A primary outcome of the United Nations Climate Change conference held in Paris in 2015 was the establishment of the long-term goal of keeping global warming below 2°C, with efforts to limit warming to 1.5°C. It has been well-established by climate scientists that achieving these targets will require a combination of CO2 emission mitigation plus removal of CO2 from the atmosphere through various negative emissions technologies (NETs). On average, as a global society, 35 Gt CO2 are emitted each year, a number that has been steadily increasing. Recent climate models suggest that through combined mitigation and removal, an average of 20 GtCO2/yr must be avoided in order to prevent 2°C warming by 2100.
Unlike CO2 mitigation, which prevents or limits CO2 release from point emitters, NETs like bioenergy with carbon capture and reliable storage, land management, enhanced weathering and direct air capture with reliable storage target CO2 that has already been released. This is important for mobile emitters, like the transportation sector, where mitigation is difficult. Additionally, carbon dioxide removal enables low-emitting countries – those who might not have a large impact in terms of emission mitigation – to make a significant impact toward the aforementioned reduction targets in some cases. The NETs reviewed here range in cost from on the order of $10/tCO2 to $1000/tCO2, depending on the removal technology and the magnitude on which it is applied. Given the potential high costs associated with negative emissions technologies and the technical challenges associated with capturing dilute atmospheric CO2, there should be increased efforts toward capturing CO2 at the emission source (i.e., from coal and natural gas-fired power plants) in addition to demonstrating its reliable storage on a meaningful scale (e.g., GtCO2/yr). In addition to these efforts, this study recommends initial focus on land management and bioenergy with carbon capture and reliable storage as low-cost negative emissions options with upper bounds of 6 and 12 GtCO2/yr of removal, respectively. If direct air capture with reliable storage is pursued, there may be lower-cost paths associated with uses that require low purities of CO2 (e.g, less than 50%). Viable enhanced weathering would require low-cost sources of alkalinity on the scale of the amount of CO2 to be mineralized.
The current global dependence on using fossil fuels to meet energy needs continues to increase. If 2°C warming by 2100 is to be prevented, it will become important to adopt strategies that not only avoid CO2 emissions, but also allow for the direct removal of CO2 from the atmosphere, enabling the intervention of climate change. The primary direct removal methods discussed in this review include land management, mineral carbonation in addition to bioenergy and direct air capture with carbon capture and reliable storage. These methods are discussed in detail and their potential for CO2 removal assessed.
Text contributed by Jennifer Wilcox