Ice Core Data: Truths and Misconceptions


Overview (Scientific Content below)

There have been a variety of discussions in the past week about what ice core data tells us. There are several misconceptions about what information the ice core data contains. Some people believe that it is the air bubbles trapped in the ice that tell the temperature history of the Earth and others believe that the ice cores tell the temperature history of the location that the ice core came from.

Both of these believes are in fact incorrect. The reason is simple. It is the water itself that tells the temperature history in the ice cores. Since ice cores are a measure of the water in the ice, what the ice core is actually measuring are the conditions of the oceans that the water originally evaporated from.

There are two methods of determining the past temperature. One is to measure the ratio of heavy and light oxygen atoms in the ice and the other is to measure the ratios of heavy and light hydrogen. Since these are the two components of water there are plenty of both to measure in ice. The ratio of heavy and light oxygen is the standard measurement.

Water that is made of the light oxygen evaporates more easily. Water made of the heavy oxygen condenses more easily. This means that the warmer the oceans are near the location of the glaciers or ice sheets, the more heavy oxygen there is in the ice core.

Another way to explain it is how warm the ocean is near the location where the ice core is from. The less heavy oxygen there is, the colder the water is near the glacier. The warmer the water is near the location of the ice core, the higher the content of heavy water there will be in the core.

Heavy Oxygen Cycle: NASA Earth Observatory

The water that evaporates and falls as snow on a glacier (or ice sheet, but I will only use glacier now) records how warm or cold the water near it was. Then the layer the year after that tells the story of that year. Each layer is recorded one on top of the other. Scientists then measure each layer and count down how many layers they are from today. Then they can know the story told for an exact year. It is like counting the rings on a tree, but it is actually much more accurate. It also tells a much larger story than a tree (or forest) possibly could.

Since Greenland is near the end of the path for the Gulf Stream and most of the water vapor in the atmosphere in that region is from the Gulf Stream, ice cores from Greenland are a good indicator of the ocean temperatures in the north Atlantic Ocean. Ice cores in Alaska tell the story of the north Pacific Ocean. Different regions of Antarctica tell different stories based on the weather patterns of that region.

The simplest reason that it works this way is that cold water does not evaporate very much. Water that is 25 °C (77°F) evaporates about twice as much water vapor as water that is 15 °C (59°F). Since there is much more light oxygen than heavy oxygen, cold water evaporates very little heavy oxygen. The warmer the water, the more heavy oxygen is released.

So ice cores are really telling a larger story than they are often given credit for. That is why they are so useful in understanding the global climate. A coral reef can tell a story, but only at the exact location that the coral exists. Since many of the coral reefs are near the equator, they see less of the change in the Earth’s climate and so they tell a very small portion of the story. This is especially true for the past couple of million years as most of the climate changes have been far stronger in the Northern Hemisphere than they have in any other place on Earth. There are very few corals in that part of the world.

This is also why ice cores in Antarctica can measure changes that are mostly happening in the Northern Hemisphere. Temperatures in Antarctica do not change as much as the ice cores indicate. What does change is how much warm water is close to Antarctica. During a glacial period (ice age) the oceans near both poles are much colder so the amount of heavy oxygen is very small. When the the northern hemisphere is warmer (like now) the oceans have a higher sea level and warmer water is closer to the both poles.

Even if a location in Antarctica stayed exactly the same temperature for 100,000 years, the ice core at that location would tell the temperature record of the ocean that evaporated the water that fell as snow at that location. In this way ice cores do not reflect the temperature of the location they are drilled. Ice cores primarily tell the record of the ocean the snow evaporated from and how far that water vapor traveled.

Any type of record that involves the ratio of oxygen that has fallen as rain can also tell the same story. Water that drips in caves to form stalagmites and stalactites can also be used to determine information like this. In places where there are no glaciers this type of thing can be done, but it is more complicated.

Ice cores give the broadest temperature reconstruction because of how the record accumulates. Each layer is distinct and can provide a wide view of the climate for the region for that specific year. This information is recorded for the period that matches the age of the glacier. The bottom and older layers do get squeezed by the weight above, but reliable ice cores that are hundreds of thousands of year old have been recovered.

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Scientific Content….

