In recent years, a number of climate change commentators, non-governmental organizations, and intergovernmental organizations have discussed the potential need for so-called “negative greenhouse gas emissions” strategies. It is also anticipated that Working Group III of the Intergovernmental Panel on Climate Change will include a discussion of this approach in its upcoming report pursuant to its contribution to the Fifth Assessment Report. The rationale advanced for focusing on negative emissions approaches are usually the threat posed by burgeoning emissions, which could result in exceeding of critical climatic thresholds in a few decades, as well as system inertia, which could lock in temperature increases associated with radiative forcing for many centuries. The processes that could effectuate permanent removal carbon dioxide from Earth’s atmosphere include air capture, bioenergy and carbon capture and storage, ocean iron fertilization and soil mineralization, and are usually classified as carbon dioxide removal (CDR) geoengineering approaches.
In my next few postings on the site, I’d like to highlight some of the excellent peer-reviewed literature on carbon dioxide removal strategies that has been released in the past few years. In 2010, Stanford professors Long Cao and Ken Caldeira published a study in the journal Environmental Research Letters (open access) that sought to assess both the long-term consequences and level of commitment required to effectuate massive removal of carbon dioxide from the atmosphere. The researchers employed a coupled-climate carbon cycle model, initially integrated under a fixed pre-industrial atmospheric concentration of 278ppm for 5000 years. The model was subsequently integrated under prescribed historical carbon dioxide concentrations between 1800-2008, and then forced with carbon dioxide emissions from 2009-2049 following the IPCC’s A2 emissions scenario. The study then simulated cessation of carbon dioxide emissions and two extreme carbon dioxide scenarios, one in which carbon dioxide was instantaneously set to its pre-industrial level of 278ppm at the beginning of 2050 by removing all carbon dioxide from atmosphere, with carbon dioxide levels in the atmosphere permitted to evolve freely thereafter, and the other scenario in which carbon dioxide was set at 278ppm in 2050 and then held at that level thereafter. In both scenarios, integration of the simulations was continued until 2500.
Among the study’s conclusions:
1. Following an extreme one-time removal of all anthropogenic carbon dioxide from the atmosphere, atmospheric concentrations are restored under the simulation to pre-industrial levels of 278ppm.
a. However, due to efflux of carbon from land associated with responses of net primary production and soil respiration, as well as releases of carbon from oceans, atmospheric concentrations experience an overshoot, with a peak concentration of 362ppm 30 years after the removal. Overall, 27% of removed carbon returns to the atmosphere. Thus, if society would wish to maintain atmospheric concentrations of carbon dioxide at a specified level, it would have to commit itself to long-term removal of carbon dioxide released from land and ocean sources;
b. A one-time removal of anthropogenic carbon dioxide also reduces warming by a little less than 50% at the time of removal, and radiative forcing by two-thirds on centennial timescales.
2. If atmospheric concentrations of carbon dioxide were restored to 350ppm, it would result in surface warming of 1.2°C, which would last for several centuries;
3. The simulated reduction in temperatures in the study yields a cooling of 0.16°C for every 100 PgC CO2 removal. The conclusion that the concept of proportional temperature change to cumulative carbon dioxide emissions is apposite to carbon dioxide removal has implications for assessing the potential effectiveness of such approaches.
4. The study also contains a cautionary note: if effective heat capacity should prove to be less than estimated in the study, or the carbon dioxide degassing timescale proved longer, it could result in temperature overshoots in which initial temperature decreases are reversed when carbon dioxide re-accumulates in the atmosphere. However, the research concluded that they did not observe this result in their simulations.
Of course, it is not likely that deployment of negative emissions approaches would, or in the case of most prospective technologies, could, follow what the researchers themselves characterize as an “extreme” scenario, i.e. a one-time removal of all carbon dioxide at a discrete point. Moreover, it’s far from clear that society would seek to return to pre-industrial climatic conditions, even if that could be effectuated. And, of course, the study did not address issues associated with feasibility, However, this study is very valuable for a number of reasons. First, it provides a preliminary estimate of anticipated reductions in temperatures per 100 PgC CO2, providing a guide to policymakers who might contemplate more limited uses of negative emissions strategies than contemplated in this study. Second, the study provides a pointed reminder of the fact that a negative emissions strategy would likely necessitate a multi-generational societal commitment, with all of the implications that this would hold for governance, ethics and practical logistics. Finally, the study could provide students with an excellent window into the methods, as well as challenges, of simulating the climatic impacts of geoengineering strategies with climate models.
