Abrupt Climate Change: Blog Conclusions

As this blog winds down, this post will summarise what the blog has discussed. It began with an overall assessment of abrupt climate changes, where they have occurred in the Earth's recent history and the effects they have presented.

From that point, research on the Younger Dryas and 8,200 year abrupt climate events illustrated the effect of the thermohaline circulation (THC) - Broecker's "Great Ocean Conveyor". The mechanisms behind the THC were described in "Short circuiting the thermohaline circulation". In the same post, the Younger Dryas (YD) stadial was introduced, a period of abrupt cooling through a period of deglaciation. After much research on the possible trigger-points for the YD, I reached a general consensus that an abrupt freshening of the North Atlantic, caused by meltwater outbursts from the glacial Lake Agassiz was the likely cause. However, on reflection, there is a significant time-lag between the outbursts and the climate change suggesting that the outbursts may have weakened the THC for another factor to cause its collapse. Evidence presented in "Point to Ponder, Perhaps?" from Steffenson et al (2008) and others, showed that the YD collapsed to an extremely abrupt (years-decades) increase in temperature of c. 7 C over just 50 years.

In a natural progression, the 8200-year event was discussed in "Fusing the thermohaline circulation - the 8200-Year Event". Evidence for a huge, much greater, series of meltwater outbursts occurred in coincidence with a 400-year cooling event with its trough at c. 8,200 years BP. Despite a larger freshwater forcing than the YD, the shorter event occurred when Northern Hemisphere insolation levels were higher because of orbital forcing thus being able to recover faster than the YD.

From this point, the early Holocene was left behind to discuss the centennial-scale variability which has occurred in the last 2000 years. The Medieval Warm Period (MWP) (800-1200AD), Little Ice Age (LIA) (1200-1850) and the transition between these two events were discussed through multi-proxy research. Although the literature has not come to a consensus on the overall cause of these events, the global nature of the temperature fluctuations cannot easily be explained by a reduction to the THC. Instead, the possible transition between Holocene ~1,500 year solar insolation cycles would explain the global evidence for the MWP and LIA.

Finally, in my penult post, I brought my research to the present in an assessment of whether abrupt climate change events could occur within the 21st century. Through analysis of the USGS 2008 report on the likelihood Abrupt Climate Change. It is unlikely that a large scale THC/AMOC collapse would occur within the next century partly because there is no Lake Agassiz from which freshwater forcing could travel. However, some papers, including the study from Schwartz and Randall (2003) suggests that AMOC collapse could potentially occur causing 5-10F temperature change over just a decade. Conversely, it is perfectly feasible that sea level rise from ice-cap melting could occur on a grand-scale and zones of drought could travel northwards.

This blog has successfully detailed the abrupt climate events in the Holocene and Late Glacial. From this research, the possibility of abrupt climate events has been brought into the context of the present and indeed the near future. I would encourage readers to continue to contact me for more information about any of my research. Alternatively, papers on abrupt climate change are common in popular journals such as Science and Nature, as well as the Journal of Climate and Oceanography.

Although major changes are unlikely in the near future, there is a real possibility of tangible change within centuries. This blog, therefore, should add further weight to the need for energy sustainability and greener practices to reduce anthropogenic warming and to allow for natural rather than anthropogenic climate variability.

"Now the Pentagon tells Bush: climate change will destroy us"

Much has changed since February 2004 when the Observer published this rather sensational tabloid-beating headline. For one, the USA, still the largest economy in the world with the greatest fossil fuel emissions, has a President who actually believes in global warming. Climate change predictions have been updated through several reports detailing the current climate situation and the likelihood of abrupt climate warming (IPCC 2007, USGS 2008 and the upcoming IPCC 5th Report in 2014) and the world has started to attempt to actually do something about global warming through COP15, the EU ETS and other similar agreements rather than simply accepting such sensational headlines as fact. 

However, there is still the issue of how likely an abrupt climate change event could take place over the next century. Parts of this blog have discussed changes which have occurred over periods less than human life expectancies including the breakdown of the Younger Dryas. Therefore, full understanding of abrupt climate change likelihood in the future is particularly important.

First let's look at the USGS 2008 report on abrupt climate change. The report is summarised into 4 sections to assess whether there will be an abrupt sea level rise, a change in the hydrological cycle, a weakening of the AMOC or an abrupt change in atmospheric methane.

Taking the AMOC first, since this blog has reported its weakening as a major cause of abrupt climate change events in the past, the USGS suggests that the AMOC will decrease by between 25-30%. This will be caused by both natural variability and anthropogenic warming. As a result, there will be less heat transfer to the North Atlantic from the thermohaline circulation. Despite this, it is expected that a warming trend will still occur over Europe and North America as temperatures from global warming increase. At present, there are no models which suggest that a complete collapse of the AMOC will occur within this century. However, if the AMOC were to collapse, an 80cm sea level rise in the North Atlantic and a net cooling of 1-2C on top of increased temperatures in response to greenhouse gas forcing. Vellinga and Wood (2008) (2) suggest that temperatures could decrease by as much as 8C locally and that, once the climate had stabilised, the AMOC would take 100 years to recover.

