Results from carbon cycle experiments

Validating the model: El Ninos and Volcanoes

Because this is the first time the two-way interaction between climate change and the carbon cycle has been included in a full climate model (i.e. a coupled atmosphere-ocean general circulation model), there is much uncertainty in the results. Recent work has involved validating the model's behaviour against observations of how the carbon cycle responds to El Nino, and the volcanic eruption of Mt. Pinatubo.

El Nino is a climatic event which occurs about once every four years. It is characterised by unusually light winds in the equatorial Pacific region, and unusually warm ocean surface temperatures there. It affects weather patterns across the globe, for example causing droughts in Australia and increased rainfall in western USA.

El Nino has a large effect on the natural carbon cycle too: the normally large release of carbon dioxide from the equatorial Pacific is reduced causing the sea to become a sink of carbon during El Nino events, but the land becomes a source of carbon due to increased respiration of carbon dioxide due to increased temperatures throughout the tropics. The overall result is that atmospheric carbon dioxide levels rise during an El Nino event and fall during a La Nina event (La Nina is the opposite of El Nino).

The figure shows how changes in carbon dioxide concentrations correlate with El Nino. The red triangles indicate observed changes in carbon dioxide and observed El Nino strength, the blue crosses show the same measurements taken from the model. The gradients of the dashed and dot-dashed lines represent the sensitivity of the carbon cycle to El Nino, as given by the observations and the model respectively. It is clear that the model has very similar sensitivity to that seen in the real world.

More details can be found in Jones et al. [2000].

Graph showing observed and modelled El Nino behaviour

Modelled and observed interannual variability in the atmospheric carbon dioxide concentration. Anomaly in the growth rate of atmospheric carbon dioxide versus Nino3 index (a measure of El Nino strength: the annual mean sea-surface temperature anomaly in the tropical Pacific, 150W-90W, 5S-5N) from the pre-industrial control simulation (blue crosses) and the Mauna Loa observations (red triangles).

A major volcanic eruption, such as that of Mt. Pinatubo in June 1991, can have a significant effect on the world's climate for a number of years afterwards. The surface of the Earth typically cools by about 0.5 degrees Celsius after such an eruption and this has an effect on the carbon cycle. The cooling causes a decrease in the respiration of carbon dioxide from the land biosphere and so the concentration of carbon dioxide in the atmosphere decreases. A series of carbon cycle model runs with a simulation of such an eruption found that the model was able to simulate this behaviour. This figure shows the decrease in atmospheric carbon dioxide after the eruption of Mt. Pinatubo once the effect of El Nino and man-made emissions have been removed. The figure shows that the model response to an eruption is realistic.

More details can be found in Jones and Cox [2001 (a)].

Atmospheric carbon dioxide anomaly following a major volcanic eruption. Mean of nine model runs (blue line) and values derived from observations (red line) with El Nino and anthropogenic effects removed.