How Much Carbon Needs to Be Captured?

Summary

Historical CO₂ emissions since the Industrial Revolution total over 2,000 gigatons - equivalent to 1.3 trillion cars. If liquefied, this would form a cube 12 km (7.5 miles) on each side. Distributed evenly over the United States, this volume would create an 18.6 cm (7.3 inches) deep layer. The remaining carbon budget to limit warming to 2°C with 87% likelihood is 900 gigatons, comparable to 600 billion cars. This remaining budget would form a 9.3 km (5.8 miles) cube of liquid CO₂, or distributed evenly over the United States it would create an 8.3 cm (3.3 inches) deep layer.

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When discussing carbon capture technologies and feasibility, it can often be difficult to visualize just how much carbon dioxide (CO₂) would need to be removed.

Total CO₂ Emitted Since the Industrial Revolution

In 2021, it was estimated that since the industrial revolution, humans have emitted more than 2,000 Gt (2,000,000,000,000 tons) of CO₂ into the atmosphere ¹.

Mass Comparison

For comparing this mass, we can use a common object such as a car. If we assume that the average car weighs 1.5 tons:

$$\begin{align*} \text{number of cars} &= \frac{2,000,000,000,000 \text{ tons}}{1.5 \text{ tons/car}} \\ &= 1,333,333,333,333 \text{ cars} \\ &= 1.333 \text{ trillion cars} \end{align*}$$

Volume Comparison

For visualizing the volume, we can calculate the volume of CO₂ if it were liquefied. For compressed CO₂, the density is approximately 1,101 kg/m³.

To calculate the volume:

$$\begin{align*} \text{volume} &= \frac{2,000,000,000,000 \text{ tons}}{1,101 \text{ kg/m}^3} \\ &= 1,816,530,408,720 \text{ m}^3 \\ &\approx 1.82 \times 10^{12} \text{ m}^3 \\ \end{align*}$$

If we want to calculate the size of a cube of liquid CO₂, we can take the cube root of the volume to find the side length:

$$\begin{align*} \text{Side length} &= \sqrt[3]{1.82 \times 10^{12}} \\ &= 12,201 \text{ m} \\ &\approx 12 \text{ km} \end{align*}$$

Which is a cube that is approximately 12 km (7.5 miles) on each side.

To visualize this volume of liquid CO₂, we can calculate how deep a layer of liquid CO₂ would be if it were distributed evenly over the surface of the United States (~9.8 million km² or 3.8 million square miles):

$$\begin{align*} \text{depth} &= \frac{1.82 \times 10^{12} \text{ m}^3}{9.8 \times 10^{12} \text{ m}^2} \\ &\approx 0.186 \text{ m} \\ \end{align*}$$

This would mean that if we were to compress all of the CO₂ produced by humans since the industrial revolution into a liquid and distribute it evenly over the entire United States, it would cover the whole country in a layer approximately 18.6 cm (7.3 inches) deep.

Total CO₂ from the Remaining Carbon Budget (RCB)

As biological processes and natural carbon sinks remove CO₂ from the atmosphere over time, it would not be necessary to remove all 2,000 Gt of CO₂. One way of quantifying how much carbon we would need to remove is through the remaining carbon budget (RCB). The remaining carbon budget is a measure derived by the IPCC that signifies the amount of CO₂ that can still be emitted while remaining within a specified global average temperature rise. This value is useful because it does not assume any future emission reduction scenarios or pathways such as attempts to meet net zero emissions by 2050.

In the IPCC's Sixth Assessment Report (2023), it was estimated that the remaining carbon budget to limit warming to 2°C with an 87% likelihood is 900 Gt (900,000,000,000 tons) of CO₂ ². As a comparison, if CO₂ emissions continued to rise at the same level as in 2019, the carbon emitted between 2020 - 2030 is projected to be just under 500 Gt of CO₂ ². Using this figure of 900 Gt of CO₂, we can repeat the visualizations.

Mass Comparison

For comparing the mass, we can use a common object such as a car. If we assume that the average car weighs 1.5 tons:

$$\begin{align*} \text{number of cars} &= \frac{900,000,000,000 \text{ tons}}{1.5 \text{ tons/car}} \\ &= 600,000,000,000 \text{ cars} \\ &= 600 \text{ billion cars} \end{align*}$$

Volume Comparison

Using the same method of calculating the volume as above, we can calculate the volume of liquid CO₂ that would be produced by 900 Gt of CO₂.

$$\begin{align*} \text{volume} &= \frac{900,000,000,000 \text{ tons}}{1,101 \text{ kg/m}^3} \\ &= 817,438,692,098 \text{ m}^3 \\ &\approx 8.17 \times 10^{11} \text{ m}^3 \\ \end{align*}$$

To calculate the size of a cube of liquid CO₂, we can take the cube root of the volume to find the side length:

$$\begin{align*} \text{Side length} &= \sqrt[3]{8.17 \times 10^{11}} \\ &\approx 9,348 \text{ m} \\ &\approx 9.34 \text{ km} \end{align*}$$

Which is a cube that is approximately 9.34 km (5.81 miles) on each side.

To calculate the depth of a layer of liquid CO₂ distributed evenly over the surface of the United States, we can use the same method as above:

$$\begin{align*} \text{depth} &= \frac{8.17 \times 10^{11} \text{ m}^3}{9.8 \times 10^{12} \text{ m}^2} \\ &\approx 0.083 \text{ m} \\ \end{align*}$$

This would mean that if we were to compress all of the CO₂ left in the remaining carbon budget into liquid and distribute it evenly over the entire United States, it would be approximately 8.3 cm (3.3 inches) deep.

Sources

Footnotes

1. Ozkan, M., Nayak, S. P., Ruiz, A. D., & Jiang, W. (2022). Current status and pillars of direct air capture technologies. iScience, 25(4), Article 103990. https://doi.org/10.1016/j.isci.2022.103990 ↩

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