Secondly, the USGS report suggests that an abrupt sea level rise caused by the melting of ice caps is "possible". Evidence has been presented showing that the West Antarctic and Greenland ice sheets are both thinning and breaking off which would cause a sea level rise much larger than predicted by the IPCC 4th Assessment Report. Hydrological changes causing water scarcity and drought in many sub-tropical areas are extremely possible. Global water extraction is set to increase by 31% by 2020 as predicted by the OECD (3) in 2003 from 1995 levels. Indeed, my university dissertation which modelled the climate change effects on the volume of discharge in Rio Grande and Arkansas rivers, two of the largest rivers in the USA supporting millions of people, found a possible 5% decrease in discharge volume by 2050 despite population size doubling (Shefford, 2011) (4).

The threat of a methane concentration rise is also real. Methane hydrates, which lie trapped in permafrost and in the deep oceans, may be released to the atmosphere causing further global warming. It appears to be very unlikely that such changes will occur in the near future but changes within 1,000-100,000 years may well be likely.

The USGS report has suggested that changes to the AMOC are unlikely along with the release of methane hydrates. However, the possibility of a northward movement of drought zones and sea level rise could well give some communities severe issues in the 21st century. 

Now there are always the sensationalists out there but this paper by Schwartz and Randall (2003) (5) pushes the boundaries. They suggest exercising caution in the likelihood of their scenarios but they have pushed their models over a temperature threshold so that the worst possible event occurs. When temperatures rise above a threshold, abrupt changes will occur causing a 5-10 F temperature change in just a decade. They suggest that these consequences could last 100 years and be analogous to the 8200-year event or even the Younger Dryas. This blog has taught me to be critical and unless they can find an invisible Lake Agassiz from which freshwater outbursts to the North Atlantic can occur, I see no support of a future Younger Dryas or 8200-year event. Yes the North Atlantic has freshened, as reported by Dickson et al (2002) (6), but I can't imagine where the freshwater forcing as seen in the Younger Dryas or 8200-year event would come from to cause the collapse of the AMOC.

So was the Pentagon correct? In some ways yes, drought will likely cripple parts of the USA with sea level rise also set to affect its low-lying cities. If we're talking about the world over, the AMOC should survive anthropogenic warming at least until the 21st century ends. However, maybe after the USA's performance after Kyoto and COP15, "us" really does mean "U.S."! 

In my final post, I will conclude this blog by summarising what my research has found and where this leaves us for the near future of abrupt climate change events.

(1) Delworth et al., (2008) USGS-CCSP 3.4

(2) Vellinga and Wood (2008) doi: 10.1007/s10584-006-9146-y

(3) OECD (2003) Improving Water Management: Recent OECD Experience

(4) Shefford (2011) unpublished.

(5) Schwartz and Randall (2003) Environmental Defense Fund

(6) Dickson et al (2002) doi: 10.1038/416832a

Were the MWP/LIA events global?

Good Afternoon, 

Over the last few posts I have brought my discussion on abrupt climate changes towards the present. The Medieval Warm Period (MWP) and Little Ice Age (LIA) have both been discussed with evidence provided detailing their Northern Hemisphere presence and impacts.

However, today I thought that a post may convince you that both the MWP and LIA were indeed global phenomena. Let me take you to the Andes in South America. Work by Licciardi et al. (2009) (1) set about taking moraine samples once covered by glaciers. By taking samples to find evidence of ¹⁰Be, a cosmogenic nuclide derived by sunlight interaction, scientists can determine when glaciers retreated. Indeed, they found a strong indication that glaciers were larger around the time of the LIA across 25 sites in the Andes. Another paper from Thompson et al. (1986) (2) reconstructed temperature through a multi-proxy approach of oxygen isotopes and electrical conductivity in another tropical glacier in the Andes. Thompson and his colleagues also found evidence for a cooling period analogous to the LIA.

So I've provided evidence for the LIA in South America...you may argue that this isn't global and would still be very responsive to AMOC variations, if indeed, AMOC circulation changes were the cause of the LIA. So here's evidence for the LIA in varved lake sediments from Lake Malawi, Southern Africa. The study was undertaken by Johnson et al (2001) (3) as they found warmer conditions throughout 1300-1520 A.D. which provides support to a Medieval Warm Period, although not well temporally correlated. However, they found distinct cooler conditions between 1570 and 1820 A.D. to add further support that the LIA was a global phenomenon. The lag between these dates and the dates for the MWP and LIA recommended by Mann et al. (2009; as posted in the "The second millennium: The Medieval Warm Period" post) are interesting however leading me to possibly consider that the Northern Hemisphere drove the temperature changes and it took the rest of the world some time to catch up. Further African evidence for the MWP and LIA is provided by Huffman (1996) (4) who looked at crop abundance in southern Africa. Reconstructing temperatures from relic evidence appears to show that this area was warmer than present - this is a new technique to me but the author seems convinced that this provides evidence of a warming period between 1100-1400 A.D. in correlation with the Medieval Warm Period of Europe followed by a cooling period into the LIA. 

