January 22, 2007When the mites go up…
Filed under: Climate Science, Paleoclimate —
globalwarming awareness2007 @
3:11 pm
Guest Commentary from Andy Baker, U. of Birmingham It doesn’t seem obvious really. Going underground into caves, removing stalagmites and analysing their isotopic composition isn’t the first thing you would do to look for past climate information. But for nearly 40 years, there has been an active, and growing research community that investigates the climate records preserved in these archives. Stalagmites have recently received high profile use in climate reconstructions, for example records from China and Norway have featured in Moberg’s last millennium temperature reconstruction; in a northern hemisphere temperature reconstruction of the last 500 years and even been debated here on RealClimate. So it seems timely to review why on (or even under) earth should research go underground to look at surface climate.
Thus there could be some climate signal preserved within stalagmites, the question is how to decode it. The surface rain will interact with the surface soil and vegetation, which may alter any climate signal containing in the rainwater, or create new soil derived signals, or probably a mixture of the two. After a period of time that will depend on climate, seasonality, vegetation, etc. the water will reach the ground water. In the ground water, it will probably mix with waters of differing ages, smoothing any climate signal it contains. The nature of ground water flow may also introduce non-linearities into the signal. The simplest example is the overflowing bath scenario - imagine that filling your bath up at an increasing rate represents increasing rainfall, and that the bath is your groundwater store, and the plughole and the overflow are the outlets feeding stalagmites. The plughole stalagmite will respond first to the rainfall, the overflow stalagmite will be delayed untill the bath is full. As the bath fills and the storage time increases, the plughole stalagmite will preserve an increasingly smoothed water signal. Therefore, it is a complex system. Over the last 40 years of stalagmite palaeoclimatology, the bulk of the research community was interested in the timescales of ice ages, as one of the big advantages of working with stalagmites is that they can be dated by the natural decay of uranium and thorium isotopes back to around 500,000 years. Over those timescales and temperature changes of 10°C or more, stalagmites are pretty convincing at recording the 1st order climate changes (e.g. glacial-interglacial changes, Dansgaard-Oeschger events, etc..) as the climate signal is much greater than the noise induced in the soil/vegetation-groundwater-cave system, and several papers a year can be read in journals such as Science and Nature (in particular, the Hulu cave record was truly exceptional). Over the last 10 years or so, with increased interest in climate change and climate variability over the last millennium, researchers have started to use stalagmites to look at the more recent past and at higher time resolution. This is despite the climate signal being much smaller (e.g. changes in temperature of less than 1 °C over the last 1000 years) yet the same level of noise in the soil/vegetation-groundwater-cave system. So this is a much harder task, requiring careful sample and site selection. Given the complexity of the signal transfer from the surface to the cave, the only approach is to: (1) work with stalagmites that were actively growing when sampled (so we know the precise age of the top of the archive, and that they were deposited over the period of instrumental rainfall and temperature data), (2) that we use stalagmites which have annual growth rings (in the same manner as tree rings, many stalagmites have annual rings too) so that a precise chronology can be obtained, and then (3) to analyse whichever proxy we are interested in at highest resolution possible to be able to calibrate that proxy against instrumental climate records over the period they exist. If that methodology sounds familiar, then it is because it is very similar to that used with other proxy climate records such as tree rings. The figure (from Proctor et al, 2000) shows an example of that kind of calibration - in that case against regional sea surface temperature records.
Stalagmites are sure to feature in forthcoming reconstructions of climate over the last millennium, as the best samples can form continuously for thousands of years, sometimes with continuous annual growth rings, and with little or no growth related trends, all of which are good conditions for preserving low frequency climate variability. But the complexity of the climate transfer function means that the three tests of chronology and calibration need to be fulfilled for any stalagmite record to be demonstrated to be both accurate and precise enough to be used in a proxy climate reconstruction of the last 1000 years. Additional reading: January 15, 2007Calling All Science Teachers
Filed under: Climate Science —
globalwarming awareness2007 @
4:02 pm