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The specific oxygen isotopes are 16O and 18O. The hydrogen isotopes are hydrogen and deuterium. These are often called the stable isotopes. They are used precisely for the reason that they are stable over time. There can also be combinations of the the different isotopes in a water molecule.. The heavier the overall molecule is, the less it will evaporate and the quicker it will condense. Anything that triggers condensation will drop the amount of 18O that is present. Altitude is another factor that makes a difference as it also decreases the 18O content of the ice.

The Inconvenient Skeptic

What this does is complicate the comparison of ice cores. One location will have a different general path that the water molecules take. An ice core from a lower location will generally have a higher ratio of 18O. This makes comparing ice cores difficult. Each one must be separately calibrated to temperature. For this reason ice core data is not often converted to temperature, but left as isotope ratios. Plotting the isotope ratio’s will show the temperature history, but not calibrated to temperature. So scale is a factor that can be ignored or calibrated. Precise scale calibration is usually not needed though as the relative changes are sufficient.

This is also why different ice cores have different ranges in the isotope record. The farther they are from the ocean source, the less the range will be. Some of the Greenland ice cores have a very large range as the Gulf Stream can get warm water close to Greenland. The Taylor ice core from Antarctica has about half the oxygen isotope range that those in Greenland do.

The Vostok or EPICA ice cores deep in Antarctica use the deuterium ratio’s because those will resolve at that distance since so little heavy oxygen makes it that far. That makes the oxygen isotopes less useful in those locations. The light oxygen with the deuterium can make it that far though and those ratios can be used instead of oxygen.

This should make it clear that ice cores do not tell the local temperature. There is no mechanism for the temperature of a location on an ice sheet to dictate the stable isotope ratios in the snow that falls there. It is truly a function of the distance and path from the location of evaporation to the location that the water molecules became part of the ice sheet.

No single type of record carries as much of a global temperature signal as the ice cores do. That is why they are so often used in paleoclimatology. A single ice core reveals a broader picture than any other method. The main limiting factor for ice cores is of course location. They only exist where there is permanent ice sheets. Glaciers that have a flow are also useless as the record from a core is not a time series. That is why only certain glaciers or parts of glaciers can be cored.

Another problem is glaciers on locations that have warm rock (ie on a volcano). The bottom of the glacier is often melting and the age of the glacier is difficult to determine. That makes the temperature of the bottom ice of a core important. Cold ice at the bottom is an indication that the bottom ice was the forming layer of the glacier. That information can also be very, very useful.

Main Source.

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Posted in Science Articles - Global Warming by inconvenientskeptic on October 21st, 2010 at 2:21 am.

18 comments

This post has 18 comments

  1. Phil Scadden Oct 21st 2010

    Scientific content is better when backed by journal references. Some of what you say is in considerable variance to say Jouzel et al 1997 or say Alley 2000

    Furthermore you use that interpretation to push the claim that temperatures for most of the holocene were warming than now which can hardly be supported by evidence of say arctic ice extent, glacial retreat levels etc, never mind other proxies.

  2. Phil Scadden Oct 21st 2010

    Actually I want to retract part of the last comment – I have a local SH view I find when I check with the IPCC report. The NH does indeed appear to have been warmer for the most of the holocene based on evidence presented in WG1.

    However, I stand by published science that I see in saying that isotopes largely reflect the temperature of the site where precipitation occurs. I am aware of the calibration issues associated with ocean wide changes in water sources though.

  3. inconvenientskeptic Oct 21st 2010

    I appreciate your willingness to accept the the early Holocene was warmer than we are today. That is something that not many people with your view very willingly accept.

    There is complexity in the path of the water vapor to the location, but the distance the water vapor travels is very important to the heavy oxygen isotope. The way I stated it is unusual, but my purpose is to help people understand. I indeed succeeded with this article.

    The article I used as a source does have several journal references. Here they are. I did link the source at the bottom of the article.

    # Alley, R., 2000: The Two-Mile Time Machine, Princeton University Press, Princeton, New Jersey.

    # Bradley, R., 1999: Paleoclimatology, Academic Press, Harcourt Brace and Company, San Diego, California.

    # Cole, J. E., R. B. Dunbar, T. R. McClanahan, and N. Muthiga. 2000. Tropical Pacific forcing of decadal variability in the western Indian Ocean over the past two centuries, Science 287: 617-619.

    # Jouzel, J., R. D. Koster, R. J. Suozzo, G. L. Russell. 1994. Stable water isotope behavior during the last glacial maximum: A general circulation model analysis. Journal of Geophysical Research, 99: 25791-25802.

