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| Fig. 1: Direct Air Capture Process Flow Diagram using NaOH as the absorbent and including solvent regeneration. (Source: Wikimedia Commons). |
In the face of climate change, many solutions have been proposed to reduce the net carbon emissions of human activities. One such example that has attracted attention recently are voluntary carbon offsets. [1] Voluntary carbon offsets are often purchased by companies in order to offset their greenhouse gas emissions. They are called voluntary because companies are currently not required to offset their carbon emissions. Even so, investment in this space has grown as companies are anticipating a growing market due to the potential of a carbon tax in the future. The supply of carbon offsets increased from about 6.2 million tons in 2004 to about 10.2 million tons in 2007. [2] In 2021, ExxonMobil launched their Low Carbon Solutions business to commercialize carbon capture technology, and proposed a $100 billion plan to capture and store the carbon emissions of other industries. [3]
Currently, companies may buy carbon offsets in order to appear more environmentally friendly, to meet company goals for carbon emissions, or to become eligible for tax credits. [4,5] Companies that take carbon out of the atmosphere can also create carbon offsets to be sold to others on the open market. However, there are several issues with these projects, including those dealing with scalability, accountability, and regulation.
In general, there are two types of carbon offsets: direct air capture and bioenergy carbon capture and storage.
Direct air capture uses chemical reactions to capture carbon dioxide from the atmosphere, which can then be stored underground or used for industrial applications. An example of one of the chemical processes used can be found in Fig. 1. A report by the American Physical Society evaluated the feasibility of direct air capture using sodium hydroxide absorption. It estimated that it would cost around $600 to remove one metric ton of carbon dioxide from the atmosphere. [6]
Another factor to take into consideration is that this method requires electricity to run, and because much of the world's electricity is generated via fossil fuels, it is important to verify that the amount of carbon dioxide captured is significantly greater than the amount released by the energy source powering it. [6]
Bioenergy carbon capture and storage is a method that involves planting trees and other bioenergy crops and capturing the carbon dioxide released as they are burnt to produce energy, thereby removing the carbon dioxide from the atmosphere. [7] There are many bioenergy crops that could be used for this purpose, such as fast growing grasses, woody trees like Eucalyptus, and sugarcane.
The downside of this method is that it requires the use of large areas of cultivated land, even more so than direct air capture. Researchers have evaluated the potential of bioenergy carbon capture and storage for limiting global warming to 1.5°C in two scenarios outlined in the IPCC Special Report on Global Warming of 1.5°C. [8] In the first scenario, S2: Middle of the road, researchers found that bioenergy carbon capture could only keep up until 2066 and would require 0.8 Gha (8.0 × 1012 m2) of land, which equals 5.1% of the total land surface area of Earth. In the second scenario, S5: Fossil-fuelled development, researchers found that bioenergy carbon capture could only keep up until 2050 and would require 2.4 Gha (2.4 × 1013 m2) of land, which equals 16% of the total land surface area of Earth. The estimated amount of carbon that this method could sequester is 408 Gt (4.08 × 1014 kg) and 1,207 Gt (1.08 × 1015 kg), for S2 and S5 respectively. However, the land requirements would take away from agriculture, as well as use lots of water for irrigation, which is quickly becoming a scarce resource. [9]
It is important that there is high accountability in the carbon offset market, because cheating would cast doubt onto the efficacy of such projects in reducing the overall amount of greenhouse gases in the atmosphere. A report by the U.S. Government Accountability Office found that there was no regulatory body that had oversight of this market in the United States at the time. [2] It also highlighted the difficulty in ensuring the credibility of carbon offsets. These included the problem of determining if an offset led to a reduction in emissions that would not have occurred without the purchase of the offset, as well as stopping double counting, both of which continue to be problems today. [1]
© William Zhang. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] P. Guigon, P., "Voluntary Carbon Markets : How Can They Serve Climate Change Policies," Organization for Economic Co-operation and Development, OECD Environmental Working Paper No. 19, 2010.
[2] "Carbon Offsets," U.S. Government Accountability Office, GAO-08-1048, August 2008.
[3] C. Krauss, "Exxon Mobil Makes a Pitch for Capturing Emissions and a Carbon Price," New York Times, 19 Apr 21.
[4] M. A. Delmas and V. C. Burbano, "The Drivers of Greenwashing," California Management Review, 54, 64 (2011).
[5] A. Jones, "The Tax Credit for Carbon Sequestration (Section 45Q)," Congressional Research Service. IF11344, June 2021.
[6] R. Socolow et al., "Direct Air Capture of CO2 With Chemicals," American Physical Society, June 2011.
[7] S. V. Hanssen et al., "The Climate Change Mitigation Potential of Bioenergy With Carbon Capture and Storage," Nat. Clim. Change 10, 1023 (2020).
[8] V. Masson-Delmotte et al., eds., "Global Warming of 1.5°C," Intergovernmental Panel on Climate Change, 2019.
[9] G. C. van Kooten et al., "How Costly Are Carbon Offsets? A Meta-Analysis of Carbon Forest Sinks," Environ. Sci. Policy 7, 239 (2004).
