The volcanic pollution results in a substantial reduction in the direct solar beam, largely through scattering by the highly reflective sulphuric acid aerosols. This can amount to tens of percent. The reduction, is however, compensated for by an increase in diffuse radiation and by the absorption of outgoing terrestrial radiation (the greenhouse effect). Overall, there is a net reduction of 5 to 10% in energy received at the Earth’s surface.

Clearly, this volcanic pollution affects the energy balance of the atmosphere whilst the dust and aerosols remain in the stratosphere. Observational and modelling studies (e.g. Kelly & Sear, 1984; Sear et al., 1987) of the likely effect of recent volcanic eruptions suggest that an individual eruption may cause a global cooling of up to 0.3°C, with the effects lasting 1 to 2 years. Such a cooling event has been observed in the global temperature record in the aftermath of the eruption of Mount Pinatubo in June 1991. The climate forcing associated with individual eruptions is, however, relatively short-lived compared to the time needed to influence the heat storage of the oceans (Henderson-Sellers & Robinson, 1986). The temperature anomaly due to a single volcanic event is thus unlikely to persist or lead, through feedback effects, to significant long-term climatic changes.

Major eruptions have been relatively infrequent this century, so the long-term influence has been slight. The possibility that large eruptions might, during historical and prehistorical times, have occurred with greater frequency, generating long-term cooling, cannot, however, be dismissed. In order to investigate this possibility, long, complete and well-dated records of past volcanic activity are needed. One of the earliest and most comprehensive series is the Dust Veil Index (DVI) of Lamb (1970), which includes eruptions from 1500 to 1900. When combined with series of acidity measurements in ice cores (due to the presence of sulphuric acid aerosols), they can provide valuable indicators of past eruptions. Using these indicators, a statistical association between volcanic activity and global temperatures during the past millennia has been found (Hammer et al., 1980). Episodes of relatively high volcanic activity (1250 to 1500 and 1550 to 1700) occur within the period known as the Little Ice Age, whilst the Medieval Warm Period (1100 to 1250) can be linked with a period of lower activity.

Bryson (1989) has suggested a link between longer time scale volcanic variations and the climate fluctuations of the Holocene (last 10,000 years). However, whilst empirical information about temperature changes and volcanic eruptions remains limited, this, and other suggested associations discussed above, must again remain speculative.

It would be fantastic if we could create an ‘ice core’ with a known concentration of atmospheric gas and then use the different methods to extract the gas, measure its composition and compare with the starting gas. I’m not sure but don’t think that such an experiment has been done in this explicit manner. It would not be easy to freeze water incorporating a known concentration of gas in bubbles. However, one experiment we can readily do is to measure the adsorption of the different gases on to ice at the crushing temperature of -25 degrees C. This will give us some handle on the magnitude of the effect.

In addition to the problems related to extraction of the gas we don’t fully understand the processes that are occuring when a gas bubble is formed in the transition from firn to ice. The first effect is thermal and gravitational separation of gas in the firn layer. The gas at the base of the layer has a slightly different composition to that at the surface as a result of gravitational settling in a gas column many 10′s of metres thick. The second problem occurs at the point of closure of the gas bubble. Imagine ice crystals gradually closing a bubble off through processes of crystal growth and grain boundary migration. There are stages where a capillary exists between free air and gas trapped in the incipient bubble. We know that there is a very marked change in composition with respect to the lighter gases. This is best seen when looking at the ratio of the light noble gases He and Ne. John A also mentions interesting effects due to the inclusion of brine, presumeably resulting from marine aerosol which is largely NaCl.

Then after the gas bubble has formed I’m very interested in what happens to it once it moves deeper into the ice column. The hydrostratic pressure increases on the bubble, ice flows and so it is also presumeably in a stress field that includes a non-hydrostatic component. The bubble might migrate, especially if it is trapped at grain boundaries. How the gas composition of the bubble changes, if at all, through these processes is not known.

I think there are a large number of fundamental studies of bubble dynamics in ice that remain to be done. I’m more than happy to keep everyone informed of progress and perhaps set up a web page so you guys can see what we’re up to.

Exactly my thoughts. Is something like this involved in the current work you mentioned, Paul?

The first reason for wanting to crush rather than melt is that you only release gas that is trapped in bubbles. There may also be a component dissolved in the ice that you don’t want. Secondly, melting the ice may lead to some dissolution of the gas into the meltwater. Believe me it is not easy to fully degas a small water sample, at least when you are looking for accurate gas compositions. Also when you melt the ice, you do so under a vacuum and it becomes necessary to strip this water from the gas sample before chromatography or mass spectrometry. This is either with a chemical trap, e.g magnesium perchlorate, or cryogenically by passing the gas sample thrrough a cold trap at -70 degrees C. This is an added experimental complication though not difficult to do in practise.

However, there are also problems due to crushing. First is you expose the gas to a very high surface area of ice at cold conditions. It is possible that there will be some gas adsorption onto the ice surface thus modifying the released gas composition. Secondly, not all bubbles may be opened and there may be cracks through the ice to some of these resulting in diffusional release of gas. This will be mass dependent and lead to a modified gas composition

There’s one further BIG problem with carbon dioxide – the solubility increases if there’s a slight bit of brine in the snow (which there usually is). The result is that some of the CO2 dissolves in the brine depleting the remaining gaseous CO2. This dissolution is highly temperature dependent.

So when the gas is released either by crushing or melting, there is a finite amount of carbon dioxide that remains in solution that is simply thrown away.

What is the result? By either method described above the carbon dioxide reported is depleted by the chemical conditions, perhaps leading to the spurious result that the current level of carbon dioxide in the atmosphere is “unprecedented in the last 450,000 years”

I’ve always wondered if its possible to create an artificial ice core with a known concentration of CO2 and see if the techniques so commonly used are able to recover the correct concentration.

JerryB, the solubility of CO2 in ice is extremely low (under normal pressure, deep cores are put to rest for at least a year to expand again). Thus it doesn’t help to melt the ice for a higher yield. To the contrary, as CO2 is quite soluble in water, some unknown amount of it will be retained in the meltwater, disturbing the reading…