    # McManus, Jerry, 2004: “Collapse and rapid resumption of Atlantic meridional circulation linked to deglacial climate changes,” Nature, 428: 834-837.

  4. Phil Scadden Oct 21st 2010

    In the northern hemisphere, much of the holocene was warmer. Down here is different as you see in the WG1 synthesis. I’m familiar with the local glacial geomorphology and watch my colleague’s high-resolution Be10 work with great interest.

  5. inconvenientskeptic Oct 22nd 2010

    Phil,

    I would expect that the early Holocene in the SH and NH were very different. It was the NH that was experiencing warming. It was only after the warming in the NH and the associated changes to the ocean from the ice water melt affected the sea levels that the SH would have experience warming.

    I obviously don’t have access to the same high-resolution data, but, if the SH was not warming and did not experience warming at the same time, then why do the Antarctic ice cores show the warming pulse in sync with the NH warming if the ice cores do not collect long range ocean temperature data?

    There is no reason for the two hemispheres to warm at the same time. I believe they have different and out of sync cycles of warming and cooling.

    The ice cores show the same Holocene warming pulse 16,000 years ago. That is because ice cores are not location specific to temperature.

    Also… If CO2 is a critical feedback to the Holocene warming pulse, then why didn’t the 40% increase in the CO2 concentration during the early Holocene trigger warming at the same time in the SH?

  6. Rob Honeycutt Oct 22nd 2010

    I don’t have the information right at hand but isn’t it understood that in the early holocene the earth was on a slightly different axial tilt with the north pole pointed slightly more toward the sun? That would explain a warmer NH and a cooler SH than today.

  7. Rob Honeycutt Oct 22nd 2010

    John,
    Again, I’m coming upon conflicting information. I understand what you’re saying is being said in this Earth Observatory article regarding the measuring of oxygen isotopes. But I’m also reading the Alley 2000 paper that Phil posted and Dr. Alley very clear states:

    “Paleothermometry. Ice cores are local paleothermometers, telling past temperature where they are (or where the snow fell, if glacier flow has caused ice in a core to have come from a significant distance). The classic paleothermometer is the stable-isotopic composition of water in the ice core (10). Natural waters typically contain a fraction of a percent of isotopically heavy molecules (in which the hydrogen or oxygen contains one or two “extra” neutrons). The vapor pressure of this heavy water is less than for “normal” light water. As an air mass is cooled and precipitates, it preferentially loses heavy water and must increasingly precipitate light water. At very low temperatures, heavy water has been greatly depleted and precipitation is isotopically light. Empirically and theoretically, isotopic composition of precipitation and site temperature are strongly correlated in time and space (10, 11); colder places and colder times have isotopically lighter precipitation.”

    When I read this I believe he is very clearly stating that it is NOT the point of evaporation that determines the isotope ratio but the point of condensation or precipitation. That would make ice cores a record of those specific locations rather than a broader measure of the sea where the evaporation occurred.

    Thoughts?

  8. inconvenientskeptic Oct 22nd 2010

    What Alley says is correct and that is why ice cores in different locations have different ratio’s and sets of ratio’s.

    It is the variation at the source of evaporation that causes most variation at a specific location.

    For example. Glaciers in Montana, USA always get snow that evaporated in the Pacific. The mountains between the ocean and Montana is decrease the heavy preferentially along the way in a comparable manner over centuries. So Montana will have less heavy than Washington. But the variation in the initial content is what gets recorded in the glacier.

    Hope that helps.

  9. Rob Honeycutt Oct 22nd 2010

    No, I got that about different locations. But that’s why they often don’t calibrate to temperature and leave the data as ratio data.

    But what I hear you saying is that ice cores are a broader measurement related to changes in SST. I hear Dr Alley saying that isotope ratios are specifically a measurement of location of the ice core.

    I’m assuming here that you’re missing a step in how the calibration is done. If you read Alley 2004 that Phil linked to you see that there are a number of processes involved past just straight measurements of isotope ratios.

  10. inconvenientskeptic Oct 22nd 2010

    Think of a chemical reaction where one species decays over time. If the initial concentration is varying over time, that variation will show up later as well.

    Both the initial concentration of heavy and specific conditions at the glacier will impact on the amount in the glacier. Phil was mentioning cooler Holocene in the SH. Ice cores there still indicate warming in the isotopes. Warmer water closer to the glacier will cause increased heavy in the ice core, even if the temperature is constant.

