scale of carbon capture required under each scenario, highlighting the urgency of emission reductions.

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When considering the feasibility of carbon capture technologies and more specifically Direct Air Capture (DAC), it is important to clarify what is trying to be achieved and what is achievable. Much of this depends on specific emission reduction pathways, which are essentially the levels of atmospheric carbon dioxide (CO₂) that we as a global community have decided to limit our emissions to and what we need to do to achieve this. The necessity for this is the fact that the amount of carbon dioxide that would need to be removed from the atmosphere to return to preindustrial levels is almost inconceivable and therefore, a more realistic goal is necessitated. The Remaining Carbon Budget (RCB) is another measure that is independent of any emission pathway that can provide an idea of how much more carbon we can 'safely' emit; however, it does not assist in providing a roadmap on where emissions need to be removed or the timescale in which this needs to be achieved.

Carbon reduction pathways can be hard to predict and visualize as they are often enacted over long periods of time and are often based on assumptions and models that predict other future emissions reductions such as the use of electric vehicles or a transition away from fossil fuels over the course of decades. The International Panel on Climate Change (IPCC) has developed a framework for categorizing these different pathways, which are known as Representative Concentration Pathways (RCPs) that detail the emission reduction pathways required to limit the rise of global average temperatures to specified values.

What are Representative Concentration Pathways (RCPs)

The Representative Concentration Pathways (RCPs) are a set of scenarios developed by the IPCC that describe future global emission pathways. The RCPs are critical in understanding future climate scenarios as they don't simply describe a final state such as "net zero" by 2100, but instead, they describe possible trajectories of emissions and concentrations of CO2 in the atmosphere. There are 4 main RCPs as well as a handful of intermediate scenarios, and the naming is based on the level of radiative forcing (measured in watts per square meter, or W/m²) projected by the year 2100 under each scenario. Radiative forcing describes the balance of incoming and outgoing energy in the earth's atmosphere. When radiative forcing is positive, the Earth is absorbing more energy than it is emitting, and when radiative forcing is negative, the Earth is emitting more energy than it is absorbing. In the case of positive radiative forcing, the imbalance will cause temperature to change until a new equilibrium is reached. Currently, greenhouse gases are estimated to be increasing forcing by 4 W/m², while aerosols are reducing forcing by around 1 W/m², resulting in a net imbalance of around 3 W/m²; however, there is a range of disagreement on this value ¹. From the radiative forcing values, it is also possible to derive a range of projected mean global temperature rise values. The four RCPs are as follows:

RCP 2.6: This scenario corresponds to a radiative forcing of 2.6 W/m² by 2100 and is achieved through almost immediate and large-scale reductions in emissions as well as large-scale carbon dioxide removal from the atmosphere. RCP 2.6 is the only scenario that is attempting to reach the Paris Agreement goal of limiting global warming to well below 2°C above preindustrial levels, and pursuing efforts to limit the increase to between 0.9°C – 2.4°C by the end of the century with a CO₂ concentration of 400 parts per million (ppm) in 2100. RCP 2.6 is considered an extremely ambitious scenario given the recent emission trends and stalled progress on mitigation efforts ^{2 3 4}.

RCP 4.5: This scenario corresponds to a radiative forcing of 4.5 W/m² by 2100 and is achieved through a combination of emission reductions and carbon dioxide removal from the atmosphere, which result in peak global CO₂ emissions in 2035. This scenario relies on limiting annual global CO₂ emissions to around half of 2010 levels in the second half of the century. The RCP 4.5 scenario is consistent with the goal of limiting global warming to between 1.7°C – 3.3°C above preindustrial levels by the end of the century with a CO₂ concentration of 525 ppm in 2100 ^{2 3 4}.

RCP 6.0: This scenario corresponds to a radiative forcing of 6 W/m² by 2100 and is achieved through a combination of emission reductions and smaller scale carbon dioxide removal from the atmosphere, which result in CO₂ emissions declining from 2080 onwards. The RCP 6.0 scenario is consistent with the goal of limiting global warming to between 2.0°C – 3.8°C above preindustrial levels by the end of the century with a CO₂ concentration of around 670 ppm in 2100 ^{2 3 4}.

RCP 8.5: This scenario corresponds to a radiative forcing of 8.5 W/m² by 2100 and is essentially achieved through a continuation of current trends in emissions and no significant carbon dioxide removal. The RCP 8.5 scenario is consistent with the goal of limiting global warming to between 3.2°C – 5.7°C above preindustrial levels by the end of the century with a CO₂ concentration of above 900 ppm in 2100 ^{2 3 4}.

