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.
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| 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.
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.
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| 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.
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| 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.
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| 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