I could go on...there are countless articles which claim to have found evidence that the LIA and MWP were global phenomena (see Williams et al. (2004) (5) for similar evidence for the LIA/MWP in New Zealand). However, we have not come closer to one dominant forcing factor to explain the abrupt shift between the two periods. I've come across another paper by Bard and Frank (2006) (6) which suggests that the transition from MWP to LIA is down to solar forcing. Citing Bond et al (1997, 2001) and Hu et al. (2003) who promote solar cycles which cause atmospheric and oceanic fluctuations on 1-2 thousand year cycles, Bard and Frank suggest that the transition from MWP to LIA is evidence of this cycle. However, they do not describe the abrupt nature which drives my skepticism. Now that the cooling period has occurred, they suggest that enhanced solar activity is pushing us towards a warmer climate albeit possibly exacerbated by anthropogenic forcing. Having read through more papers ascribing the cause of the MWP and LIA to solar forcing, I came across Solanki et al. (2000)(7) who found that sunspot numbers were lowest during the LIA and Shindell et al. (2001) (8) found that their model of solar irradiance was highest during the MWP


Such a cause for the MWP/LIA would also explain the global nature of phenomena but without further research, it is impossible to know for sure.

This discussion brings an end to abrupt climate shifts of the past. In my next post, I will consider how likely these events are in the future before concluding what my blog has found over its duration.

(1) Licciardi et al. (2009) doi: 10.1126/science.1175010

(2) Thompson et al. (1986) doi: 10.1126/science.234.4774.361

(3) Johnson et al. (2001) doi : 10.1130/0091-7613(2001)​029<0083:DROCVS>​2.0.CO;2

(4) Huffman (1996) doi: 10.1016/1040-6182(95)00095-X

(5) Williams et al. (2004) doi: 10.1191/0959683604hl676rp

(6) Bard and Frank (2006) doi: 10.1016/j.epsl.2006.06.01
(7) Solanki et al. (2000) doi: 10.1038/35044027
(8) Shindell et al. (2001) doi:  10.1126/science.106436

Recent centennial climate variability: possible explanations for the LIA

Last week, I presented evidence to support the Medieval Warm Period (MWP). Although a consensus has not been reached as how the event occurred, the effect of atmospheric circulation changes led from solar variation has been suggested.

Today I will continue with evidence for the Little Ice Age (LIA), the period of cooling which started to occur at the end of the MWP c. 1250 AD. If we return to last week's graph of climate over the past 1500 years (Fig. 1), you can see the blue box showing the LIA lasting from 1400 - 1700 AD with some variability from 1700-1850AD.

Fig. 1. Proxy compilation showing temperature reconstructions over the past 1500 years (Mann et al., 2009

So what are the possible causes of the LIA? My research has led me to 4 possible factors:

Solar variability, volcanic activity, surface albedo and the ocean-atmosphere led changes introduced for the MWP. 


Looking at Solar variability first, several papers have attempted to reconstruct cosmonuclide production, materials such as ¹⁴C and ¹⁰Be, which are used as a proxy for solar activity (Bard et al., 1999) (1). Magnetically charged solar winds deflect the particles from the Earth's atmosphere and therefore lower records of cosmonuclide on Earth should be correlated with higher periods of solar activity. Indeed total solar irradiance records based on such reconstructions reveal a drop in solar insolation from 1200 AD with the trough at 1400 AD when the LIA began as shown on Fig. 2. 

Many of the clear changes in Fig. 2 reflect the record of Fig 1 such as the fall in insolation at the end of the 18th century and the abrupt rise at the end of the 17th century. However, there is a clear fall in solar insolation at approximately 1175 AD when the MWP was supposedly recovering for one last bout of warming if compared to Fig. 1. 

Fig. 2. 1200 years of cosmonuclide total solar irradiance reconstruction (Bard et al., 1999)

Therefore, despite general agreement between Mann's multi-proxy and Bard's solar insolation approaches, there are areas of disagreement suggesting that there cannot be a perfect correlation between solar irradiance and temperatures - there must be other forcing factors.

Another potential forcing factor could be from enhanced volcanism. Times of strong volcanic activity tend to increase average particle size in the upper atmosphere. Because of this, more non-selective and Mie atmospheric scattering is likely to occur thus preventing sunlight from reaching the lower atmosphere and the surface. The sulphur content of the volcanic eruption is also important as this merges with water vapour to form sulphuric acid, dense clouds which absorb and re-emit incident solar radiation (The link between volcanoes and climate is well documented on a San Diego State University Geology page).

