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.

Click for full size

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.

Click for full size

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.

A brief introduction to... Ice Core Records (Part 2)

bldgblog.blogspot.co.uk; Photo by Planet Taylor

I didn't keep you waiting too long for the follow up to ice cores pt1, did I? As promised, today we will discuss the cycles of climate change throughout the Quaternary and what we think the reasons for this are. We may need to go off on a tangent on this post too, but it's worth it.

So we've established that we can use ice cores as a record of regional temperature through the Quaternary. But regional temperature is subject to many different variables. If only we had information about global temperature change? Well, the last post mentioned that we'd concentrate on just the polar ice cores. Isotope ratios from ice cores in both Greenland and Antarctica have been compared and it seems that they correlate relatively well, implying that whatever is forcing the climate in the Northern Hemisphere is doing exactly the same thing in the Southern Hemisphere, a global climate forcing. Now for the promised tangent.

We mentioned solar forcing. Cyclic changes in received solar radiation (known as insolation) are caused by the rhythmic variation in the Earth's orbit and axial tilt. These are known as the Milankovitch cycles, named after Milutin Milankovitch. The eccentricity of the Earth's orbit (how elliptical it is) fluctuates on a 100,000 year cycle and affects both the total solar radiation received in a year  (due to the change in distance from the sun) and seasonality (difference in temperature between Summer and Winter). Obliquity is the angle of the tilt of the Earth's axis. This fluctuates between 22.1 and 24.5 degrees on a cycle of 42,000 years and is another control on seasonality. If the North Pole is further away from the sun during Southern Hemisphere Summer, Northern Hemisphere Winter is longer and colder than if it were closer to the sun. Do you follow? The third cycle is the Precession of the Equinoxes. This occurs on a cycle of 26,000 year cycle and is the cycle at which the Earth's axis rotates about an imaginary axis perpendicular to Earth's orbit. The precession is caused by the combined gravitational forces of the Sun and the Moon and is another control on seasonality. For a really great tutorial on Milankovitch Cycles check out this link, it's a great introduction to the concept (you do need flash player I think).

How do these Milankovitch cycles relate to the fluctuations in the climate proxy records we see in ice cores? It is the current paradigm that these cycles in the relative position of the Earth to the Sun control the cycles in the Earth's climate from glacial conditions back to interglacial conditions.

The prevailing cycle in the climate proxy record is the 100,000 year glacial cycle. However, it has been noted by many researchers that the fluctuations in received solar radiation caused by the 100,000 year eccentricity cycles would not be enough to control the massive fluctuations in global air temperature seen between the interglacial and glacial climate extremes. Many summarise that it is probably the combined effect of the precession and obliquity cycles that control or trigger the 100,000 year climate cycle observed over the past 400-800,000 years.

There is however, a hitch. Work by Carl Wunsch analysing the temperature proxy record leads to the conclusion that most of the temperature variation over the past 400,000 years is indistinguishable from a stochastic (internally complex with unidentified feedback mechanisms) system and not a result of solar forcing. In my opinion, the jury is still out on this. Also, as highlighted in the work by Wunsch, an 800,000 year sample size isn't that big when you're only considering eight 100,000 year cycles.

So, make of that what you will! The best place to start reading about this is probably from the beginning, with the Hays et al. (1979) paper in Science, "Variations in the Earth's Orbit: Pacemaker of the Ice Ages". I know it talks about marine sediment cores, but the concept is exactly the same. It's definitely a great introduction to the concept of solar forcing of past climate.

There we have it, a brief introduction to ice core temperature proxies. Tomorrow I will discuss some of the other climate proxies we extract from ice cores: CO2, CH4 and dust flux, and what these tell us about the environments at the time.