Atmospheric carbon dioxide removal: long-term consequences and commitment

Abstract

Carbon capture from ambient air has been proposed as a mitigation strategy to counteract anthropogenic climate change. We use an Earth system model to investigate the response of the coupled climate-carbon system to an instantaneous removal of all anthropogenic CO 2 from the atmosphere. In our extreme and idealized simulations, anthropogenic CO 2 emissions are halted and all anthropogenic CO 2 is removed from the atmosphere at year 2050 under the IPCC A2 CO 2 emission scenario when the model-simulated atmospheric CO 2 reaches 511 ppm and surface temperature reaches 1.8 • C above the pre-industrial level. In our simulations a one-time removal of all anthropogenic CO 2 in the atmosphere reduces surface air temperature by 0.8 • C within a few years, but 1 • C surface warming above pre-industrial levels lasts for several centuries. In other words, a one-time removal of 100% excess CO 2 from the atmosphere offsets less than 50% of the warming experienced at the time of removal. To maintain atmospheric CO 2 and temperature at low levels, not only does anthropogenic CO 2 in the atmosphere need to be removed, but anthropogenic CO 2 stored in the ocean and land needs to be removed as well when it outgasses to the atmosphere. In our simulation to maintain atmospheric CO 2 concentrations at pre-industrial levels for centuries, an additional amount of CO 2 equal to the original CO 2 captured would need to be removed over the subsequent 80 years.

Adjustment of the natural ocean carbon cycle to negative emission rates

“…On the other hand, the scenario simulation PPM-0S shows that the oceanic release of anthropogenic carbon is much higher if the CDR action is done with a higher atmospheric CO 2 concentration with a INE of 48 Gt C (Table 1). This trend confirms the results of Cao and Caldeira (2010), who performed one-time CDR experiments at year 2050 and obtained a peak ocean flux of about 25 Gt C/year.…”

“…In the case of the one-time removal of carbon dioxide at year 0 (PPM-0, Fig. 2a), there is a sudden release of inorganic carbon from the ocean that returns to neutral flux values in less than 30 years as previously reported by Cao and Caldeira (2010) using an intermediate complexity model. The imposition of a linear CO 2 decrease (PPM-10 and PPM-30, Table 1) removes the initial shock and therefore reduces the total ocean carbon release of the CDR as reported in the figure legend (see also Section 3.2 below).…”

“…The involved fluxes may thus be different: this is why we added the experiments in Section 3.4, which indicate that the amount of the ocean carbon release may depend on the initial mean oceanic flux, which may also be a function of the mean ocean state. It is also confirmed that performing a CDR with a larger atmospheric concentration (that is waiting for the ocean to store more anthropogenic carbon) would lead to a substantially higher release of carbon in response to the CDR measure (as found in Cao and Caldeira 2010). In summary, the release of carbon from the ocean may be smaller than found here if the ocean is a larger sink than currently estimated and it is anticipated to progressively increase if the CDR will start at the end of the century in a higher CO 2 world.…”

“…This is clearly an approximation of the full carbon cycle system: the terrestrial part is quicker to react and largely uncertain (at least according to model results, Friedlingstein et al 2006;Johns et al 2011), but the response strongly depends on the parameterization of CO 2 fertilization in the modelled terrestrial vegetation. Cao and Caldeira (2010) found in their simulations that terrestrial net primary production (NPP) is largely sensitive to the atmospheric removal and this eventually drives soil respiration. A large reduction in NPP is first observed because of the reduced atmospheric CO 2 value that stops the fertilization effect, and then a reduction in soil respiration follows because less carbon is transferred to this stock.…”

“…This ocean outgassing counteracts the planned removal effort, while the role of the terrestrial sink is more difficult to evaluate given the fact that the land sink is not directly known. Cao and Caldeira (2010) used an Earth System Model (ESM) of intermediate complexity to study the long term adjustment of the carbon cycle to an idealized one-time instantaneous CDR to preindustrial concentrations occurring at year 2050, using the IPCC SRES A2 emission scenario as baseline. The usage of a more simplified ESM allowed to perform longterm simulations, as the study addressed the temperature changes and carbon flux adjustment between the climate components up to year 2400.…”