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Long-term consequences of storing captured CO2

Klimatické změny
Long-term consequences of storing captured CO2

A recent study has examined the implications of CO2 leaking from storage sites under a variety of scenarios over 100,000 years. The findings suggest that ocean storage of CO2 (prohibited in the EU) would lead to long-term warming of the atmosphere, higher sea levels and acidification and dead zones in the ocean. In contrast, geological storage with low-leakage rates (1 per cent per 1000 years) would lead to conditions similar to a low-emissions future. This implies that selection of low-leakage sites is key to long-term climate stability.

Capturing and storing CO2 emitted from power stations or directly from the air is considered to be a feasible way to combat climate change. However, the selection of storage sites will have crucial implications regarding how effective and safe the long-term storage of the CO2 will be, as CO2 could leak back into the atmosphere from storage sites.

The study took business-as-usual fossil fuel use as its basis, but with emissions sequestered so as to match a low-emission trajectory with global warming peaking at 2°C. This projects much greater use of geological sequestration than usual; carbon capture and storage (CCS) is usually considered as just one of a range of mitigation options, including energy efficiency and increased use of renewables. The study analysed the consequences of a range of CO2 leakage scenarios from this very large-scale sequestration over the next 100,000 years. It considered leakage from different storage sites: deep in ocean waters, in marine sediments on the floor of deep oceans and in naturally occurring geologic underground sites found both on-land and offshore. Future scenarios were based on the continued use of fossil fuels but with full CCS and varying leakage rates.

All future scenarios initially showed mitigation of warming in the short-term. Some of the results suggest:

    • Storing CO2 in the ocean at depths of 2,500-3,500 metres leads to acidification of the water and damages marine life by increasing 'dead zones'. In addition, the atmosphere would warm by 3.5°C by the year 4000 as the CO2 leaks back to the atmosphere relatively quickly due to ocean circulation patterns. This would result in a situation similar to a business-as-usual projection where no CO2 had been sequestered. Sea levels are also projected to rise significantly.
    • For storage of CO2 in deep underground sites with rapid leaking (1 per cent per 10 years), it would take several thousand years for atmospheric warming to become greater than if there had been no CO2 sequestration.
    • With moderate leakage rates of 1 per cent per 100 years from underground storage, there would be greatly reduced atmospheric and ocean warming, ocean acidification and ocean dead zones for more than 10,000 years before conditions would exceed a business-as-usual projection with no CCS. Only with weak leakage rates of 1 per cent per 1,000 years from underground storage, would conditions be similar to a low-emissions future, where the global temperature rise is kept to about 2°C above pre-industrial levels.
    • Storage in deep ocean sediments may be preferable to onshore geological storage or ocean storage, although it is not known if there are enough suitable storage sites. Storage in such locations would significantly reduce and delay atmospheric warming and ocean acidification.

The study suggests that, for geological sequestration on the scale proposed, CO2 would have to be stored for tens of thousands of years if global warming is to be delayed and other major environmental changes such as ocean dead zones are to be avoided. This exceeds previous estimates that CO2 must be stored for 4,000 years to be effective and would entail the use of low-leakage sites.

An alternative option would be to re-sequester CO2 that has leaked from higher-leakage sites. However, apart from difficulties in matching leakage rates with re-sequestration rates, the study suggests this approach would place a burden on future generations for many thousands of years, comparable with problems associated with the long-term management of nuclear waste.

The policy implication of the analysis is that careful selection of geological storage sites is needed to ensure low-leakage rates, thus avoiding the long-term consequences of CO2 leaking from storage in the future.

Source: Shaffer, G. (2010). Long-term effectiveness and consequences of carbon dioxide sequestration. Nature Geoscience. DOI: 10.1038/NGEO896.

Contact: gshaffer@dgeo.udec.cl

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