Category Archives: ecology
|We are what we eat
Reference paper: Christina J. Adler et al., Sequencing ancient calcified dental plaque shows changes in oral microbiota with dietary shifts of the Neolithic and Industrial revolutions. Nature 2013. Pay per view → LINK [doi:10.1038/ng.2536]
Greenland ice-core δ18O-temperature reconstructions suggest a dramatic cooling during the Younger Dryas (YD; 12.9–11.7 ka), with temperatures being as cold as the earlier Oldest Dryas (OD; 18.0–14.6 ka) despite an approximately 50 ppm rise in atmospheric CO2. Such YD cooling implies a muted Greenland climate response to atmospheric CO2, contrary to physical predictions of an enhanced high-latitude response to future increases in CO2. Here we show that North Atlantic sea surface temperature reconstructions as well as transient climate model simulations suggest that the YD over Greenland should be substantially warmer than the OD by approximately 5 °C in response to increased atmospheric CO2. Additional experiments with an isotope-enabled model suggest that the apparent YD temperature reconstruction derived from the ice-core δ18O record is likely an artifact of an altered temperature-δ18O relationship due to changing deglacial atmospheric circulation. Our results thus suggest that Greenland climate was warmer during the YD relative to the OD in response to rising atmospheric CO2, consistent with sea surface temperature reconstructions and physical predictions, and has a sensitivity approximately twice that found in climate models for current climate due to an enhanced albedo feedback during the last deglaciation.
The problem is that, when compared with other records, the Greenland Ice cores’ oxygen isotope ration does not hold. The explanation is complex and related to CO2 levels, the North American Ice Sheet (which was already in retreat) and the different composition of oxygen isotopes when they arrived from the Pacific Ocean.
Working with UW-Madison climatologist Zhengyu Liu, collaborators at the National Center for Atmospheric Research and others, Carlson found their computer climate model breaking down on the Younger Dryas.While it could reliably recreate temperatures in the Oldest Dryas — a similar cooling period about 18,000 years ago — they just couldn’t find a lever in the model that would simulate a Younger Dryas that matched the Greenland ice cores.“You can totally turn off ocean circulation, have Arctic sea ice advance all the way across the North Atlantic, and you still will have a warmer climate during the Younger Dryas than the Oldest Dryas because of the carbon dioxide,” Carlson says.By the time the Younger Dryas rolled around, there was more carbon dioxide in the air — about 50 parts per million more. The warming effects of that much CO2 overwhelmed the rest of the conditions that make the Oldest and Younger Dryas so alike, and demonstrates a heightened sensitivity for Arctic temperatures to rising greenhouse gases in the atmosphere.The researchers zeroed in on the Northern Hemisphere’s temperature outlier, Greenland ice cores, and found that the conversion of oxygen isotope ratio to temperature typically used on the ice cores did not account for the sort of crash climate change occurring during the Younger Dryas. It assumes prevailing winds and jet streams and storm tracks are providing the moisture for Greenland precipitation from the Atlantic Ocean.
“The Laurentide ice sheet, which covered much of North America down into the northern United States, is getting smaller as the Younger Dryas approaches,” Carlson says. “That’s like taking out a mountain of ice three kilometers high. As that melts, it allows more Pacific Ocean moisture to cross the continent and hit the Greenland ice sheet.” The two oceans have distinctly different ratios of oxygen isotopes, allowing for a different isotope ratio where the water falls as snow.
Hat tip: Pileta.
|Some temperature proxies for the Younger Dryas|
|Pines from Tierra de Pinares (source)|
|Cuéllar, above the “sea of pines” (green area behind).|
The researchers analyzed the cores for elements like hydrogen that leave distinctive signatures in sediment. These geochemical markers correspond with past precipitation levels, which influence weathering. They also examined ratios of aluminum and potassium, which indicate weathering intensity, because potassium is a highly mobile element whereas aluminum is one of the most immobile. As expected, the weathering patterns closely followed precipitation levels—that is, until about 3000 years ago. At that point, Bayon says, the pattern became completely different. The sediment appeared to have undergone intense chemical weathering, which the climate alone could not explain. So the team began suspecting another factor was responsible.
Hat tip to Stone Pages’ Archaeonews.
|Based on Svendsen 2004|
|Author: Väino Poikalainen, horizontal stripes are lakes|
Two locations in Norway have proved particularly lucrative for the researchers. One of them, Andøya Island, in north-western Norway, is the source of material dated between 17,700 and 22,000 years-old. During the last ice age, the island was an ice-free pocket, one “refuges” on the edge of the enormous ice sheet, which blanketed at that time nearly all of Scandinavia.“The other evidence, which supports the surviving conifers in the midst of an ice age, originates in Trøndelag, central Norway. One hypothesis is that trees were able to survive on the top of nunataks, the exposed ridges or peaks of mountains protruding from glacial cover, or in more sheltered areas close to the coast where proximity to the temperate conditions of the Atlantic Ocean favoured survival.
Source: Science Daily.