Mazzotti et al. (2013), Vichi et al. (2013), and Smith and Torn (2013), our three papers dealing with CDR science and technology, all raise issues that complement the integrated assessment models. Mazzotti et al. addresses the “net carbon” aspects of direct air capture (DAC)—DAC’s carbon intensity—by exploring a benchmark CO2-absorbing system where CO2 emissions associated with accomplishing DAC can be a significant fraction of CO2 removed from the atmosphere; Chen & Tavoni treats this generalizable feature of CDR explicitly.
Vichi et al. raises and quantifies the important issue of ocean outgassing associated with CO2 removal from the atmosphere (Kheshgi et al. 2005). Just as CO2 emissions into the atmosphere result in CO2 passing from the atmosphere to the ocean to sustain equilibrium at the ocean’s surface, so too do flows from ocean to atmosphere accompany atmospheric removal. Chen & Tavoni incorporates the results of Vichi et al.
Smith & Torn presents a systematic quantitative discussion of inputs to biological CDR strategies: land, nitrogen, phosphorus, and water. The authors also quantify the CO2 and nitrous oxide (N2O) emissions associated with these inputs; emissions of N2O, a potent greenhouse gas, are sensitive to nitrogen inputs and have received little attention. The falling water table usually associated with afforestation is also discussed, as well as soil salination and biodiversity. This paper adds an important dimension to CDR research and raises the prospect that biological versions of CDR may largely transfer environmental risk from the atmosphere to the land.
In the rest of this section, we identify further features of these three papers.
Direct air capture with chemicals (Mazzotti et al.)
Mazzotti et al. presents an optimization of the same benchmark system that was studied, but not optimized, in the report on DAC released in 2011 by the American Physical Society (Socolow et al. 2011). Optimization across three variables (fraction of CO2 captured, air flow velocity, and sorbent solution velocity) reduces the estimated capture cost only slightly, because key cost assumptions for capital equipment are not changed. The cost estimate (in the range of $500 per ton of CO2) is used as an input in the integrated assessment papers by Fuss et al. and Chen & Tavoni.
The APS report and Mazzotti et al. propose that CO2 removal from the atmosphere will almost certainly be substantially more expensive than reducing (by retrofit or rebuild) nearly all of the emissions from large and concentrated sources of CO2 at coal and natural gas power plants, refineries, cement plants, and many other industrial facilities. The fundamental reason is that the concentration of CO2 in the flue gas at these facilities is much higher than in air. The ratio for a coal power plant is roughly 300–12 % CO2 in the flue gas, 0.04 % in air. This ratio affects the frontal area facing the incoming gas, but this is only one of several contributors to the cost ratio. The path of flue gas at a coal plant can be longer, the percent of CO2 removed can be larger, materials can be allowed to be more expensive, and operating costs can be a higher fraction of total costs. In Mazzotti et al., the ratio of total costs is 6 to 8.
Several start-up companies are seeking to reduce the cost ratio further. Lackner, for example, speculates that DAC could take the form of large numbers of low-unit-cost factory-produced absorbers. (Lackner et al. 2012) Arguably, DAC could experience greater cost reductions than flue-gas capture, because a DAC facility has only a single objective, whereas conventional CCS at a large industrial facility is an add-on, complicating and compromising the core activity (e.g., producing power). Nonetheless, it is hard to imagine a future where many coal power plants spew their CO2 into the atmosphere while, simultaneously, many DAC plants are removing CO2 from the atmosphere. It seems far more likely that DAC deployment will not begin in earnest until very few large concentrated sources remain.
Perhaps DAC can expand greatly with little change in unit cost, making DAC a “backstop” technology. By contrast, BECCS will eventually reflect land constraints. Moreover, DAC may require much less land per ton of CDR than BECCS and may create fewer land-use conflicts. In common, BECCS and DAC require low-cost, publicly acceptable, and abundant geological storage capacity for CO2.
Ocean outgassing (Vichi et al.)
CO2 is removed from the atmosphere by natural processes that restore equilibrium. Extra CO2 in the atmosphere leads to some fraction dissolving into seawater at the ocean’s surface, and a similar redistribution (though much more complicated) occurs for vegetation. Over the past several decades, on average (but with large year-to-year variation) the net atmospheric removal rate has been about half of the emissions rate from fossil fuel combustion (with roughly equal contributions from ocean and land removal). One can anticipate, therefore, that deliberate removal of CO2 from the atmosphere should be accompanied by ocean outgassing of CO2 and (with less certainty) reduction in the global stock of carbon in biomass. These are negative feedbacks on the CDR enterprise. Vichi et al. may be the first paper to estimate ocean outgassing when the CO2 is removed over an extended period of time. Cao and Caldeira estimated ocean outgassing when CO2 is removed from the atmosphere at a single moment (Cao and Caldeira 2008).
Vichi et al. studies two related extended campaigns that in the absence of outgassing would remove about 490 GtCO2 (130 GtC) from the atmosphere over a 30 year period (about 16 GtCO2/yr, about half the current rate of fossil fuel emissions). As discussed in the previous section, CO2 removal rates this large and larger are foreseen by the IAMs. In the first campaign, a global CDR industry follows a prescribed emission-reduction schedule: it removes 16 GtCO2/yr for 30 years and ignores the outgassing; in response 50 GtCO2 outgases from the ocean, a 10 % negative feedback. In the second campaign, a global CDR industry follows assures that the net CO2 removal is 16 GtCO2/yr when outgassing is taken into account. This requires removing somewhat more than 16 GtCO2/yr, because the concentration difference between ocean and atmosphere is always larger; the associated outgassing is slightly greater, 66 GtCO2. The authors warn that their results are specific to an initial atmospheric concentration of CO2 of 348 ppm (the concentration in 1988) and for a specific ocean model. Nonetheless, a crude representation of outgassing in Chen & Tavoni indicates that this negative feedback can significantly reduce the scope for CDR.
