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Abstract A simple climate model is used to calculate the benefit, over time, of geosequestration of CO2 that would otherwise be released to the atmosphere. The analysis is performed relative to two reference cases. The first case is defined by a CO2 concentration profile leading to stabilisation at 500 ppm. The second case is defined by ‘business-as-usual’ (IS92a) CO2 emissions until 2100. The benefits are considered in terms of incremental change (per unit of displaced emission) in temperature and its rate of change, concentrating on the period to 2200. An automatic differentiation procedure has proved a convenient way of performing the calculations. The ‘temperature benefit’ of avoided carbon emission is found to be of order 1 mK/GtC on the time-scale of decades to centuries. This result is model-specific and would scale in proportion to the climate sensitivity of the model. Because of non-linearities in carbon-climate processes, the results have a small dependence (of order 10–20%) on the future emission scenario with a rather smaller contribution to uncertainty arising from model calibration uncertainties that reflect uncertainties in the 20th century carbon budget. Analysis over the longer term, to 2500, considers the effect of leakage of geologically stored CO2 to the atmosphere, and shows that even at 0.1% per annum leakage, about half the climate benefit remains after 500 years.

Abstract The paper describes various techniques for measuring emissions to the atmosphere from geologically stored carbon dioxide, from point, line and area sources at scales of metres to several kilometres. Flux chambers are suitable for measuring small leakage rates from sources at known locations but many samples are required because of large spatial heterogeneity in the fluxes. Micrometeorological eddy covariance, relaxed eddy accumulation and flux-gradient techniques are suitable for measuring leakage from large area sources, while integrated horizontal mass balance, tracer methods and plume dispersion approaches are applicable for line and point sources. Distinguishing between leakage signals and natural fluctuations in CO2 concentrations due to biogenic sources pose significant challenges and the use of naturally occurring tracers such as CO2 isotopologues or introduced tracers such as SF6 added to the sequestered CO2 will assist with this problem. Forward Lagrangian dispersion calculations showed that CO2 concentrations 0–80 m downwind of a point source would be readily detectable above all natural variations for point sources >0.3 g CO2 s−1 (about 10 tonnes of CO2 per year). The inverse problem involves solving for the unknown emission rate from measured wind fields and down wind concentration perturbations. An optimum monitoring strategy for inverse analysis will require continuous measurements of CO2 and tracer compounds upwind and downwind of the possible leak location, coupled with transport modelling to determine leakage fluxes, and to differentiate them from other sources. Computations using The air pollution model (TAPM) showed that expected perturbations in CO2 concentrations at distances of several hundred metres from a leak of 32 g CO2 s−1 (about 1000 tonnes CO2 per year, or about 0.01% per year of a typical amount to be stored) will be detectable, but this anomaly will be very small compared to natural variations, thereby complicating the inverse analysis. While the techniques canvassed here have proven successful for measuring fluxes in other applications, none has yet been demonstrated for geosequestration. The next step is to test them in the field.