Effectiveness and Consequences of Carbon Dioxide Capture and Sequestration

With coal likely to continue to play a very substantial role in energy production over the course of the next few decades, with critical implications for climate policy making, there has been substantial focus in many quarters on the prospects for carbon capture and sequestration (CCS). However, a new study in the journal Nature Geoscience, Gary Shaffer, Long-Term Effectiveness and Consequences of  Carbon Dioxide Sequestration, 3 Nature Geoscience 464-467 (2010) (subscription required) should serve as a cautionary tale for these prospects. This would be a good reading at the graduate school or law school level.

Among the key take-aways from the study, which makes long-term projections of the potential of CCS using the Danish Center for Earth System Science model:

  1. Under the IPCC A2 scenario, total gross and net emissions in the simulation were 3,721 GtC and 822 GtC, with 2,899 GtC sequestered over two centuries. While this is “immense” sequestration, it doesn’t exceed geological storage capacity;
  2. In assessing seve3ral different storage/leakage scenarios, the authors concluded:
  • Ocean storage between 2500-3500 meters: much of the initial warming is mitigated, but it still exceeds some IPCC projections by about 1C because of greater net radiative forcing from non-CO2 greenhouse gases. Ocean storage also leads to a mean pH decrease at 3000 meters of more than one unit and a mean pCO2 increase at this depth of almost 7000ppm; both these factors could have negative impacts on deep-sea life. Additionally, while atmospheric warming drops slightly after an initial warming peak, it rises again to 3.5C by 4000 as some sequestered carbon dioxide leaks;
  • Onshore geological storing yields several potential scenarios:
    • Under a rapidly leaking projection of carbon dioxide, atmospheric warming would exceed those of even the A2 projection with no sequestration;
    • Under a moderating leaking projected ocean warming, acidification and dead zone volume exceed those of A2 projections;
    • In a weakly leaking projection, atmospheric warming would decrease rapidly to levels slightly above present-day global warming;
    • In a best case projection with no leakage from geological storage, relevant indicators would converge to very near AG projection after several hundred years.
  • Carbon dioxide storage would have to last for tens of thousands of years to avoid strong, delayed warming and vast expansion of ocean dead zones, much longer than the 4000 years projected by previous studies. Deep-ocean sequestration can’t meet this storage criterion. Moreover, a leakage rate of 1% or less per thousand years from an underground storage reservoir would be required to maintain conditions close to low-emission projections with no sequestration. Another possible option would be continuous re-sequestration; however this process would have to be carried on for thousands of years, much like the burden imposed by storing spent nuclear fuel.

CCS, even if concerns about energy use and economic viability can be overcome, thus faces some serious questions about the technological viability of minimizing leakage to extremely minimal levels, and imposing this task on many generations to come. The ethics of this approach vis-a-vis alternatives could stimulate some very good classroom discussion.

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