Carbon capture and storage: climate impacts
One proposal for meeting targets for the reduction in emissions of greenhouse gases (GHGs) is to capture and store CO2 that is emitted by fossil fuel burning power stations. In a recent study, researchers demonstrated that carbon capture and storage (CCS) could help mitigate against climate change, especially in the short term.
Using CCS, CO2 emitted from energy or industrial plants could be captured and stored in secure underground reservoirs. Four main factors that influence the effectiveness of CCS in limiting the amount of CO2 released are:
- The energy penalty, required to capture and store the CO2; this is the extra energy consumed during the capture and storage processes.
- The proportion of CO2 emitted to the atmosphere from a CCS plant (i.e. not captured and stored).
- The overall proportion of global fossil fuel emissions captured by CCS.
- The retention time of CO2 in storage reservoirs. There may be some leakage of CO2 over time.
The researchers modelled a number of scenarios using these factors and various leakage rates, to explore the impact of CCS on climate change. The scenarios included a range of climate responses to different levels of CO2 in the atmosphere. This determines how much leakage could be tolerated to ensure that CCS leads to a reduction in CO2 in the atmosphere, rather than any increase. The extra energy required to capture the CO2 in the first place was also taken into account.
The study revealed that the benefits of CCS in helping to mitigate temperature rises from climate change are significantly affected by different levels of CO2 in the atmosphere: the higher the surface temperature of the earth (in response to higher atmospheric concentrations of CO2), the greater the temperature reduction can be made by CCS. Other important findings from the study suggest that:
- The storage time of CO2 in reservoirs has the most impact on climate change, rather than the fraction of CO2 removed and the energy penalty.
- Lower global surface temperatures could be achieved with CCS up to the year 2100, but by 2500, using CCS in some cases could cause a rise in temperatures. This is because leakage rates over the longer term could offset the benefit of capturing and storing the CO2.
- With an annual leakage rate of 1 per cent over the next 100 years, CCS would still contribute to reduced global temperatures.
- The advantages of using CCS depend mainly on how long the CO2 is stored, but this benefit tends to decrease as retention times are increased because there is some CO2 leakage from the storage sites.
- The absolute benefit of using CCS is greatest when emissions are highest and when a greater sensitivity of the climate to carbon dioxide levels is assumed.
The study suggests that some storage sites may be unsuitable as they allow too much leakage. The researchers compared this idea with the theory that the Earth's climate system can cope with an increased abundance of GHGs in the atmosphere for a short time without a corresponding rise in temperature, provided the amount of GHGs quickly dropped back to the stabilisation level. Using CCS with higher leakage rates could be considered the same as storing CO2 in the atmosphere temporarily, but at lower risk.
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