I have been working on soil and vegetation carbon sequestration, and on the potential of bioenergy for climate mitigation for over 20 years, so when I heard about “negative emissions technologies” (NETs) a few years ago, I was pleased to find that I had some expertise on a topic that had become very trendy! Negative emissions technologies are practices and technologies that lead to a net removal of greenhouse gases from the atmosphere. Before the term negative emission technologies came into common usage, the term “carbon dioxide removal” (CDR) was used, and the new fashion appears to be to use the term greenhouse gas removal (GGR) as this encompasses technologies that remove other, non-CO2 greenhouse gases from the atmosphere. So much for cutting down on the over-use of acronyms!
Figure 1 shows the main flows of carbon associated with GGR technologies (shown from panels D to I), compared with fossil fuels (panel A), bioenergy (panel B) and carbon capture and storage (panel C).1
GGR technologies have become the subject of such intensive scrutiny because of the increasing difficulty of limiting warming to well below 2°C above pre-industrial temperatures (the aim of the Paris Climate Agreement2) through emission reduction alone. Because we have failed to effectively tackle climate change to date, and greenhouse gas emissions have continued to increase despite efforts to reduce them,3 it is getting increasingly difficult to hit the 2°C target. Indeed, a majority of scenarios submitted to the Intergovernmental Panel on Climate Change Scenarios Database show that often very significant amounts (20 Gt CO2e/yr) of greenhouse gas removals are required to reach a 2°C target by 2100.3,4,5 Given that most models fail to reach a 2°C target without GGRs,3 it seems impossible that the aspirational target of 1.5°C of the Paris Agreement could be met without GGRs. For this reason, it has become necessary to assess the global potential, feasibility, barriers and impacts of GGRs.
I have recently been involved in efforts to examine and compare the global implications of widespread implementation of GGRs on land competition, greenhouse gas emissions, physical climate feedbacks (e.g., albedo), water requirements, nutrient use, energy and cost.5,6 These studies suggest that no GGR is a magic bullet, and each has its limitations. For direct air capture using amines or, for example, sodium hydroxide solution, and for enhanced weathering on minerals that naturally absorb CO2, the GGR potentials are large, but the energy and monetary costs are high.5 For bioenergy with carbon capture and storage, and for afforestation / reforestation, the potentials are also large, but the land, water and nutrient footprints are high.5 For soil carbon sequestration and biochar addition to the soil,6 there is significant GGR potential (though lower than for the other GGRs mentioned above), but the potential could be realised with much less competition for land, water and nutrients than, for example, BECCS and afforestation, and at much lower cost than enhanced mineral weathering and direct air capture of CO2.
Another recent study with which I was involved suggests that GGR potentials for soils and biochar could be greater.7 In addition, soil-based GGRs could help deliver other Sustainable Development Goals (SDGs), particularly those pertaining to poverty, hunger, climate and life on land.8 Yet constraints due to high uncertainties about the GGR achievable, the need for site-specific options and incentives, social and ecological impacts, and the risk of impermanence have limited soil-based GGRs to date.
When I first began looking in detail at GGRs, I was of the opinion that there was a “moral jeopardy” associated with them, i.e. that relying on GGRs could lower our ambition on emission reduction actions, and might be used as an excuse to continue with business-as-usual emissions for longer.5,9 After the Paris Agreement was signed, I have changed my mind. Given the stringent targets, it now seems to me that we will need both immediate and aggressive mitigation actions plus implementation of GGRs10.
We could start implementing some of these GGRs now: soil carbon sequestration could be started, and could be incentivised through the “4per1000” Initiative that arose from the Paris Agreement,11 with co-benefits for food security and other ecosystem services. Reforestation of land not used for food production and peatland/coastal wetland restoration could also be begun immediately. We could begin large-scale demonstration of bioenergy with carbon capture and storage, or biochar production, in cases where biomass is already being burned for bioenergy – for example, paper and pulp industry and other forestry “waste” stream, thereby doing so with no additional land/water footprint. And we could invest heavily in research and development of other GGRs such as direct air capture and enhanced mineral weathering, to find ways to reduce energy and financial costs that seem to be barriers to implementation at present.10
We also need a better evidence base for GGRs. Although the Smith, et al. (2016)5 and Smith (2016)6 reports collated the best available evidence, the evidence base for many of the impacts of GGRs was weak. More fundamental research on GGRs is necessary, and well as better representation of a variety of GGRs in ecosystem models and integrated assessment models.10
I expect interest in GGRs to continue to grow in the coming years, so we as a scientific and engineering community must be ready to meet the R&D challenges presented, and to help in designing large-scale demonstration projects that will allow us to learn by doing, and to realise economies of scale. And we need to be ready to sense-check the proposed implementation of GGRs and to assess and advise on potential negative externalities.
Getting into the world of GGRs has introduced me to a whole range of new collaborators with very different areas of expertise to my own. This interdisciplinarity has been stimulating and rewarding, and I look forward to working on these issues for a number of years to come.
1 Smith et al. (2016) Environ. Sci.: Processes & Impacts 18, 1400–1405.
2 UNFCCC (2015) The Paris Agreement: http://unfccc.int/files/essential_background/convention/application/pdf/english_paris_agreement.pdf
3 Edenhofer, O. et al. (eds) (2014) Climate Change 2014: Mitigation of Climate Change. CUP, UK.
4 Fuss, S. et al. (2014) Nature Clim. Change 4, 850–853.
5 Smith, P. et al. (2016) Nature Clim. Change 6, 42-50.
6 Smith, P. (2016) Glob. Change Biol. 22, 1315–1324.
7 Paustian, K. et al. (2016). Nature, 532, 49-57.
8 UNDP (2015) Sustainable Development Goals: undp.org.
9 Anderson, K. (2015) Nature Geoscience 8, 898–900.
10 Fuss et al. (2016) Environ. Res. Lett. 11, 115007. doi: 10.1088/1748-9326/11/11/115007.
11 The 4 per 1000 Initiative: 4p1000.org.
Figure 1. Schematic representation of carbon flows among atmospheric, land, ocean and geological reservoirs1,5,6.