    A colder winter will also increase the heavy content at a location. One does not preclude the other. Changing the initial conditions will generally have the larger affect.

    Not sure how else to explain this. I understand that really high resolution cores can separate summer and winter accumulation based on the isotopes. There is a lot of different analysis that can be done with ice cores. They are very useful in many ways.

  11. Phil Scadden Oct 23rd 2010

    John, I did NOT imply SH was in glacial while NH was warming – BOTH hemisphere warmed in the holocene. I only stated that evidence of past temperature indicates that now is warmer than most of the holocene in SH. If solar was the only force at play, and milankovich forcings were strong enough without feedbacks to control the glacial cycle, then the SH and NH would have antiphased glacial cycles. Instead the pacemaker for glacial cycles is insolation at 65N. With tremendous amount of work in progress in understanding the glacial cycle, it will become clearer, but even qualitatively, you see that CO2 (probably initially CH4) is the feedback that turns a local event in part of NH into a global effect. Try doing a heat balance with solar and albedo forcing alone – this was the objection to the Milankovich forcings when proposed. In short, the 40% increase DID induce warming in SH.

  12. Phil and John: “CO2..the feedback that turns a local event in part of NH into a global effect…the 40% increase DID induce warming in SH.” Undoubtedly there was a CO2 feedback effect. But from the ice core data alone we have no way of determining the actual size of this feedback; it could have been responsible for the bulk of the glacial-interglacial-glacial temperature changes, or almost none of them. Only when we have an INDEPENDENT method of calculating the the marginal CO2 effect on global T can we assign a value to the glacial-interglacial CO2 feedback effect.

  13. inconvenientskeptic Oct 25th 2010

    HR,

    I agree that the 40% increase in energy did turn a local effect global. The melting of the ice sheets and the change in albedo and the change in the sea levels were all feedbacks as a result of the insolation change. That the insolation change was enough to warm the NH while it had a lower albedo is an indication of how much it matters.

    Separating the CO2 portion when such massive changes are taking place is impossible. There is no possible way to determine the feedback factor of CO2. To claim that it was a dominant part of the global feedback is unprovable.

  14. Phil Scadden Oct 25th 2010

    Everything in science is “unprovable”. That belongs to mathematics. However, you can try modelling with and without CO2 and see which gives you the results. While an energy budget has no predictive skill, you must also be able to create a probable global energy budget at any point. Lets you do that for the turning point of LGM while assuming that GHG effect was exactly the same as that. We can make good estimates of global temperature so can estimate planck radiation (on global level), good estimates of albedo from ice extent, reasonable estimates of evaporation and convection from temperature contraints, now try closing that surface budget with GHG.

    As always, produce a better model (with the numbers) and publish it. (Check what others have done first if you think it can done with albedo in NH only)

  15. inconvenientskeptic Oct 25th 2010

    Phil,

    That is a little out of context. Determining a precise value for one factor out of many changes at the same time is difficult. As hr stated, you would have to have independent measurements of CO2 to nail down the impact of the increasing CO2 levels during the beginning of the interglacials.

  16. Phil Scadden Oct 25th 2010

    What do you mean by “independent measurement of CO2”. Do you imply that you cant calculate radiative forcing for known GHG concentrations (despite so many papers that demonstrate their validity – this would be pure denialism in my books unless you can produce a basis for invalidating the experimental evidence). Or you imply that GHG concentration at say LGM cant really be determined within any useful measure of accuracy?

    Also, the statement “CO2 is the critical feedback:” is yet ANOTHER strawman. I am not aware of anyone being confident of the forcing attributions. Only Crowley’s group have looked at LGM from high res modelling point of view and a lot to digest from that study. As for IPCC, WG1 Pg 447, GHG forcing for LGM to pre-industrial is estimated at -2.8W/m2. Radiative perturbation from icesheet and changed sealevel is estimated -3.2W/m2 (with higher uncertainties than GHG), Vegatation and aerosols estimated -1 W/m2.

  17. Phil Scadden Oct 25th 2010

    I should also add that an alternative model for glacial cycle assuming GHG feedback was insignificant would have to produce some physics to show how the radiative effect from change in GHG managed to have no effect. Now that would be interesting reading.

  18. Really nice post,thank you

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