Therefore, one would expect periods of enhanced volcanism to cause cooler temperatures in the subsequent years. Such a relationship was found by Crowley et al. (2008) (2) as they reconstructed aerosol optical depth (AOD), which measures the transparency of aerosols and if therefore a proxy of how easily solar radiation can reach the surface, comparing it to tree-ring temperature reconstructions by Jones et al. (1998). On 16 occasions when there were large volcanic eruptions, as measured by sulphate peaks from 13 Greenland and Antarctican ice cores, AOD was found to rise sharply, after volcanic events, as temperatures fell suggesting that greater stratospheric sulphate blocked incoming solar radiation as shown on Fig. 3.

Fig. 3. Aerosol Optical Depth (AOD) plotted against Tree-ring reconstructed temperature from 1600-1850
(Crowley et al., 2008)

Crowley and his colleagues have presented evidence to support the link between cooler temperatures as a result of enhanced volcanism. However, during times where no major eruptions are taking place, or certainly have not been identified in the record, temperatures are still fluctuating suggesting that volcanism cannot be the sole reason for cooler periods during the LIA. It is more likely that a combination of increased volcanism and reduced solar activity caused such a cooling period.

Assuming the LIA did occur, as now widely accepted, a build up of ice would have likely occurred on the Greenland and Antarctica. This could be measured either by ice accumulation volume or by lower ¹⁸O concentrations in ice core records. Such an ice build-up would have increased surface albedo, the Earth's reflectivity of solar insolation thus reducing heat absorption and leading to a feedback of further cooling. Taking evidence from documented records, several pictures including Fig. 4's representation of a frozen Thames (Wikipedia), point to more ice build up in Europe which would support a southward movement of ice accumulation and thus higher Earth surface albedo.

Fig. 4. Frozen Thames in 1677 (1607 had a Thames "Frost Fair") (Wikipedia)

Finally I return to the atmospheric circulation changes which may have both caused the MWP and relaxed into the LIA. Having sampled foraminifera in sediment cores, Lund et al. (2006) estimate that ocean circulation declined by as much as 10% during the LIA (Lund et al., 2006) (3). This reduction in Gulf Stream transport would have reduced the heat transport to the high latitudes thus causing reduced temperatures and ice accumulation thus increasing surface albedo into further positive feedbacks. 

My research on the LIA hasn't yielded any single factor to have caused the LIA. However, the combination of all four factors may well have caused the LIA. 

In my next post, I will tackle another question to provide evidence suggesting that the LIA and MWP were either global or just Northern Hemisphere events. At this stage, I have found evidence to suggest that cooling events have been seen in the Southern Hemisphere. However, take a look at the ocean drilling program map of the cores taken to date. You will find that most of the areas around Europe and the North Atlantic have been sampled with spatial sampling considerably more coarse in more remote ocean areas - studies are often first carried out in areas of interest based around scientists-individual locations. Therefore, there may be a huge archive of untapped evidence supporting Southern Hemisphere changes which have not yet been documented. 

In my final two posts after the global LIA/MWP evidence post, I will consider the chances of abrupt climate change in the 21st century before concluding what my research has found.

(1) Bard et al. (1999) doi: 10.1034/j.1600-0889.2000.d01-7.x

(2) Crowley et al. (2008)                                                                                                             http://www.geos.ed.ac.uk/homes/tcrowley/crowley_PAGESnote_volcanism.pdf

(3) Lund et al. (2006) doi: 10.1038/nature0527

The second millennium: The Medieval Warm Period

Further evidence for Northern Hemisphere (NH) climate variability in the past 1000 years has been shown by Mann et al (2009) (1). In Fig. 1, a compilation of proxies from ice cores, dendrochronology, corals and ocean/lake sediments have been used to show the temperature anomalies against the 1961-1990 record from 1500 years BP to present. 

Fig. 1. Proxy compilation showing temperature reconstructions over the past 1500 years (Mann et al., 2009)

The red box on Fig. 1. details the timeline of the Medieval Warm Period (MWP), also known as the Medieval Climate Anomaly (MCA) occurring from c. 950-1250 AD. This is largely only seen in the NH as SH records are sparse.

In the MWP, Mann et al. suggest that mean temperatures were significantly higher than the 1961-1990 average and possibly higher than the 1990- present period. Indeed, multi-proxy temperature reconstruction undertaken by Goosse et al (2006) (2) confirm that European summer temperatures 1000 years ago were similar to temperatures seen in the past 25 years.

Several reasons for this warm period have been presented. A paper by Graham et al. (2011) (3) developed a global physically-based model to represent temperatures since 800 AD. In that model, changes to atmospheric circulation, including a widening of the Hadley Cell as well as North Atlantic Oscillation (NAO) strengthening were presented as possible reasons for the warming of the NH climate. They suggest that this may have been caused by warming to the Indian and Western Pacific Oceans which would also affect monsoon-led circulation. As Trouet et al (2009) (4) explain, such an atmospheric change would result in a net cooling of the Eastern and Central Pacific causing La Nina like conditions. This would strengthen the NAO causing enhanced tropical westerlies, push the ITCZ further North and enhance the AMOC thus increasing heat transfer to the NH (see 'Short circuiting the thermohaline circulation' post for more information).

