A perturbation analysis of the climate benefit from geosequestration of carbon dioxide

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.