The Younger Dryas

19 06 2012

We have an interesting seminar here at VUW this Thursday (21/6/12), where Professor James Kennett will be speaking about the ‘Younger Dryas Impact Hypothesis’ (YDIH). In short, the YDIH suggests that approximately 12900yrs ago, an extraterrestrial impact or ‘fireburst’ in the North American region caused widespread fires, the extinction of the Clovis culture and contributed to the rapid, short-lived regional cooling event observed in climate records covering that time frame. I was an undergraduate student on an exchange at a Norwegian university when this theory was announced at and I remember watching the press conference (see video below) with my lecturers.

This is a controversial theory that challenges the longer held beliefs that the Younger Dryas resulted from the breakup of the last North American ice sheet. It is based on the discovery of a widespread carbon layer in sedimentary deposits that date to the start of the Younger Dryas at around 12900yrs ago. This is thought to represent the widespread burning that resulted from the extraterrestrial impact. The theory has sparked much debate and has also been widely criticized since it was announced at the AGU meeting in 2007. This went so far as a ‘requiem‘ being published for the YDIH. It will therefore be very interesting to here one of the original authors of the study talk to us firsthand about the evidence upon which this theory is based.

I will post another blog about the seminar next week, and if I remember to bring my generic tablet device I may even do one of those ‘live-tweet’ things on my twitter feed. In the mean time here’s a bit of background to the Younger Dryas….

The Younger Dryas

Following the peak of the last glaciation between 19000-26000years ago (commonly referred to as the ‘Last Glacial Maximum’ or ‘LGM’, the Earth’s climate was warming and the large ice sheets that had covered much of northern Eurasia and North America were retreating northwards. Similarly in the Southern Hemisphere, glaciers that had extended out from the mountain ranges such as the Southern Alps and the Andes were also retreating in response to the global increase in temperature. These climatic changes are thought to be caused by changes in the Earth’s position relative to the sun, which varies over known timescales.

Temperature change in Greenland from 23000-8000 years ago (Source:climateshifts.org)

Following the LGM, a number of more rapid (c. 1000-2000 yr long) cooling-warming cycles occurred, until global climate stabilised at around 11000yrs ago. These rapid events were first  identified in polar ice cores, where high-resolution records of past temperature changes are preserved in the layers of snow accumulation. The Younger Dryas represents one of these climatic events and covers a  period of time  approximately 1300 years long, between 12900 and 11600 years ago. It was first defined in fossil pollen assemblages in Scandinavia in the 1970s, where the relative abundance of pollen from plant species that inhabit cooler climatic zones increased, suggesting a decrease in local temperature. Subsequent work has shown that the Younger Dryas represents a period of dramatic cooling in the North Atlantic region, where a cooling of approximately 10degC caused glaciers to readvance in many parts of Europe and North America. One of these species is called Dryas octopetala, which is where the Younger Dryas gets its name (there is an Older Dryas, but that’s another story). The ice core records show that the Younger Dryas ended very abruptly, with rapid warming of several degrees occurring in a matter of decades. The fact that these changes occurred so rapidly and the fact that this occurred relatively recently (geologically speaking), has meant that the Younger Dryas is one of the most studied time-periods of the whole Quaternary. The main question surrounding this event is, ‘what caused it?’. The timescale over which it occurred is too short for it to have been caused by changes in the Earth’s orbit of the sun – as was the case at the LGM, therefore another explanation is needed.

Early work (and work still being undertaken today) focused on trying to establish the geographic extent of this cooling event (i.e. was it restricted to the North Atlantic, or was it a global event?), in order to try to identify the likely cause.  In the mid-1990s, radiocarbon dates of a glacial deposit suggested glacier advance in New Zealand during the Younger Dryas. The author’s of this influential study (cited 192 times and counting), interpreted this as meaning that the Younger Dryas was a time of global cooling and suggested that this was driven by changes in the atmosphere, which are rapidly translated around the globe. However, refined dating techniques, such as surface exposure dating, and a greater range of climate records now suggest that the opposite is the case and that the mid- to high-latitudes of the Southern Hemisphere actually experienced warming during the Younger Dryas, whilst the North Atlantic experienced cooling. This is also seen in polar ice cores where Antarctica is seen to experience warming whilst Greenland cools during this period.

