There is a new paper on Science Express that examines the constraints on climate sensitivity from looking at the last glacial maximum (LGM), around 21,000 years ago (Schmittner et al, 2011) (SEA). The headline number (2.3ºC) is a little lower than IPCC’s “best estimate” of 3ºC global warming for a doubling of CO2, but within the likely range (2-4.5ºC) of the last IPCC report. However, there are reasons to think that the result may well be biased low, and stated with rather more confidence than is warranted given the limitations of the study.
A. Schmittner, N.M. Urban, J.D. Shakun, N.M. Mahowald, P.U. Clark, P.J. Bartlein, A.C. Mix, and A. Rosell-Mele, "Climate Sensitivity Estimated from Temperature Reconstructions of the Last Glacial Maximum", Science, vol. 334, pp. 1385-1388, 2011. http://dx.doi.org/10.1126/science.1203513
A group of colleagues have succeeded in producing the first continuous proxy record of sea level for the past 2000 years. According to this reconstruction, 20th-Century sea-level rise on the U.S. Atlantic coast is faster than at any time in the past two millennia.
Good data on past sea levels is hard to come by. Reconstructing the huge rise at the end of the last glacial (120 meters) is not too bad, because a few meters uncertainty in sea level or a few centuries in dating don’t matter all that much. But to trace the subtle variations of the last millennia requires more precise methods.
This months open thread. There are some Items of potential interest::
or whatever you like.
This is the strong conclusion of a new paper in the Earth Science Reviews by Pinter et al (via Scribd). From their abstract:
The Younger Dryas (YD) impact hypothesis is a recent theory that suggests that a cometary or meteoritic body or bodies hit and/or exploded over North America 12,900 years ago, causing the YD climate episode, extinction of Pleistocene megafauna, demise of the Clovis archaeological culture, and a range of other effects.
The physical evidence interpreted as signatures of an impact event can be separated into two groups. The first group consists of evidence that has been largely rejected by the scientific community and is no longer in widespread discussion…. The second group consists of evidence that has been active in recent research and discussions:…. Over time, however, these signatures have also seen contrary evidence rather than support.
In summary, none of the original YD impact signatures have been subsequently corroborated by independent tests. Of the 12 original lines of evidence, seven have so far proven to be non-reproducible. The remaining signatures instead seem to represent either (1) non-catastrophic mechanisms, and/or (2) terrestrial rather than extraterrestrial or impact-related sources.
The YD impact hypothesis made a big splash at AGU in 2007, and we’ve written about it a few times since. Our assessment was (in 2007), that this would need a lot of confirmatory evidence to get accepted, and even if it was, it did not provide much explanation for other, very similar, abrupt changes in the record. In 2009, we were still skeptical and noted that “the level of proof required for this extraordinary idea will need to be extraordinarily strong”. Unfortunately, as this paper makes clear, neither a lot of confirmatory evidence nor extraordinarily strong proofs have been forthcoming.
This paper is unlikely to the very last word on the subject, but it is likely to be the last time the mainstream paleo-climatologists are going to pay this much heed unless some really big new piece of evidence comes to light.
However, while the specifics of this particular hypothesis and its refutation are interesting in many ways…
The YD impact hypothesis provides a cautionary tale for researchers, the scientific community, the press, and the broader public.
Let’s be specific…
A few items of interest this week.
1. A new study by Spielhagen and co-authors in Science reconstructs temperatures of North Atlantic source waters to the Arctic for the past two millennia, adding another very long-handled Hockey Stick to the ever-growing league.
2. From last week, an article in Science Express by Buntgen et al reconstructing European summer temperature for the past 2500 years, finding that recent warming is unprecedented over that time frame, and providing some historical insights into the societal challenges posed by climate instability (listen here for an interview with mike about the study on NPR’s All Things Considered).
3. The team of ice core researchers at WAIS Divide reaches its goal of 3300 meters of ice. [WAIS Divide, central West Antarctica, is a site of significant warming in Antarctica, over at least the last 50 years, a result recently confirmed by the study of O’Donnell et al. (2010); Stay tuned for more on the that soon].
Other Miscellaneous Items:
1. More in Nature on data sharing.
2. A great primer in Physics Today on planetary energy balance from our very own Ray Pierrehumbert (link to pdf available here).
3. Now shipping are David and Ray’s The Warming Papers and Ray’s Principles of Planetary Climate.
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