By Sabine Fuss, Mercator Research Institute on Global Commons and Climate Change (MCC) and IIASA Ecosystems Services and Management Program
Current strategies for limiting climate change to no more than 2°C above pre-industrial levels are centered around a shift towards less carbon-intensive technology, increases in energy efficiency, and changes in management and behavior.
This won’t be enough.
Global carbon dioxide concentrations have exceeded the benchmark of 400ppm, and it is clear that we’re headed for an overshoot. This means that to have a chance of stabilizing climate change below 2°C, we will actually need to extract greenhouse gases from the atmosphere, thus achieving what we call “negative emissions.” This is even more evident when we look at continued population growth, our dependence on existing infrastructure in the near future, and rising living standards in many emerging regions.
In a session on negative emissions at this year’s CFCC conference in Paris jointly organized by members of the Global Carbon Project at IIASA, MCC and CSIRO, and CO2-GEONET, a group of leading international researchers discussed the need for negative emissions and the implications of large-scale removal of CO2 from the atmosphere, and took a closer look at the outstanding questions and uncertainties on the topic.
A wide range of possibilities – but many open questions
The IPCC’s AR5 scenarios show that negative emissions could be achieved by combining carbon-neutral Bioenergy with Carbon dioxide Capture and Storage (BECCS), but also through afforestation. Most of the ambitious climate stabilization pathways show that we would need BECCS by the middle of the century, even though the removed emissions would not outweigh the remaining positive emissions at that point, that is, we would not yet see net negative emissions.
More precisely, the most recent scenarios of Integrated Assessment Models (IAMs) show that to achieve the 2°C limit, negative emissions of up to 13.2 GtCO2-eq./yr in 2100 are needed. This could be reached by BECCS, which might run into problems as competing for land with other demands, or a technology known as Direct Air Capture, which is more energy-intensive. Enhanced Weathering and afforestation might also deliver negative emissions, though of a smaller magnitude. However, all the presented negative emission technologies have their limits and none is a silver bullet. Clearly, there are more cards in the deck than just BECCS and we will have to aim for a portfolio respecting limits and trade-offs with other policy goals, but also opportunities and synergies.
One glaring clear point: negative emissions cannot be used to continue “business as usual” and then remove the bulk of the emissions mid-century. The required carbon flows would simply be too large. At the same time, such a high-emissions world would bring with it major environmental feedbacks, such as ocean acidification. Thus, negative emissions have to be understood as just one element of a mitigation portfolio complementing drastic GHG emission reductions in the near term.
While the large-scale use of biomass and its impacts have been at the center of bioenergy discussions for a while, CCS will also need to be scaled up to massive amounts of up to 25 GtCO2 per year by 2100. However, geology experts at the meeting were optimistic with respect to the storage potentials for these large amounts. The only challenge would be to find enough viable storage sites with assured capacity.
Other challenges include the need to investigate negative emission options that are not yet included in the AR5 scenarios, such as Enhanced Weathering, Direct Air Capture, and a method to improve CCS and BECCS with geothermal energy. How much the combined potential of these negative emissions options will indeed reduce temperatures also depends on the response of the climate system. However, two modelling teams presented new insights on reaction to overshoot, and negative emissions physically needed to keep global warming below 2°C.
While negative emissions are needed at large scale, many questions remain, which will need to be addressed very soon in order for scenarios meet reality. Communication must improve between scientists, politicians, practitioners, but also media and the public. Existing misunderstandings, for example, that negative emissions are just an excuse to continue on a business as usual pathway, or that negative emissions carry the same risks as geo-engineering, need to be resolved.
Read the full session report (PDF)
Sabine Fuss is leading the working group “Sustainable resource management and global change” at the Mercator Research Institute on Global Commons and Climate Change (MCC) in Berlin and holds a guest affiliation with IIASA’s ESM program. She is co-leading (with D. v. Vuuren) the research initiative “MAnaging Global Negative Emission Technologies (MaGNET)” hosted at the GCP Tsukuba Office
Note: This article gives the views of the author, and not the position of the Nexus blog, nor of the International Institute for Applied Systems Analysis.