Representative Concentration Pathways (RCPs) and Carbon Capture

To reiterate, the RCPs are a set of possible future emissions pathways, they are not a prediction of what is going to occur in the future and they are also not a climate goal. Because of this, it is possible to evaluate a goal such as 'atmospheric concentration of CO₂' in relation to the individual RCPs. For the purpose of this analysis, we will define the goal as "limiting the atmospheric concentration of CO₂ to 450 ppm by 2100" which is slightly higher than the current atmospheric concentration of 420 ppm in November of 2024 ⁵. Note there are other systems on Earth that act as carbon dioxide removal mechanisms such as reforestation and changed land usage which are already contained within the RCP scenarios.

RCP 2.6: Under the RCP 2.6 scenario, no additional CO₂ removal would be required to meet the goal of 450 ppm. However, it should be noted that the RCP 2.6 pathway already includes large-scale carbon dioxide removal from the atmosphere. Within the RCP 2.6 scenario, between 2020 and 2050, 10 Gt (10,000,000,000 tonnes) of CO₂ would need to be removed from the atmosphere each year and between 2050 and 2100, 20 Gt of CO₂ would need to be removed each year ⁶. This value is not to be confused with decarbonization or reductions in emissions, but rather the actual amount of CO₂ that would need to be removed from the atmosphere to achieve the goal of 400 ppm. To calculate the amount of CO₂ that would need to be captured to achieve this goal, we can use the following formula assuming that our starting year is 2020:

$$\begin{align*} \text{CO}_2 \text{ captured} &\approx [(2050 - 2020) \space \text{years} \times 10 \text{ Gt/year}] \space \\ &\space \space + [(2100-2050) \space \text{years} \times 20 \text{ Gt/year}]\\ &\approx [30 \text{ years} \times 10 \text{ Gt/year}] \space + [50 \text{ years} \times 20 \text{ Gt/year}]\\ &\approx 300 \text{ Gt} + 1000 \text{ Gt}\\ &\approx 1300 \text{ Gt} \end{align*}$$

RCP 4.5: Under the RCP 4.5 scenario, atmospheric concentrations of CO₂ are projected to stabilize at around 525 ppm by 2100. We can calculate the amount of CO₂ that would need to be captured to achieve the goal of 450 ppm by 2100 as follows:

Given that

• The generally accepted estimate of the mass of the Earth's atmosphere is: 5.15 x 10^18 kg ⁷
• Amount of CO₂ represented by one ppm: 10⁻⁶
• The molecular weight correction factor (ratio of the molecular weight of CO₂ to the molecular weight of dry air) is approximately 1.52

First, we can calculate the mass of CO₂ in the atmosphere at 525 ppm:

$$\begin{align*} \text{Mass of CO}_2 \text{ in the atmosphere} &\approx \text{CO}_2 \text{ concentration} \times \text{CO}_2 \text{ represented by one ppm}\\ &\space \space \times \text{Mass of the atmosphere} \times \text{Molecular weight correction factor}\\ &\approx 525 \times 10^{-6} \times 5.15 \times 10^{18} \times 1.52\\ &\approx 4.389 \times 10^{15} \text{ kg}\\ &\approx 4389 \text{ Gt} \end{align*}$$

Next, we can calculate the amount of CO₂ in the atmosphere at 450 ppm:

$$\begin{align*} \text{Mass of CO}_2 \text{ in the atmosphere} &\approx \text{CO}_2 \text{ concentration} \times \text{CO}_2 \text{ represented by one ppm}\\ &\space \space \times \text{Mass of the atmosphere} \times \text{Molecular weight correction factor}\\ &\approx 450 \times 10^{-6} \times 5.15 \times 10^{18} \times 1.52\\ &\approx 3.5226 \times 10^{15} \text{ kg}\\ &\approx 3522.6 \text{ Gt} \end{align*}$$

Finally, we can calculate the amount of CO₂ that would need to be captured to achieve the goal of 450 ppm by 2100:

$$\begin{align*} \text{CO}_2 \text{ captured} &\approx \text{Mass of CO}_2 \text{ in the atmosphere at 525 ppm} \\ &\space \space - \text{Mass of CO}_2 \text{ in the atmosphere at 450 ppm}\\ &\approx 4389 \text{ Gt} - 3522.6 \text{ Gt}\\ &\approx 866.4 \text{ Gt} \end{align*}$$