Although the terrestrial sink is much less well understood than the ocean sink, by a combination of measurement and modeling a coarse sketch of the geographical distribution of the terrestrial carbon sink is emerging. Eventually, models will incorporate a CDR-efficacy factor that accounts for all sources and sinks. For now, a conservative estimate—aligned with the strength of the negative feedbacks from the ocean and land observed today—would assume that a one-ton reduction in the atmospheric stock of CO2 requires two tons of CO2 to be removed from the atmosphere.
Land, nutrients, and water for biological CDR (Smith & Torn)
Smith & Torn studies the environmental aspects of two biological CDR strategies: BECCS and afforestation. Their BECCS system is based on switchgrass grown in a temperate climate. Afforestation is modeled as 50 years of carbon build-up in tropical eucalyptus plantations. In both cases, these choices are intended only to be illustrative: to illuminate qualitative issues common to many biomass strategies and to provide templates for analogous calculations bearing on other species and locations. They estimate demands on land, water, nitrogen, and phosphorus and they identify impacts on albedo, biodiversity, and land tenure. All quantitative estimates are scaled, helpfully, to a global enterprise removing carbon dioxide from the atmosphere at a rate of 3.7 GtCO2/yr (1 GtC/yr). Here, we report a few of their sobering results.
The key estimate is the productivity of land, i.e., the yield in tons per hectare-year (t/ha-yr). For the Smith & Torn switchgrass plantation producing feedstock for BECCS, the estimated yield is 5.6 to 23 tbiomass/ha-yr. Note that the high estimate is four times larger than the low estimate. Assuming that the carbon intensity of energy from switchgrass is 15 GJ/tbiomass, the estimated yield of primary energy in the form of switchgrass feedstock (rounding off) is 80 to 350 GJ/ha-yr. Compare this range with the primary biomass energy yield at the end of the century assumed for two scenarios in Edmonds et al., 600 GJ/ha-yr (about 300 EJ/yr grown on about 500 Mha, see Table 2). A rate of improvement of yields of 0.6 %/yr to 2.3 %/yr for 90 years would be required for Smith & Torn’s estimated yield today to grow to the yield in 2100 in Edmonds et al. This range brackets the 1.6 %/yr long-run record of yield improvement reported by the Food and Agriculture Organization of the United Nations.
The “net carbon” issues for DAC explored in Mazzotti et al. reappear in the BECCS system examined by Smith & Torn. The authors estimate that about 2.2 tons of carbon must be fixed in switchgrass by photosynthesis for each ton of carbon sequestered as CO2 in a geological formation via BECCS, to compensate for emissions associated with harvesting, transport, and electricity production. Smith and Torn report that switchgrass is 42 % carbon; as a result, to sequester one ton of carbon per year via BECCS will require growing 5.2 tons of biomass per year, corresponding to 0.23 to 0.94 hectares.
As for land demands for afforestation at eucalyptus plantations, Smith & Torn assume that carbon builds up in trees for 50 years at an average rate of 1.3 to 4.0 tC/ha-yr. Thus, each ton per year of CDR via afforestation will require 0.25 to 0.77 hectares. Evidently, in Smith & Torn, the land required for an identical level of CDR activity is nearly the same for a fifty-year afforestation project and for BECCS. At the end of the 50 years, new land will be needed for afforestation (the eucalyptus trees will have reached maximum size), but, in principle, the land planted in switchgrass for BECCS can keep producing.
If hundreds of millions of hectares are dedicated to carbon management, other land will experience knock-on effects. For example, a change from pasture to energy crop for BECCS will require that fodder be grown elsewhere and may elicit feedlot cattle-raising. These knock-on consequences are subsumed under the label, “indirect land-use change,” or “ILUC” (Searchinger et al. 2008; Fargione et al. 2008). They will have their own climate impacts—not only CO2 emissions but also changes in albedo and evapotranspiration. Including ILUC in the assessments of BECCS and afforestation was beyond the scope of Smith & Torn.
Smith & Torn estimates that an average application rate of 80 kgN/ha-yr of nitrogen fertilizer is required to achieve the assumed switchgrass yields. The Smith & Torn reference CDR level of 1 GtC/yr (3.7 GtCO2/yr), therefore, will require an annual input of 19 to 75 million tons of nitrogen. By comparison, today’s annual global fertilizer production provides about 100 million tons of nitrogen. Today’s rate also roughly equals the global rate of biological fixation of atmospheric nitrogen by microorganisms. Increased nitrogen availability has fed the world, but it has also had many adverse environmental consequences, such as eutrophication of coastal waterways and increased production of nitrous oxide (N2O), a potent greenhouse gas. BECCS can be expected to exacerbate these adverse consequences, even as it retards climate change. The authors speculate, however, that future nitrogen requirements to achieve the same yield will decline as fertilizer is delivered more effectively.
Hundreds of millions of hectares dedicated to biomass CDR is an immense demand on land. Integrated assessment models that call for large demands on land have long incorporated ecological constraints that reduce the scope for bio-energy in climate change mitigation (van Vuuren et al. 2009). It is already time for broad discussion of these ecological constraints and their implications. Climate mitigation modelers, land use planners, agricultural economists, agronomists, foresters, plant physiologists, and genetic engineers—this is only a partial list of those who must participate.