The cause of such atmospheric circulation changes, with enhanced La Nina and NAO, may have been caused by enhanced solar irradiance, as solar irradiance cycles are accepted in the literature, coupled with lower volcanism, which eject particles high into the atmosphere to reflect and block solar energy as suggested by Mann et al. (2009).

The relaxation of these atmospheric anomalies may have swung the NH climate into the LIA, shown by the blue box on Fig. 1. Evidence for the LIA will be presented in the next post as well as further theories to explain both events.

(1) Mann et al. (2009) doi: 10.1126/science.1177303

(2) Goosse et al. (2006) doi: 10.5194/cp-2-99-2006

(3) Graham et al. (2011)  doi: 10.1007/s00382-010-0914-

(4) Trouet et al. (2009 doi: 10.1126/science.1166349

Closer to home: Climate variability in the past 1000 years

As this blog enters into its final third, I will be leaving past records to focus on Northern Hemisphere climate variability over the past 1000 years. These considerably differ from the 8200 yr and Younger Dryas events since there is no dammed freshwater Lake Agassiz to cause freshwater outbursts. Therefore, it will be interesting to see the mechanisms causing abrupt climate shifts in this time period.


This post will serve as an introduction to the past 1000 years' climate. Temperature data can be seen in a 2004 paper by Keller (1). He compiles model results from 7 model runs showing temperature anomalies compared with the observed 1961-1990 average, shown in Fig 1. This graph helps to identify three established key periods within the record, the Medieval Warm Period (MWP) from 800-1200 AD, the Little Ice Age (LIA) from 1200-1800 AD and the recent anthropogenic forced warming since 1850. 

Fig. 1 Graph showing temperature variability in past 1200 years (Keller, 2004)

These three events will be discussed separately with a final post relating to the possible future of abrupt climate change. 


(1) Keller (2004) doi:10.1016/j.asr.2004.01.020

Short-Circuiting the AMOC - A poster summary of this blog

Morning All, 

As other pieces of work saw their deadlines come and go, blog posts have been light. I apologise for this but an update will come later in the week. I've developed a poster to summarise what I've done so far. It also looks at the role of the oceans in changing climate in the 21st century. Hopefully the clear poster structure and informative summary will serve as a refresher to those who haven't read my earlier posts.

Fusing the thermohaline circulation - the 8200-Year Event

Having concluded the Younger Dryas in my last post, it is time to move onto the second major abrupt cooling event within my time-series. 8400 cal yr BP, an abrupt cooling occurred before temperatures rebounded to 8000 cal yr BP. Today's post will consider the causes of the 8200-yr Event by looking at one paper in particular, a review by Barber et al (1997) (1).

Although the reasons for this abrupt change are still unknown, several papers have attributed the event to the final retreat of the Laurentide Ice Sheet releasing a series of outbursts from Lake Aggasiz through the Hudson Strait to the Northern Atlantic. This freshwater flux has been estimated at >10¹⁴m³ occurring c. 8470 cal yr BP. Just like in the case of the Younger Dryas, such a deposit of freshwater into the North Atlantic would prevent the formation of North Atlantic Deep Water preventing heat transfer to the high-latidudes of the Northern Hemisphere.

Evidence for this event can be found through many sources. Alley et al. (1997) (2) present a useful diagram as shown in Figure 1. Across many proxies, the 8200 year downward spike can be seen through ice accumulation, a decrease in methane, temperature and other substances. Further evidence from Klitgaard-Kristensen et al., (1998) (3) present data from marine records in the North Sea and dendrochronology in Germany which show evidence for a 2 C drop in temperature in correlation with the Greenland ice core data.

Figure 1. - Holocene climate clearly showing 8200-yr Event (2)

Today's short post has continued to explain abrupt climate events within this blog's time-series. My next post will prevent further evidence of the event found in the literature. From then on, I will look at a few less pronounced climate changes, including the Medieval Warm Period and the Little Ice Age, before relating the changes in the thermohaline circulation to what could potentially occur in our future.

1. Barber et al., 1997 doi: 10.1038/22504
2. Alley et al., 1997 doi: 10.1130/0091-7613(1997)​025<0483:HCIAPW>2.3.CO;2
​3. Klitgaard-Kristensen 1998 doi: 10.1002/(SICI)1099-1417(199803/04)13:2<165::AID-JQS365>3.0.CO;2-#

Point to Ponder...Perhaps?

Afternoon All, 

The last few posts have generally accepted that the Younger Dryas (YD) was caused by freshwater outbursts disrupting the thermohaline circulation. A few theories have been proposed, some have been rejected but to date I have not posted about the end of the stadial.  