RCP 8.5: Under the RCP 8.5 scenario, atmospheric concentrations of CO₂ are projected to stabilize at around 900 ppm by 2100. We can calculate the amount of CO₂ that would need to be captured to achieve the goal of 450 ppm by 2100 in the same way as above:

First, we can calculate the mass of CO₂ in the atmosphere at 900 ppm:

$$\begin{align*} \text{Mass of CO}_2 \text{ in the atmosphere} &\approx \text{CO}_2 \text{ concentration} \times \text{CO}_2 \text{ represented by one ppm}\\ &\space \space \times \text{Mass of the atmosphere} \times \text{Molecular weight correction factor}\\ &\approx 900 \times 10^{-6} \times 5.15 \times 10^{18} \times 1.52\\ &\approx 7.0452 \times 10^{15} \text{ kg}\\ &\approx 7045.2 \text{ Gt} \end{align*}$$

Next, we can calculate the amount of CO₂ in the atmosphere at 450 ppm:

$$\begin{align*} \text{Mass of CO}_2 \text{ in the atmosphere} &\approx \text{CO}_2 \text{ concentration} \times \text{CO}_2 \text{ represented by one ppm}\\ &\space \space \times \text{Mass of the atmosphere} \times \text{Molecular weight correction factor}\\ &\approx 450 \times 10^{-6} \times 5.15 \times 10^{18} \times 1.52\\ &\approx 3.5226 \times 10^{15} \text{ kg}\\ &\approx 3522.6 \text{ Gt} \end{align*}$$

Finally, we can calculate the amount of CO₂ that would need to be captured to achieve the goal of 450 ppm by 2100:

$$\begin{align*} \text{CO}_2 \text{ captured} &\approx \text{Mass of CO}_2 \text{ in the atmosphere at 900 ppm} \\ &\space \space - \text{Mass of CO}_2 \text{ in the atmosphere at 450 ppm}\\ &\approx 7045.2 \text{ Gt} - 3522.6 \text{ Gt}\\ &\approx 3522.6 \text{ Gt} \end{align*}$$

Conclusion

The RCPs are a set of future emission pathways used by the IPCC to describe possible future emissions scenarios. RCP 2.6 is considered the most stringent with immediate and large-scale reductions in emissions and large-scale carbon dioxide removal from the atmosphere. RCP 4.5 is a more moderate scenario with emission stabilization in the second half of the century. RCP 8.5 is considered the "business as usual" scenario with no significant emission reductions, which is the current course that the world is on. If the world were to follow these scenarios but still wanted to limit the atmospheric concentration of CO₂ to 450 ppm by 2100, the amount of CO₂ that would need to be captured is summarized in table 1 below.

Table 1: Atmospheric CO₂ removal required to achieve 450 ppm or less for each RCP scenario

Scenario	Projected 2100 CO₂ concentrations (ppm)	Estimated average global temperature Rise Range (°C)	CO₂ Removal required (Gt)
RCP 2.6	400	0.9 – 2.4	1300
RCP 4.5	525	1.7 – 3.3	866.4
RCP 8.5	900	3.2 – 5.7	3522.6

Sources

Footnotes

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2. Intergovernmental Panel on Climate Change (IPCC). (2023). Climate change 2023: Synthesis report. Contribution of Working Groups I, II, and III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change (Core Writing Team, H. Lee, & J. Romero, Eds.). IPCC. https://doi.org/10.59327/IPCC/AR6-9789291691647 ↩ ↩² ↩³ ↩⁴

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4. IPCC. (2019). Summary for policymakers. In H.-O. Pörtner, D.C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, A. Alegría, M. Nicolai, A. Okem, J. Petzold, B. Rama, & N.M. Weyer (Eds.), IPCC special report on the ocean and cryosphere in a changing climate. In press. ↩ ↩² ↩³ ↩⁴

5. Lan, X., Tans, P., & Thoning, K. W. (2024). Trends in globally-averaged CO₂ determined from NOAA Global Monitoring Laboratory measurements. https://doi.org/10.15138/9N0H-ZH07 ↩

6. McQueen, N., Gomes, K. V., McCormick, C., Blumanthal, K., Pisciotta, M., & Wilcox, J. (2021). A review of direct air capture (DAC): Scaling up commercial technologies and innovating for the future. Progress in Energy, 3(3), 032001. https://doi.org/10.1088/2516-1083/ac1d6c ↩

7. Verniani, F. (1966). The total mass of the Earth's atmosphere. Journal of Geophysical Research, 71(2), 385-391. ↩