A brief introduction to... Ice Core Records (Part 3)

Hello again. This is the last installment on ice core records and arguably the most interesting. I'm going to talk about the CO2 record, CH4 record and dust concentrations found in ice cores. Before we get into the details, I'll explain what CO2 and CH4 concentrations mean.

Remember the brief introduction to glaciation? We discussed how snow becomes buried under more snow. Under enough pressure (ie. at a certain depth) snow becomes firn. This is a kind of state between snow and ice, where snow crystals interlock, closing up pores and trapping bubbles of gas within the firn. When an ice core is extracted, it is the gas within these bubbles that is analysed for CO2 and CH4 concentrations, as these are assumed to be tiny samples of the atmosphere at that point in time. You may have noticed a flaw. The atmosphere at the time the pores are closed up as the snow turns to firn does not reflect the atmosphere when the snow was deposited. There is no standard time lag between the deposition of snow to the transition to firn because precipitation rates are different for different regions, times and climates. These things have to be estimated based on precipitation rates.

A lot of the concern about CO2 concentrations in the atmosphere causing climate change is derived from estimations of CO2 concentrations in the past. In the last two posts we discussed the use of isotopes as climate proxies and most specifically temperature. I have a couple of lovely graphs for you. Again, these are from the EPICA Dome C core, and show deuterium ratio in the black series and CO2 concentrations from the gas bubbles in the ice core in the red series.

Click image for full size

Click image for full size

As you can see, CO2 concentration increases in the atmosphere seem to be a response to an increase in temperature. I've chosen these two graphs because we can see the Last Glacial Maximum in the top one, followed by the Holocene (what we're in now), and something called the Mid-Bruhnes Event in the second graph. This is the interglacial that is considered the most similar to the one we are in now, and may be used to predict how the climate will change in the near future. However, you can see CO2 concentrations in the Holocene continue to increase steadily with temperature remaining around constant.

I'm going to skip over to methane concentrations now because this is swiftly turning into something more than a brief introduction! CH4 concentrations in the atmosphere are thought to be a result of greater wetland extent globally, indicating greater precipitation at lower latitudes and greater temperatures at higher latitudes. The next graph shows methane concentrations from the EPICA Dome C ice core against deuterium ratio in the ice. It shows a clear time lag in methane concentrations following temperature changes, which may be a result of the time it takes for vegetation to respond to climate change.

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The next graph almost confirms the idea that it gets wetter during interglacials and drier during glacials. It shows dust concentrations in the ice. We know that dust will only become suspended in the atmosphere when it is dry, so it stands to reason that it's drier with increased dust fluxes.

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In summary, it is obvious that there is a lot of information to be obtained from ice cores. Whether we can make any useful predictions about climate change in the future from these records is up for debate. The changes we are making to the atmosphere and biosphere are unprecedented and we are certainly not in a state of equilibrium with the climate system, if there is such a thing.

One thing you should almost certainly check out is the Mauna Loa Keeling Curve, it shows year on year CO2 concentrations from the top of Mt Mauna Loa since 1958. See this link for more information. Compare the concentrations seen on their graphs in comparison to CO2 concentrations of the past on the two CO2 graphs above.

As always, thank you for reading. If you have any questions, comments, suggestions, corrections etc etc. just get in touch! Any discussion is good discussion! Staying with the glacial theme, I'm going to explain the concept of till fabric analysis and interpretation and over the weekend I'll introduce the periglacial environment and start explaining some landforms and processes.