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