Reality check on old ice, climate and CO2

by ANDREW C. REVKIN, February 8, 2010

Richard Alley’s name has been thrown around a bit by bloggers asserting that ice-core records from Greenland show  that carbon dioxide has scant, if any, influence on climate. Dr. Alley, a glaciologist and climate scientist at Penn State, is a longtime contributor to the Intergovernmental Panel on Climate Change, author of a nice history of ice and climate, “ The Two-Mile Time Machine,” and — as many Dot Earth readers are aware — a teacher with musical and terpsichorean talents (see the YouTube video below for his orbital dance explaining how ice-age cycles help show the amplifying power of greenhouse gases).

Apture™

There have been repeated references to his work here by skeptics of human-driven warming, most notably by “wmar.” Here’s an example (link to full comment is here):

The ice tells us about the past, and from Dr. Alley of the I.P.C.C., it is entirely clear that the carbon/temperature link is either a fallacy or negligible. Unlike the I.P.C.C. or any such pro AGW group, the ice cores have no emotions or agendas and simply are what they are … let’s have a look shall we?
WattsUpWithThat…

I sent a query to Dr. Alley about such interpretations of his work and the ice-core record and he sent a reply, the heart of which is pasted below. Where he refers to  GISP2, he’s describing a particular ice core extracted during what was called “Greenland Ice Sheet Project 2.”

First off, no single temperature record from anywhere can prove or disprove global warming, because the temperature is a local record, and one site is not the whole world. One of the lessons drawn from comparing Greenland to Antarctica and many other places is that some of the temperature changes (the ice-age cycling) are very widespread and shared among most records, but other of the temperature changes (sometimes called millennial, or abrupt, or Younger-Dryas-type) are antiphased between Greenland and the south, and still other temperature changes may be unrelated between different places (one anomalously cold year in Greenland does not tell you the temperature anomaly in Australia or Peru). After scientists have done the hard work of working out these relations, it is possible to use one ice-core record to represent broader regions IF you restrict consideration to the parts that are widely coherent, so it is O.K. to plot a smoothed version of an Antarctic temperature record against CO2 over long times and discuss the relation as if it is global, but a lot of background is required.

Second, although the central Greenland ice-core records may provide the best paleoclimatic temperature records available, multiple parameters confirm the strong temperature signal, and multiple cores confirm the widespread nature of the signal, the data still contain a lot of noise over short times (snowdrifts are real, among other things). An isotopic record from one site is not purely a temperature record at that site, so care is required to interpret the signal and not the noise. An extensive scientific literature exists on this topic, and I believe we are pretty good in the community at properly qualifying our statements to accord with the underlying scientific literature; the blogospheric misuses of the GISP2 isotopic data that I have seen are not doing so, and are making errors of interpretation as a result.

Thirdly, demonstration that there have been large climate changes in the past without humans in no way demonstrates that humans are not now responsible. Many people have died naturally but murder still exists; it is up to the police to learn whether a given mortality was natural or not, and up to climate science to learn what is causing ongoing changes (and we have good confidence that most of what is happening to climatic global average surface temperature is being caused by humanity now). Similarly, demonstration that life, and humans, survived warmer temperatures in the past in no way shows that warmer temperatures in the future are good for us. If you don’t care about humans and other things with us here, making a big change in climate might be an interesting experiment. Evolution does respond to climate change and produce novel results. I just happen to have a personal bias (shared, I believe, by the majority of the six-plus billion people on the planet) that we should ask what is best for humanity, and pursue that. An opinion, surely, and not purely scientific, but that’s my bias.

So, what do we get from GISP2? Alone, not an immense amount. With the other Greenland ice cores (which demonstrate that the GISP2 record is quite good and reproducible), and compared to additional records from elsewhere, an immense amount.

> More sunshine from orbital changes produces warming. The magnitude looks consistent with our understanding of the climate system.

> Some of the “wiggles” in temperature (such as the Little Ice Age signal) correlate with changes in solar output. The beryllium-10 record provides an imperfect but useful estimate of the past variations of solar output, after correction for effects of magnetic-field variation on beryllium-10 production. The resulting solar fluctuations have been small over the times of good climate records, with small climate response, as expected. Again, there is no solid evidence for any weirdness, special sensitivity of climate to the sun, or large solar variations, but instead a generally good match to expected behavior of the climate system. (I’m among those who have looked very hard to find weirdness, too.)

> Nothing else really weird appears in forcings of climate change. No major changes are found in space dust, which remains rare enough that it cannot have been very important. Large changes in cosmic rays are documented in response to magnetic-field variations (the Laschamp event of about 40,000 years ago is especially prominent) with no corresponding change in climate, so any cosmic-ray influence on the climate must be very small (a weak correlation can be obscured by noise; a strong control is almost always visible “by eye,” and clearly is absent). Volcanic eruptions and local climate response are recorded, and again appear consistent with expectations of climate science. There may be small but interesting time-variations in eruptions, but the record is almost entirely one of “noise”–if volcanoes could get organized they could be very important agents of climate change, but they aren’t organized. (The recent work of Huybers and Langmuir suggests that on ice-age time scales, the loading and unloading of the planet by ice growth/shrinkage and sea-level fall/rise may weakly organize the volcanoes, but not a lot, and with nothing interesting for our time.)

Climate is surely a lot of things. The data show that the sun’s variations have been small over the times we care about, the climate responds to variations in sunshine caused by orbital changes, but these are slow. CO2 matters a lot. Volcanoes make “noise.” With those in your pocket, you’re a long way to understanding changes in Earth’s climate—not done, but well on your way.

The abrupt-climate-change story remains interesting, though. Today, the salty north Atlantic waters sink before they freeze in the winter. The data indicate that at times in the past, the north Atlantic was fresher so the waters froze before they sank. The resulting wintertime cooling in the north Atlantic was rather severe, and the influences far from the north Atlantic included a general southward shift of the tropical circulations and drying of monsoonal and northern-tropical regions where billions now live. The IPCC gives >90% chance that the melting of Greenland’s ice and other changes in the future will not be fast enough to trigger such a discontinuity over the next century, but >90% is not necessarily 100%. The implications, that slowing down or stopping the melting may buy insurance against a rare but catastrophic outcome, are interesting.

So, using GISP2 data to argue against global warming is, well, stupid, or misguided, or misled, or something, but surely not scientifically sensible. And, using GISP2 data within the larger picture of climate science demonstrates that our scientific understanding is good, supports our expectation of global warming, but raises the small-chance-of-big-problem issue that in turn influences the discussion of optimal human response.

 Link:  http://dotearth.blogs.nytimes.com/2010/02/08/richard-alley-on-old-ice-climate-and-co2/