Oxygen isotope proxy errors corrected in Greenland ice cores

Scientists, mariners, and weather hobbyists started directly measuring temperature with thermometers globally in the late 1800s. When modern climatologists want temperature data farther back in time than those first global measurements, they have to use things called “proxies.” A proxy for temperature is something that, when calibrated properly, indirectly measures temperature. The most common proxies that are used as temperature stand-ins tend to be tree rings, the amount of an oxygen isotope in ice cores, and coral growth rings.

There are a couple of problems with proxies, however. The first problem is that scientists have to develop an appropriate and accurate calibration method to convert the width of a tree ring to an average annual or summer temperature. The second problem is that a given proxy may well be influenced by other factors beyond temperature, and so calibrating the proxy becomes a difficult and potentially error-prone process. For example, tree rings are a proxy for both temperature and moisture, and so any climatologist who wants to extract just the temperature information needs to discover a way to independently estimate the effect of moisture changes on the tree ring before the effect of temperature on the tree ring can be accurately determined.

A new study published September 17th as a letter in the journal Nature describes a new method to compensate for proxy changes due to elevation in the Greenland ice sheet (GIS) during the Holocene (the present geologic epoch, starting about 12,000 years ago).

This basic problem is that the oxygen isotope used as a temperature proxy in ice cores, 18O, varies with regional temperature, the body of water from which the snow originated, the path the water vapor traveled from its source to where it falls as snow, what season the snow was deposited in, how close the ice core is to the pole, and even the altitude at which the snow fell. A great deal of science has been done to understand how 18O changes with all of those factors, but sometimes errors creep in anyway. In this case, a previously un-corrected error in ice core 18O data from the GIS had confounded understanding the response of the GIS to warming during the Holocene.

The main problem is that the GIS used to be a lot thicker than it is today. At the start of the Holocene, the Earth was transitioning from an ice age to an interglacial, and as a result the Earth was quickly warming and sea levels were rising as a result of melting ice caps. And during that period, the GIS shrank and thinned, effectively lowering the elevation of the GIS at the same time. What this means is that there was some unknown amount of error in the 18O isotope signature in the ice cores, and that error was causing all sorts of problems. The image below, specifically part “b,” illustrates how the 18O varied significantly from one part of the GIS to another (part “a” shows where the cores were drilled, and part “c” will become important in a minute).

GISicecore

The authors set out to find sites that they could use to correct the 18O elevation effects in the GIS. They found two (Agassiz and Renland) that they could justify as being understood well enough to correct the other four sites in the GIS cores. And when they calibrated the 18O for those two cores for their known elevation and distance from the North Pole, they got the image above, part “c.” They’re not exactly the same, but they were close enough to use as calibration sources for the other four ice cores.

When the authors calibrated the other four ice core locations, they discovered that there had been significant elevation changes as the GIS thinned during the transition to the Holocene. In addition, the authors compared their new corrected proxy information to an elevation proxy, specifically the total gas content held in the ice. They compared the estimated elevation changes at two sites (GRIP and Camp Century) using the two different methods and discovered that they were qualitatively and quantitatively similar.

The authors also compared their data against models of the GIS and found that the ice sheet models did not accurately estimate the changes in elevation. In general, the models underestimated the change in GIS elevation, and thus the rate and amount of ice melt. So in addition to correcting a significant bias in the 18O temperature proxy record, the author also found that the GIS is more sensitive to temperature changes than expected.

It is therefore entirely possible that a future temperature increase of a few degrees Celsius in Greenland will result in GIS mass loss and contribution to sea level change [that is] larger than previously projected.

Thanks to Ubertramp who was kind enough to help me obtain a copy of the paper.

Image Credit
Nature

10 comments on “Oxygen isotope proxy errors corrected in Greenland ice cores

  1. Great article Brian. Thanks a lot. This reminds me of the carbon-14 dating methods that had to be calibrated to bristlecone pines back to around 5000 BCE because of a deviation of several hundred years.

    Thanks again.

    • JS – C14 is useful for dating living matter before nuclear testing started in the 1940s, but nuclear blasts created so much of the stuff in the air that C14 can’t be used much of anything these days.

      wufnik – I wrote a blurb in a Carboholic about that myself back in June 2008, although I heard about it from a Scotsman article rather than the Independent. Climate disruption won’t make everything, or everyone, worse by any stretch, but the negative changes are expected to significantly outweigh the positive ones. So planning ahead is a very good idea.

      Unfortunately, since the 1980s, people seem to have lost the capacity to plan farther out than the end of the next fiscal quarter.

  2. Mmm. Brian, on the issue of C14, I’m not an expert, but I believe that some of the archeologists I know would disagree with you. For one thing, I believe C14 dating dates internal samples that were absorbed while the organism was alive, comparing the ratio of C!2 to C14. Of course, contamination can happen on surface samples, and has happened in the past, which means one must take especial care to take only internal samples and shield them from contamination very carefully.

    On a second reading, are you saying that future researchers will be unable to use C14 dating after 1945? I guess that’s possible unless there is a calibration system in place. If that’s what you’re saying, then you may be right, especially if C14 is very unevenly distributed across the surface of the earth. But it’s not really very important at the moment. Any living object that died after 1945 can probably be dated using other methods ;-).

  3. Yep, you’re right on your re-read. IIRC, air detonations were the big cause of C14 contamination after 1945, and when those were banned, C14 contamination stopped being replenished and began to decay back down to the background amounts.

    What I don’t know is if underground nuclear tests also kick up C14. If so, then every time India or Pakistan or North Korea test detonates a nuke, the C14 calibration has to be reset again.

  4. Very nice job of making cutting edge research accessible to the general population. Unfortunately, I don’t think that the crowd who trusts Exxon-funded ‘scientists’ more than they trust the National Academy of Sciences will bother reading it.

    • Thanks.

      You’re almost certainly right that posts like this don’t hit the people who need to know it the most. But I still feel the need to write this stuff, for those rare occasions when someone with an open mind stumbles by.

  5. Thanks for this revelation; I like to keep up with these developments and I appreciate the effort people put in to honing the tools of science, especially on the important question of sea level rise. (Important for me anyway, living close to the sea.)
    Unfortunately I think Joe Public neither understands the science debate nor cares about it, and just wants to cut to the chase, which is why I turned to a mixed fact/fiction format to push the message. Take a peek at my own contribution at: http://www.strategicbookpublishing.com/TheLostEra.html and have a good laugh. (Or email me at losteranz@xtra.co.nz if you would like to see a few snippets of it.) Thanks again,
    Ian Marsden

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