“An Inconvenient Truth,” the Davis Guggenheim documentary on global warming starring Al Gore’s presentation on the subject, provides an accurate, engaging, accessible, thought-provoking and (at times) even humorous introduction to one of the most important scientific issues of our time ( see our review of the movie). In some countries, viewing “An Inconvenient Truth” has actually become a required part of the science curriculum, and with good justification, we think. Given that the DVD is currently selling for $19.99 through Amazon.com, you’d think that the National Science Teachers’ Association ( NSTA) would jump at the chance to quickly get 50,000 free copies quickly into the hands of their members. Yet, when Laurie David, one of the producers of the film, made this offer to NSTA last November, it was summarily turned down on the grounds that the NSTA has a 2001 policy against “product endorsement” (as if Laurie David were trying to shop some new deodorant to high school science teachers). What in the world is going on here? Before continuing with the history of NSTA’s bizarre decision, let us provide you with the most important information: Up to 50,000 US science teachers can receive a free copy of the DVD by filling out a simple request form here . The deadline for requesting your copy is January 18, so if you want a copy, take a few minutes to put in your request right away. Laurie David described her correspondence with the NSTA in a Washington Post Op-Ed, where she notes that an email sent to her by NSTA invoked not only the product endorsement issue, but also a fear that distributing the film would place “unnecessary risk upon the [NSTA] capital campaign, especially certain targeted supporters.” David goes on to point out that one of these supporters is in fact ExxonMobil (whose efforts to spread confusion about climate change are described in a recent report by the Union of Concerned Scientists.) Is NSTA for sale? Did concern about losing ExxonMobil funding lead to NSTA’s timidity about accepting the donation of the DVDs? The NSTA responds to David’s charges here , pointing out among other things that they offered to sell David the NSTA’s commercial mailing list and that the email to her regarding the fundraising issue was unauthorized. We ourselves find the NSTA’s defense unconvincing. While it is impossible for us to know the extent to which ExxonMobil funding has compromised NSTA’s objectivity on global warming, a perusal of the NSTA web site shows that their teacher resources are rather short on support for teaching about the fundamental science of global warming. This contrasts strongly with their in-depth support for the teaching of Evolution. Indeed, the NSTA’s “compromise” of providing a link on their homepage to the independent DVD giveaway strikes us as uncomfortably similar to placing a sticker on a biology textbook disclaiming Evolution as “Theory, not Fact.” Their willingness to link to the giveaway without providing it directly to their members conveys a distinct impression that the film is somehow tainted. Doing a search on “Global Warming” on the NSTA site turns up only a paltry supply of useful educational material. It is also illuminating to go into their “recommendations” section and type in “global warming.” That will turn up this recommended book by Kenneth Green, a fellow of the American Enterprise Institute whose article Clouds of Global-Warming Hysteria in the National Review endorsed Michael Crichton’s view of global warming and called supporters of climate change action “One-worlders and other socialist sorts.” Needless to say, the NSTA recommendations (as of today) did not turn up “An Inconvenient Truth” either in its DVD or book form. Nor did it turn up Revkin’s book directed at juveniles “The North Pole Was Here,” nor any of the other scientifically respectable introductions of which we are aware Perhaps the NSTA policy has not been compromised by its funding sources, but it will have to work a lot harder to convince us. The best way it could do that would be to bring their support for teaching about global warming up to the same standards as their support for teaching about Evolution. Meanwhile, there have been scattered reports of outright censorship of “An Inconvenient Truth” in the classroom. In a widely reported case, one Seattle school district has essentially banned the film. We have also heard from a science teacher in a populous East Coast state, who was forbidden from showing the film after some parents complained that in fact the earth was “cooling, not warming.” (We have been asked to keep this teacher’s identity confidential so as to prevent reprisals). Hopefully these are isolated instances. We are eager to hear from our readers, not only on the issue of censorship of the film, but also with regard to their experiences with teaching about human-caused climate change in the K-12 classroom (and the extent to which “An Inconvenient Truth” has proved a useful tool). January 12, 2007Arctic Sea Ice decline in the 21st Century
Filed under: Climate Science, Arctic and Antarctic —
globalwarming awareness2007 @
6:43 pm
Guest Commentary by Cecilia Bitz, University of Washington Last month a paper I co-authored received considerable media attention. Headlines read “Experts warn North Pole will be ‘ice free’ by 2040″, “The Big Melt: Loss of Sea Ice Snowballs“, and “Arctic Clear for Summer Sailing by 2040: Models Predict Rapid Decline of Sea Ice‘’. The story also reached NPR, BBC, CBC, the Discovery channel, and Fox News, among others. Dr. Marika Holland, the first author of the paper, was inundated with media attention. About a dozen journalists contacted me too. I was impressed by the questions they posed — questions that probably reflect what the public most wants to know. However, after giving lengthy interviews, I would read the resulting article and see my explanations boiled down to a few lines. In this essay, I’d like to explain the science in the paper and give my answers to the most often asked questions. In our paper (with co-author Bruno Tremblay), we examined the September Arctic sea ice cover in the 20th and 21st centuries in climate models, and found occasional decades of very rapid retreat. The most extreme case was a decrease from 6 to 2 million square kilometers in a decade (see Fig 1). This is about 4 times faster than the decline that has been observed in the past decade.
