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