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.

View the full article published in WIREs Interdisciplinary Reviews: Energy and Environment

Text contributed by Jennifer Wilcox