There’s a new study out in GRL (press release), the third (at least) in the last couple of years using plant remains that have been newly uncovered by receding ice caps in Greenland and Baffin Island to estimate the most recent date at which temperatures must have been about the same as current. I’ve now seen
four numerous notices on Twitter about it, and a popular media story on it has just popped up. That story contains the basic storyline statement: “…summer temperatures in the Canadian Arctic are higher today than they’ve been in at least 44,000 years, and are likely the warmest they’ve been in 120,000 years.”
Interestingly, the author of that piece says “I’m a journalism professor and science journalist. Miller told me about his results last spring, and I’ve been waiting for his study to be published in a peer-reviewed journal before writing about them.” Well, given that he’s a journalist and had six months, it would have been nice if he’d asked some basic level investigative questions during that time instead of taking stated conclusions on faith.
When I first started reading this I was actually excited about novel evidence that might be relatively decisive regarding how unique current temperatures are in the arctic, relative to the Holocene/Pleistocene. Alas, as I continued reading that hope faded quickly and I now instead marvel at how the authors could state the conclusions they do, given the methods and data presented.
The study dates plant and lichen remains (using C-14 methods) found right at the edge of melting ice caps, on Baffin Island, Canada. Because ice caps expand and contract only by snow accretion and ice melting, and not much by movement, they don’t disturb the ground like glaciers, and it is therefore possible to find intact plant remains that have not been moved and mixed by the ice. Dating them gives the approximate last date that no ice covered that particular piece of ground, and therefore presumably, the date that air temperatures were last about the same as current.
The authors made over 300 specimen collections, with 145 datings, along a ca 1000 km transect on the east side of Baffin Island (see their Figure 1 below). Most specimens were mosses in the genus Polytrichum, with some fruticose lichens collected from rocks at a few higher elevation sites. Of the 145 dates, 135 (93%) dated to < 5000 kya, all of which were Polytrichum, (no lichens) and ranging from 236 to 4900 years, with a mean of 1243.
Figure 1. Eastern Canadian Arctic DEM showing collection sites, with an expanded map of the southern field area. Circles (color-coded by their 14C age) identify the 135 sites where rooted plants were collected the year of their exposure along receding ice cap margins that have been 14C dated 47 ka. The margins of the LIS at the LGM and 9 ka [A. S. Dyke, 2004] solid lines, and the 1 km LGM surface elevation contour (dotted line; [A.S. Dyke et al., 2002]) show that sites dated >47 ka were elevationally above the LIS and supported only local ice caps. P: Penny Ice Cap; B: Barnes Ice Cap. A tight cluster of 5 coastal sites (circled) exhibit anomalously low changes in their ELAs and are excluded from the regressions in Fig. 2A).
The other ten samples are much older, all > 47 ky. These samples include six lichen and four Polytrichum, collected from four sites. Those sites are all clustered nearby each other (diamond symbols in the figure), and the four moss samples are from one sampling location. The authors state in another publication describing their methods, that they prefer moss over other plant materials, such as those used in Greenland in a study published by others last year, because it has a relatively short life cycle, which presumably this keeps the dating tighter. Therefore, we can assume that at the three sites where lichens were used, there was no Polytrichum available, or they would have used it. Why is there no old moss there? Does using lichens instead, which differ in longevity, make a difference?
Note the open circle near the bottom of the figure, the one that contains the three orange circles. About this the authors state: “The five largest low-elevation outliers with respect to this trend are a tight cluster of coastal sites (circled in Fig. 1), where sea ice persists longest in summer [Moore, 2006]; hence, these sites are unlikely to be representative of the regional snowline.“, and that they “exhibit anomalously low changes in their ELAs [equilibrium snowline altitudes]”. In simple terms, these sites are unusual with regard to the snow and ice dynamics in the local area. Well, as the figure clearly shows, that area is very close to the cluster of sites (diamonds) where all the older samples were taken. Further, those samples are also 270 meters higher on average than the 135 samples aged at < 5 kya, (which have a mean elevation of 900m). So, it is entirely reasonable to ask whether the combination of the higher elevation and whatever factor(s) creates the unusual ice and snow persistence in the area, aren't partly or wholly responsible for the existence of these older samples, i.e. that they are local exceptions not reflective of the larger climatic phenomenon of interest.
But even if they are not, we have four sites clustered together at one end of the 1000km sampling transect that give very anomalous results relative to the 135 samples collected all along that transect. So why in the world are they focusing on those four sites, to the exclusion of the much more geographically extensive 135? How can the authors just blow past this fact without discussing why in any way? Reviewers, HELLO?? The authors then note the oldest radiocarbon ages are near the limit of the method and then hypothesize that these are therefore probably under-estimates. But by how much? This they address by referencing Greenland ice cores that (they state) show that one has to go back at least 110,000 years to get temperatures warm enough to have open ground. And the Discovery article and press release of course, mention this larger number.
The authors also include rough calculations of time required to melt a cap of 70m maximal thickness (assumed, based on hill diameter and stated ice weight constraints), assuming current, observed melt rates of 0.5 m/yr (getting the time required wrong in the process, stating it as 100 years, when it’s obviously 140). This means that it would have taken an extended period of warmth (140 yrs) to fully remove some ice caps of a given initial thickness, in the past. But we don’t know the original thicknesses of these caps before the dated plants grew there, so what’s the point? This is a confused discussion. The rest of the paper goes on to use estimated rates of snowline change since the mid 20th century to argue that “many climate models underestimate the magnitude of Arctic amplification, even on relatively long timescales.“, which of course implies that said amplification is going to be worse than predicted.
The authors conclude with this statement, which really pretty much gives away their bias:
“These findings add additional evidence to the growing consensus that anthropogenic emissions of greenhouse gases have now resulted in unprecedented recent summer warmth that is well outside the range of that attributable to natural climate variability.”
No it does not thank you very much. The study doesn’t even address natural variability. And I thought the consensus was supposedly already pretty much full grown…that’s what I’ve been hearing anyway. And lastly, an area of a few square miles on Baffin Island upon which the thesis rests, does not deserve the general phrase “Arctic Canada” used in the title.
Addendum: Reported ages for the 135 Polytrichum samples < 5000 years old:
236 358 436 441 441 454 460 531 538 544 544 555 557 558 559 559 559 562 563 564 569 569 577 579 587 597 599 605 609 638 642 642 644 646 648 651 651 651 651 653 653 655 655 656 656 657 657 658 658 658 658 693 693 705 707 708 710 712 713 714 721 724 735 738 751 834 901 904 904 912 1003 1058 1061 1062 1066 1067 1070 1071 1073 1073 1073 1073 1073 1077 1077 1081 1102 1104 1104 1104 1113 1154 1166 1169 1228 1331 1378 1515 1534 1549 1581 1601 1603 1607 1612 1630 1633 1635 1635 1640 1664 1671 1680 1694 1698 1766 1846 1984 2000 2296 2549 2631 2636 2993 3027 3425 3712 3741 3824 4082 4166 4193 4376 4398 4900