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