Climate and Atmospheric Science (ICAS) PhD Projects

Estimating and understanding large-scale CO2 sources and sinks in a warming world

Supervisors: Professor Martyn Chipperfield and Professor E Gloor (Geography)

Introduction
Carbon released to the atmosphere due to anthropogenic activity contributes the most to manmade global warming, accounting for approximately 50% of the anthropogenic perturbation in radiative forcing. Over the last decades a fairly constant fraction (~60%) of fossil fuel carbon has accumulated in the atmosphere while the rest has either been taken up by the oceans or entered pools on land. Understanding the nature and persistence of this balance is important for predicting greenhouse warming in the future. Turned the other way around, current and future changes of this balance indicate a response of carbon pools like the land vegetation to global warming. This is important for detecting unexpected feedback processes. Possible responses could, for example, be carbon release from peat, permafrost soils and from boreal forests due to enhanced fire risk.

Our knowledge and capability for quantifying carbon sources is still quite marginal. Well-known terms of the global carbon budget include the atmospheric burden, ocean carbon uptake and fossil fuel carbon emissions. A range of records also indicate substantial carbon loss to the atmosphere due to deforestation, particularly in the tropics. Balancing the terms of the global budget indicate there needs to be substantial carbon uptake on land – of the same order as fossil fuel emissions. Nonetheless, the nature and not even location of this land sink is well known.

Information on carbon sources and sinks can be inferred from the patterns they imprint on atmospheric concentrations. In order to exploit this information the relation between sources and sinks and atmospheric patterns needs to be established, which is done using atmospheric transport models, and then inverted to deduce the sources and sinks. Atmospheric transport can be modelled fairly realistically using atmospheric chemical transport models (CTMs) which solve numerically the tracer transport equation given winds from weather forecast centres like ECMWF (European Centre for Medium-Range Weather Forecasts). Until recently a main deficiency of the approach has been the limited amount of high-accuracy data. For example, tropical land regions remain very poorly observed by atmospheric data, partially because of the logistical problems involved with transporting air samples to a central laboratory. Not only have the data been sparse but also largely limited to the surface.

In recognition of the limitations of existing data several new programmes have been initiated over the last few years, including vertical aircraft profile sampling, quasi-continuous records from tall towers and CO2 estimates obtained with remote sensing. There are also new satellite missions dedicated entirely to CO2. Besides CO2 itself there is a range of tracers like CO and CH4 that hold information about the nature of carbon sources and sinks. For example, elevated CO concentrations in the tropics are related to biomass burning. Similar to CO2, CO and CH4 have recently become much more densely sampled, partially using remote sensing methods.