It may be as slow as watching paint dry, but measuring how fast air bubbles move through slices of ice is far from boring. The slow sublimation of gas bubbles under a temperature gradient in ice cores tells Victoria University’s Dr Ruzica Dadic a complex story about how much we might be missing in measuring past climates, and what that means for the future.
What’s the difference between snow and ice?
Snow is a porous material and therefore the air in snow is always exchanged with the atmosphere. When snow is compacted through overburden pressure, the pores close off and it becomes ice. Ice is not porous and the air that is trapped in air bubbles within the ice remains at more or less the same composition as at the time that the pores were closed off. We can therefore reconstruct past air compositions from studying air bubbles in ice. Depending on temperature and snow accumulation, the bubbles in polar regions can close-off between 70 and 7000 years after the snow has fallen on the ground. In New Zealand, where snow melts and refreezes, the bubble close off can happen in one season.
So the air trapped in ice core bubbles might be hundreds of years younger than the ice itself? Is this a problem for ice core interpretation?
Yes, this is a problem for ice core interpretation and scientists have been working on this problem for a while now. The age for the ice core is extracted from the ice itself and the former atmosphere composition (e.g. greenhouse gases) is extracted from the entrapped air bubbles. But because of the delay between when the snow falls and when the air in the bubbles is isolated from the atmosphere, the difference in age between the snow and the air can be problematic. The other problem, which is the focus of this Marsden project, is that not all air bubbles are trapped at once, so some of them might be a hundred years old and others might be several thousand years old, which means that we have gas that represents different atmospheric compositions in one sample, and currently we can only measure the bulk air composition, which gives us an average air composition over a long time period.
Why is getting an average a problem?
If we are only measuring the average, we can miss peaks in the gas concentration. Let us consider an example; over a period of 1000 years, we have a constant methane concentration of 500 ppb except for a high peak in concentration (1000 ppb) during 100 years somewhere in those 1000 years. If we can only measure the average, the bulk measurement will give us a concentration of 550 ppb, and we will have completely missed the peak concentration. Considering today’s steep increase in greenhouse gas concentrations, it is important for us to know whether such rapid greenhouse gas increases have happened in the past and how they affected the climate.
Can you figure out which bubbles are older than others?
Yes, this is the novel idea of our project. We can measure the pressure in individual air bubbles within a sample, which gives us information about the age of those bubbles. Bubbles that have been closed off long ago will have higher pressure because they have had time to equilibrate with the overburden pressure of the ice, while bubbles that have been closed off recently will still be close to the local atmospheric pressure.
The method we are using is air bubble migration under a temperature gradient, where bubbles will “migrate” toward the warmer side if exposed to a temperature gradient. This migration speed is, amongst other measurable variables, pressure dependent and bubbles with high pressure will migrate faster than bubbles with low pressure.
Is this good news for the human race – if the past actually has higher peaks than we thought, then doesn’t that make today’s emission peaks less likely to cause catastrophe?
Unfortunately, it is a bit more complicated than that, because higher peaks of greenhouse gases in the past would confirm that the atmosphere is less stable than previously thought and that rapid climatic shifts are more likely to have happened in the past. They might have been caused by feedback processes of which we still know little and which may accelerate the changes we are already witnessing in today’s climate.
These interviews showcase researchers supported by the Marsden Fund which, since 1994, has been supporting fundamental, investigator-led research in New Zealand.