This post will produce evidence by three studies which suggest that the YD stadial event abruptly ended within 3-50 years. In 1989, a paper reviewing the evidence to support the abrupt termination of the YD was written by Dansgaard et al (1). In this paper, the late glacial and the early Holocene is recorded from the ice core taken by the Greenland Ice Sheet Project in South Greenland. The Younger Dryas is recorded in Delta Oxygen-18 isotope analyses as shown in Figure 1, b (Dansgaard et al). 

Figure 1: Greenland ice core evidence showing δO-18, deuterium excess and dust variation supporting a rapid end to the Younger Dryas

In Figure 1, d-f parts of the image zoom in on the end of the Younger Dryas and start of the Pre-Boreal warmer stage. d shows a sharp increase in delta Oxygen-18 (δO-18) as the over a 50 year period. When an increase in δO-18 is seen in ice cores, compared with the concentrations of standard mean ocean water (SMOW), it is assumed that this precipitation will have been enriched in δO-18 which requires greater heat energy, and thus higher surface and air temperatures, to evaporate it before falling as precipitation. There is also less likelihood that the δO-18 will fall on the ice core as it is heavier. Therefore, a higher proportion of δO-18 will increase the chances of it falling at higher latitudes. 

Part e of Figure 1 shows evidence of a decrease in deuterium excess, defined as δDeuterium - 8* δO-18, over just 20 years. Dansgaard and his colleagues attribute this decrease to the northward movement of the Polar front and sea-ice margin causing greater moisture supply to the ice cores leading to a lower deuterium signal.

Part f provides evidence for a decrease in dust in the ice core over just 20 years. The prevalence of dust in an ice core is correlated with storminess (to transport the dust to the ice core) and dry climatic conditions (to allow for increased dust entrainment). The dust signal is negatively correlated with δO-18 and so cooler conditions give rise to more dust. Such a decrease in dust over just 20 years shows that temperatures increased rapidly over this period.

Looking at the evidence, the three proxies of δO-18 isotope analysis, deuterium excess and dust concentrations all point to higher temperatures over a very rapid time-series. Dansgaard estimates the 5‰ increase in δO-18 over 50 years to be equivalent to a 7°C warming. Another paper by Alley et al (1993) (2) has found evidence for increased ice accumulation over an abrupt period of just 3 years at the onset of the YD's termination. Figure 2 (Alley et al) shows ice accumulation in the Late Glacial into the early Holocene.

Figure 2: Ice accumulation rates in Greenland showing evidence of rapid shifts during abrupt climate changes

The areas of abrupt change are focussed on each showing an abrupt change in ice accumulation as temperatures varied. Alley et al suggest that a near doubling of ice accumulation rates would require a c. 7°C warming in line with Dansgaard's predictions. Ice accumulation rate increases with temperature as more snow is able to fall when temperatures allow for greater air humidity increasing snowfall.

If the previous two articles weren't enough evidence for rapid abrupt climatic change at the end of the YD, the paper by Steffensen et al (2008) which I posted in my second post "Late Glacial and Holocene climate variability" details a deuterium excess shift occurring over just 1-3 years. 

Today's post has provided some ice core evidence to suggest that the Younger Dryas ended very abruptly. It has also shown that abrupt climate shifts can occur in human life expectancy time-scales. Towards the end of this blog, I will be assessing past changes and how they could be realised in the future. Although the YD was a significant event in Earth's recent history, a large increase in freshwater from ice-cap melting could feasibly occur again over an alarmingly rapid time-period, possible within our life-time. Next time, I will wrap up my discussion of the Younger Dryas before moving on to another large abrupt climate shift which occurred 8.2 ka BP.

(1) Dansgaard et al (1989) doi: 10.1038/339532a0
(2) Alley et al (1993) doi: 10.1038/362527a0
(3) Steffensen et al. (2008) doi: 10.0.4.102/science.1157707

R.I.P.- Younger Dryas impact event hypothesis

Avid followers, 

Last time I posted on some hypothesis behind the Younger Dryas stadial. A large part of that post described the theories of Firestone, Kennett and their colleagues who suggested (implored readers) that an impact event may have caused rapid melting of the Laurentide Ice sheet. After largely being discredited by subsequent literature, Anson Mackay has pointed me to a new article (1), still in press, which seeks to end the debate on the impact event hypothesis. I thought it was necessary to include a short post here to review that paper. 

Pinter and his colleagues, including Daulton, a previously cited opponent of the hypothesis, consider the evidence for the impact hypothesis in two sections. The first section involves those signatures of an impact event which have already been largely rejected in the literature. These include the magnetics within bones found at the YD onset, concentration variations in radioactivity, iridium and Helium isotopes. 