 

The leading hypothesis for the observed differences in climate between the hemispheres during this time is known as the ‘bi-polar seesaw’ and was developed by eminent scientist Wally Broecker. Broecker recognised the role of the oceans in distributing thermal energy around the globe via density driven currents. In the early 1990s, following the development of the polar ice cores climate records (mentioned above), a correlation was made between these rapid cooling events and large inputs of freshwater (as recognised by marine deposits showing increased iceberg discharge) to the North Atlantic from the decaying North American and Eurasian ice sheets. It was suggested that these freshwater inputs altered the density of oceanic waters in the North Atlantic to an extent where the circulation slowed, therefore reducing the heat flux from the South to the North and causing cooling the North Atlantic region and warming in the Southern Ocean. This reconciles well with the interhemispheric asynchrony observed in glacier advance records (outlined above). No iceberg deposits are found to be associated with the onset of the Younger Dryas and initially Broecker hypothesised that the freshwater input associated with this event was sourced from Lake Agassiz, a large body of terrestrial water that was dammed by the former North American (Laurentide) ice sheet. However, later work  could not reconcile the timing of the main drainage of this lake with the onset of of the Younger Dryas. Other studies have identified a different route for the lake drainage, that appears to coincide with the onset of cooling.

In summary, despite the abundance of research that has focused on this time period, great uncertainty still surrounds the precise mechanisms that caused this dramatic climatic reversal. The ‘impact hypothesis’ presents another theory of the potential mechanism(s) that caused or contributed to the climatic changes seen at this time. Recently, Broecker and others have suggested that the Youngas Dryas is not a ‘freak event’ that it has long thought to be. They identify similar events in the ice-core temperature records of previous glacial-interglacial transitions and suggest that these short but intense climatic reversals are an integral part of the deglacial process. They conclude that “there is no need to call upon a one-time catastrophic event to explain the YD” which is a statement clearly leveled at the supporters of the impact hypothesis. Of course, it does not mean that there was not a meteorite impact at this time, but Broecker and others believe that the climate changes during the YD were driven by internal mechanisms. I look forward to hearing Prof. Kennett’s views on the whole subject…

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Greenland – The Times Atlas vs Scientists

28 09 2011

There has been a big furore over the past few weeks regarding the publication of the new, 13th Edition of the Times Comprehensive Atlas of the World.  Specifically, the depiction of the Greenland Ice Sheet (GIS), and the associated press release, which said that the GIS had lost 15% of its permanent ice cover.

Different in the permanent ice cover in Greenland between 1999 (left) and 2011 (right)

This figure is a gross over-exagerration (Greenland has actually lost about 0.1% over the last decade), and was picked up by a number of scientists almost immediately.  Emails were being fired around mailing lists rapidly, as glaciologists attempted to find the best way to remove this figure from the mass media, before it became public knowledge and damaged the reputation of glaciologists working in Greenland (of which there are a lot).  The Times Atlas publishers (Harper Collins) initially refuted the claims, saying  that they:

“are the best there is … Our data shows that it has reduced by 15%. That’s categorical.”

Initially it was unclear where the “15%” figure had appeared from, but it soon emerged that the new map bared a striking resemblance to an online map of the GIS on the National Snow and Ice Data Centre’s website.  Those working on Greenland for the Times Atlas had clearly misinterpreted this map, and taken it to be an absolute measure of all ice cover in Greenland, when it actually represented something else.

The reason that this cartographer’s error spiralled out of control is simple – scientists were not consulted.  Had consultation taken place, the error on the map, and the consequent ice loss figure of 15% would have been immediately spotted and corrected.  The error, if left uncorrected would have discredited what a number of scientists from institutions across American and Europe have been working on for a number of years.  The actual picture of what is happening to the GIS is extremely complex, and remains poorly understood in areas.  It is a story of variability, with extreme thinning and increased in melting in areas, counterpoised with slight thickening in other areas.

The details are too complex to do justice in this post, but maybe another time….

The main issue that arose from this “crisis” was not the actual error made by the cartographers, this was relatively easily rectified in the end, but the ease at which this information got into the public domain with no input from scientific experts.  Such experts, who have built their career working on monitoring changes of the GIS, are subject to the rigorous peer-review system when disseminating their work to the wider scientific community. The problem with this however, is that this information is largely only available to the academic community, whose institutions provide paid subscriptions to the content. ‘Scientific’ information reaching the public through popular media is subject to no such scrutiny and due to the far greater reach of such outlets, errors can propagate much further much more quickly. With recent “fiascos” tarnishing science’s reputation (Climategate, the IPCC), science needs all the help it can get to stay favouring in the public’s eyes.  Thankfully this episode was resolved quickly, and if anything, demonstrated the ability of scientists to quash rumours with scientific evidence quickly.

At no point did the writers think to contact any scientists over the alarmingly large 15% ice loss Greenland had experienced.  Instead they simply put it to print and made a fool of themselves as a result.  They have now agreed to work with scientists in the future to correct this issue, and ensure it doesn’t happen again.

Hopefully this will be the standard position for those publishing material that should (and does) have a grounding in scientific work.

For an interesting alternative visualisation of melting in Greenland visit Cryocity.