Atlantic thermohaline currents nearly stopped in some cold spells of the Ice Age

That is what researchers claim in a new study:

Stephan P. Ritz et al., Estimated strength of the Atlantic overturning circulation during the last deglaciation. Nature Geoscience 2013. Pay per view → LINK [doi:10.1038/ngeo1723]

Abstract

The Atlantic meridional overturning circulation affects the latitudinal distribution of heat, and is a key component of the climate system. Proxy reconstructions, based on sedimentary 231Pa/230Th ratios and the difference between surface- and deep-water radiocarbon ages, indicate that during the last glacial period, the overturning circulation was reduced during millennial-scale periods of cooling. However, much debate exists over the robustness of these proxies. Here we combine proxy reconstructions of sea surface and air temperatures and a global climate model to quantitatively estimate changes in the strength of the Atlantic meridional overturning circulation during the last glacial period. We find that, relative to the Last Glacial Maximum, the overturning circulation was reduced by approximately 14 Sv during the cold Heinrich event 1. During the Younger Dryas cold event, the overturning circulation was reduced by approximately 12 Sv, relative to the preceding warm interval. These changes are consistent with qualitative estimates of the overturning circulation from sedimentary 231Pa/230Th ratios. In addition, we find that the strength of the overturning circulation during the Last Glacial Maximum and the Holocene epoch are indistinguishable within the uncertainty of the reconstruction.

Summary of thermohaline circulation (public domain, NASA)

In the North Atlantic the best known thermohaline current is the Gulf Stream, which effectively keeps Europe several degrees warmer than it would be otherwise, allowing a relatively dense population at latitudes unheard of elsewhere on Earth. This current was weak at best in the Ice Age. 

Notice that they say that they can find any difference between present day (Holocene epoch) and the Last Glacial Maximum, so it cannot be inferred, it seems, that the glaciation itself had anything to do with the thermohaline currents but only with  some particular cold spells of the late Upper Pleistocene, particularly the HE1 (c. 18-14.6 Ka ago) and the Younger Dryas (c. 10 Ka ago).

Source[es]: Servicio de Información y Noticias Científicas SINC (via Pileta). 

Sudden major rise of sea levels after last Ice Age

I cannot find the paper right now (the CEREGE research center's website is down?) but several news sites echo that in only 350 years, beginning 14,600 years ago, sea levels rose as much as 14 meters, judging from coral formations near Tahiti (Polynesia).

That means on average 1 meter each 25 years. The actual impact of this kind of change varies depending on the relief of the coast but nowadays one such change of just one meter would swallow about 1/3 of the Netherlands, for example.

Sources: CNRS, Pileta[es].

Complex speciation process of polar and brown bears?

Actual modern hybrid

According to new research, polar and brown bears could have diverged as long ago as 4-5 million years but have been hybridizing intermittently since then, specially as ice sheets receded in interglacial periods as today's.

In fact we know that they do it nowadays.

While some clades of brown bear, notably the Admiralty Islands (but also French ones) are more closely related to polar bear by mitochondrial DNA,  by overall nuclear DNA they fall wholly within the brown bear clade. That's because, mtDNA-wise polar bears are a subset of brown bears, while by nuclear DNA they are more clearly distinct.

The authors propose that this implies mtDNA introgression from brown bear into polar bears, up to the point of total displacement of the native polar bear lineages, but of course it may also be that some of their calculations are totally wrong. After all molecular-clock-o-logy is not rocket science, not at all and a total lineage replacement by just occasional inter-breeding seems a most unlikely event with the laws of probability in hand.

My impression is in fact the opposite: that the nuclear differentiation should have happened after that of the mtDNA but that molecular-clock speculations obscure this fact. But whatever. I may also be wrong, of course but I just can't accept molecular-clock-o-logy as evidence of anything - doing that is pseudoscience.

Ref. Webb Miller et al. Polar and brown bear genomes reveal ancient admixture and demographic footprints of past climate change. PNAS 2012. Open access. [Early edition link, DOI: 10.1073/pnas.121050610] 

See also: Buffalo University press release, Science Daily.

Phylogenetic labyrinth... or molecular-clock fanaticism?

Update: PConroy mentions (see comments) a previous study (Current Biology 2011, Science Now article), which states that there is even closer mtDNA affinity between extinct Irish brown bears and modern polar bears than these have with the Alaskan ABC islander ones.

Reliability of Greenland ice cores questioned for Younger Dryas only

It seems now that the main proxy to understand Northern Hemisphere glaciation, the Greenland ice cores, is not as straightforward as scientists used to think. At least that is what a new study claims in relation to the Younger Dryas (only):

Zhengyu Liu, Younger Dryas cooling and the Greenland climate response to CO2. PNAS 2012. Pay per view (6-month embargo or depending on world region).

Abstract

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.

From the Archaeology News Network:

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

More evidence in support of the Younger Dryas impact theory

Some 12,800 years ago, when the climate was warming very fast and temperatures had almost reached present day levels... they suddenly plumetted again in a matter of months and remained at Ice Age levels for more than a thousand years before warming again. That period is known as the Younger Dryas.

The Younger Dryas is the last cold gorge by the left

There used to be several explanations but since some time ago, the meteorite impact theory has been gaining weight. This evidence would seem to consolidate it.

It was already quite consolidated as a theory because, while some had questioned the earliest evidence, further data had been collected from around the world that reinforced the model by about the same time.

The new evidence comes in form of melted glass (siliceous scoria like objects, SLOs) mineral inclusions from Pennsylvania (USA), which appears to require such an impact to have been produced. This kind of product requires temperatures similar to those of a nuclear explosion. 

The glass-like grains at two different imaging resolutions

These remains have been found so far in North and South America, Europe and West Asia, suggesting several impacts from an already fragmented meteorite. It is unclear if there could be more such findings elsewhere on Earth. 

One of the places directly affected by the impact was the site of Abu Hureyra, at the Mid-Upper Euphrates (Syria), where a layer of ashes followed by an archaeological hiatus mark the boundary between an Epipaleolithic and the more important Neolithic settlement. 

The impact and the subsequent sudden cooling probably was a major influence in the extinction of some subarctic megafauna in North America and Northern Eurasia such as the mammoths. 

Source: Science Daily.

Video: visualization of ice sheets and sea level on Europe since LGM

Hat tip to Marnie for this great finding. 

The video-animation has been developed by Adrian Meyer and Karl Rege, from the Zurich University of Applied Sciences.

It begins at 19,000 BCE (or 21,000 years ago and extends several millennia into the future according to predictions). Most of the action is concentrated right after c. 0:50 mins, which corresponds with 10,000 BCE (latest Upper Paleolithic and semi-official beginning of the end of the Ice Age).

 

Credit: http://www.youtube.com/user/InITzhaw and http://radar.zhaw.ch/bluemarble3000.html