A second section involves the evidence such as carboniferous spheroids, magnetism in particles, wildfire evidence and nanodiamonds pushed so far by Kennett et al (See my previous post). Within this second section, Pinter et al. reject the origin of each materials supposedly attributed to an impact event. The carbon spheres, attributed to large-scale wildfires across North America, have been re-identified as terrestrial deposits from fungal and arthropod fauna. The paper also rejects the discovery of grains which have magnetism which could only have been produced by a meteorite as the authors, in their own work and the work of colleagues, have yet to reproduce these findings. The absence of a large fire event around the onset of the YD is also seen suggesting that there is no evidence to suggest an impact event causing fires and melting the Laurentide Ice sheet. Finally, the identification of nanodiamonds is called into question. This paper confirms evidence in my last post that the nanodiamonds were likely incorrectly identified and that they should have been identified as graphene and graphane compounds.

Overall, Pinter and his colleagues say that 7 of the 12 evidence arguments for the YD impact hypothesis are non-reproducible. This sounds to me as if they are undermining the work of Firestone et al. Indeed, further to my last post when I suggested that the hypothesis proponents may have been embarrassed, take a look at the conclusion of Pinter's paper to find a rejection of the hypothesis and a belittling of the proponents whom 'will continue their quest until the hypothesis is confirmed'.

Pinter et al. have called this hypothesis now a requiem, defined as a celebratory ceremony for the dead, hoping to end any further research on the subject. However, from this paper's utter rejection of the hypothesis, I would suggest that a less decorated ceremonial event should take place.

(1) Pinter et al (2011) doi: 10.1016/j.earscirev.2011.02.00

Alternative theories to explain the Younger Dryas

Last week, I blogged on the thermohaline circulation (THC) and the well-accepted cause of the Younger Dryas (YD). Today, I will look at two alternative causes of the YD, both of which suggest that extra-terrestrial factors forced the onset of the YD on Earth.

One such cause to consider suggests that the Earth was hit by one or many comets in the decades leading up to the beginning of the Younger Dryas 12.9 ka cal yr BP. First theorised by Firestone et al (2007) (1), this paper suggested that a black carbon-rich layer in many North American sediment cores may have been formed by a meteorite or series of meteorite events. This theory is supported by Kennett et al (2009) (2) who found evidence for nanodiamonds in the ground boundary layer dated to the YD, termed the 'black mat', in several North American sites. This layer was supposedly caused by continent-wide wildfires. The paper suggests that the nanodiamonds are left behind after comet impact events having been prevalent in cores dated to the Chicxulub impact event which is thought to have caused the Cretaceous-Tertiary extinction event. Both papers suggest that such an impact event would have destabilised the Laurentide ice sheet causing rapid melting and therefore freshwater floods to the Atlantic.

However, this theory has since been, essentially, destroyed. Other studies have found evidence for nanodiamonds in a YD dated layer in other locations including Tian et al (2010) (3). However, Tian and his colleagues found that the nanodiamonds present were consistent in all layers of their Lommel core in Belgium and therefore not attributable to an impact event. No evidence to support Kennett et al's theory was found by Surovell et al (2009) (4) who found no peak in magnetic minerals or substances which could have been left by a cosmic impact event. If this theory required any further rebuttal, Daulton et al (2010) (5) cast further doubt on the impact hypothesis by suggesting that what Firestone et al and Kennett et al identified as nanodiamonds, were actually graphene - naturally occurring single planes of graphite. Daulton et al found no evidence for nanodiamonds posing strong challenges to the impact hypothesis. I believe this may have caused some embarrassment for the supporting authors and they have yet to respond to support their theory.

In another theory, Renssen et al (2000) (6) argue that the main freshwater outburst occurred 1,000 years before the onset of the YD. Therefore the THC shutdown must have been helped by another factor which they suggest could be decreased solar activity.

This paper compiled several studies of cosmogenic isotope identification, the prevalence of Beryllium-10, from the GISP2 ice core, and Carbon-14, from dendrochronology, both produced by cosmic rays in the upper atmosphere and therefore an indication of solar activity. During the late glacial, the abundance of Beryllium-10 and Carbon-14 are well correlated pointing to three possible forcing mechanisms. Firstly, strong increases in both isotopes around the YD would suggest a decrease in solar activity. Such a sharp Carbon-14 increase can be seen in Figure 1 around the onset of the YD. Secondly, there is evidence for a c.2500 year cold periods which correlates with the Carbon-14 record. Such a sudden decline in Carbon-14, and therefore solar activity could have triggered the YD. Finally, Renssen and his colleagues argue that the current THC shutdown does not explain the evidence found for the YD in the tropics and the mid-latitudes of the Southern Hemisphere. A solar minimum could explain the global effect of the YD although the global nature of the YD is well disputed.

Figure 1. Link between Carbon-14 and Oxygen-13 (Renssen et al, 2000).

The mechanisms behind this theory are further explained by Renssen et al by suggesting that reduced solar activity could have caused a decrease in ozone content possibly resulting in a reduced latitudinal effect of the Hadley cell. This would cause cooling in all non-tropical areas and would shift precipitation belts. It is then explained that the precipitation shift may destabilise the polar ice sheets, increasing icebergs and increasing the freshwater flux to the THC causing a shutdown. Another possible mechanism suggests that cloud cover would increase from enhanced cosmic rays. More cloud cover would increase reflection of incoming radiation thus cooling Earth. Greater cloud cover may also increase precipitation and therefore freshwater input into the Atlantic Ocean.