It is common practice to run climate models multiple times with slight variations to the initial conditions. Because the system is chaotic, the natural variability in each run is random and uncorrelated from one run to the next. When an ensemble of runs is averaged, the natural variability is reduced in the ensemble mean, and it is easier to detect a significant trend. An ensemble of runs offers an opportunity to evaluate rare events too, such as extreme sea ice decay. We were in search of evidence for “tipping points“, which several authors have speculated might exist in sea ice. RealClimate places sea ice in the category of systems with “known unknowns” with regard to tipping points. This means we know there are thresholds involving sea ice (e.g., it can cease to exist), but we don’t know when, or if, the climate will arrive at one. Only one of seven ensemble members had an event as extreme as quoted above, and it resulted in near ice-free conditions for September by 2040 (see Fig 1d). (The sea ice grows back at least for some portion of winter for the duration of the 21st century.) However, every ensemble member had an event 5 years or longer at some time in the 21st century when the sea ice retreat was about 3 times faster than the observed retreat since 2001 (see Fig 2). These ensemble members took about 5–10 years longer to become nearly ice-free in September than the most extreme case. As illustrated in Fig 1, the sea ice retreat accelerates during the 21st century as the ice decays and more sunlight is absorbed by the ocean (the positive ice-albedo feedback). Increasing ocean heat transport under the sea ice adds to the melt back. The retreat appears abrupt when natural variability in the ocean heat transport into the Arctic Ocean is anomalously high. We did not find clear evidence of a threshold, which can be difficult to identify given the variability and complexity of the climate system. Therefore we can neither verify or rule-out the existence of a tipping point. Regardless, the rapid declines seen in our runs are a serious concern.
Most common questions asked by journalists 1) How does our model compare with the trend in the observed record? The trends in the seven ensemble members for 1979-2006 span the trend in the observations: Some members retreat a little faster and some a little slower, as expected from the random natural variability in the runs (see Fig 2). The model also reproduces the mean and variance of the observations with good fidelity. 2) Other scientists are predicting an ice-free Arctic in September by the year 2060-2080, why is this model predicting it 20-40 years sooner? First consider estimates based on extrapolation from the observational record. I’ve heard these numbers quoted in the media, but I have not seen a reference to a scientific paper that discusses the analysis in any detail. Figures 2 and 3 illustrate the danger of making an estimate of the future from the observational period. The future trend is not linear, the observational record is too short and the ice-free time is too far in the future to trust extrapolation. If one carries out such an exercise anyway, extrapolation from a linear fit to 1979–2006 gives a zero intersect (indicating the first ice-free year in the future) at about 2110 (see Fig 3). If instead one uses just the last decade, the extrapolation gives 2060. Both estimates are questionable, and so instead we turn to climate models.
3) Is sea ice in our model retreating faster than in other models? Figure 4 shows September ice retreat in 16 models that were archived for the IPCC AR4. The most extreme predictions are from models that have too much or too little sea ice extent compared to observations, so it is important for a model to produce the correct sea ice coverage in the past. Some of the spread is expected from natural variability, but much depends on differing model sensitivity relating to the representation of sea ice, heat transport by the ocean, and cloud cover. It is not possible to identify the most accurate model prediction, although I think it is safe to rule out some of the outliers owing to their poor match to the observations. About half of the models become ice-free in September during the 21st century. I included one ensemble member from our model, CCSM3, which is in the middle of the pack until about 2020. Our model run retreats faster than most after about 2020, but it isn’t radically different. There is considerable uncertainty in future model projections, and Figs 2 and 4 illustrate why it would be better not to focus too much on the year 2040, which to our dismay was highly publicized. The more important message from models is that all but a few outliers predict enourmous sea ice retreat this century. At least a few respectable models predict a nearly ice-free Arctic by midcentury, with a retreat that may be punctuated by rapid events.
4) Is it too late to save the sea ice? The future emissions scenario discussed here is one that assumes modest increases in emissions. If humans can reduce the rise in emissions compared to this, then sea ice retreat would be slower and rapid events would be rarer, according to the IPCC AR4 models. 5) Have we crossed a tipping point? I don’t think we have yet. If we fix the greenhouse gas and aerosol levels at year 2000 values and run the model into the 21st century, the sea ice retreats for only another decade or two and then levels off (some of the ensemble members even recover a little bit). So according to our model, the sea ice does not appear to have passed a threshold yet. We have not done an exhaustive study of any years beyond today, so unfortunately we cannot say with certainty that no tipping points exist. The bottom-line: The retreat can be surprisingly rapid even without clear evidence of a tipping point. I thank Dr. Holland for valuable suggestions to improve this post and providing Fig 1. I thank Ian Eisenman for computing ice extent from the IPCC AR4 models shown in Fig 4. I look forward to reading your comments and questions. |
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To do that, we need briefly to explain how stalagmites are formed. Most simplistically, to grow a stalagmite you need water, and that water has to be saturated with carbon dioxide. Then this water drips from a cave roof, the carbon dioxide in the water will ‘degas’ into the atmosphere, and as part of that process calcium carbonate will form, which will form a stalagmite. Both the presence of water, and the fact that the water is saturated with carbon dioxide, can provide information about the surface climate. The water was, at one time in the past, surface rain or snow, and should contain information about that rain or snow through the composition of its isotopes. And the carbon dioxide saturation comes, not from the atmosphere, but the soil above the cave. Soil carbon dioxide concentrations are orders of magnitude greater than atmospheric, and there is a complex relationship between the concentration of soil carbon dioxide in cave drip waters, temperature and soil moisture.