Today's two additions to the THC shutdown theory posted last week both largely support a shutdown of the THC. However, the discredited cosmic impact event suggests that the THC shutdown is an effect of a series of meteorite whereas the theory of depressed solar activity seeks to operate alongside the freshwater outbursts from Lake Agassiz. It is still unknown what caused the THC shutdown but, having reviewed a wide-range of literature, this mechanism is still best positioned to explain the YD.

Next time, I will consider the abrupt termination of the YD before leaving the YD behind to look at another abrupt climate change event.

(1) Firestone et al (2007) doi: 10.1073/pnas.0706977104

(2) Kennett et al (2009) doi:10.1126/science.1162819

(3) Tian et al (2010) doi: 10.1073/pnas.1007695108

(4) Surovell et al (2010) doi: 10.1073/pnas.0907857106

(5) Daulton et al (2010) doi: 10.1073/pnas.1003904107

(6) Renssen et al (2000) doi:10.1016/S1040-6182(00)00060-4

Short circuiting the thermohaline circulation

Ladies and Gentlemen,

For this post, I will look at how the Younger Dryas (YD) the most-well researched of the Late Glacial abrupt climate shifts, supposedly cut-off the thermohaline circulation (THC) causing an abrupt cooling event. The THC, also known as the Atlantic Meridional Overturning Circulation (AMOC), is the method by which the ocean regulates global energy budgets by transporting heat and water across the globe and through the water column. By transporting heat from the equator polewards, together atmospheric circulation, heat is transported to mid- and high-latitude areas. The THC is driven by ocean currents which travel as a factor of sea-water density, affected by temperature and salinity. 

Cold, saline water at high latitudes is transported to low latitudes via the oceans' deep currents, warmer water is then transported from low-latitudes to replace this deficit. Water from the Northern Atlantic sinks and flows to the Southern Hemisphere and eventually to the conveyors circulating the Antarctic continent. Here more cold, saline water joins and is transported to the Indian Ocean before interactions with the Pacific basin. In areas of upwelling, especially in the Pacific, cold deep water rises to the surface and is heated and evaporated leaving saltier water behind. Such water flows North to join up with the Gulf Stream which travels from the Gulf of Mexico along the North American Eastern Seaboard and eventually towards NW Europe. The evaporated heat from this maintains the relatively mild British climate for its latitude. Evaporation, sea-ice formation and cooling within this process leaves very cool, saline water behind which sinks to the deep to re-start the process.  Since the THC relies on the sinking of cold, saline water in the polar regions, if a large volume of freshwater was dumped into the system, the water would become too light to sink. In this case, no warmer water would replace the regular sinking cold water and so the heat transfer to the polar regions would cease from the THC causing a rapid return to glaciation. Please see Figure 1 for a visual representation of the Earth's ocean currents.

Figure 1: Thermohaline circulation (Source: 1. TSC)

Interposed between the start of the Holocene and the Allerod/Bolling warming stages, the YD cooled the Earth from c.12,800 cal yr BP before coming to an abrupt stop c.11,500 cal yr BP. As temperatures rose through the Allerod and Bolling warm stages, the Laurentide ice sheet over North America retreated creating the largest North American lake by volume, Lake Agassiz. In these warm stages, the Lake periodically released water to the North (Arctic Ocean), South (Gulf of Mexico) and East (North Atlantic Ocean) as shown by Figure 2. 

Figure 2: Suggested overflow routes from Lake Aggasiz causing the Younger Dryas (2. Broecker, 2006). Note: the axes show latitude (y) and longitude (x)

However, it is widely believed that a large outburst of freshwater from Aggasiz into the North Atlantic caused the YD. This disrupted the THC plunging the Northern Hemisphere, and especially Europe into a period of cooling once more. The evidence for such large outbursts affecting the THC and causing the YD is well summarised by Teller et al. (2002). Teller et al suggest that freshwater inputs to the THC as low as a 0.1 Sv  flux (where 1 Sverdrup = 1 x 10⁶ m³s^-1) may interrupt the formation of North Atlantic Deep Water (NADW) which drives the cool, saline water in the deep THC of the North Atlantic. Data from Lake Agassiz outbursts suggest that a 0.3 Sv flood flux of 9500km³, the second largest recorded outburst, occurred 12.9 ka cal yr BP in line with the beginning of the Younger Dryas. Its route was through the Great-Lakes to the East and the St. Lawrence River flowing NW into the North Atlantic. If this flux was seen over a period of 1 year, it would be at least 6 times higher than the regular flow into the St. Lawrence from Agassiz.

Other authors have proposed different reasons for the inception of the YDs which will be explored next time. Following this, I will look at evidence for the YD's abrupt termination.

1. TSC: thermohaline circulation
2. Broecker (2006) doi:10.1126/science.1123253