Medieval Warm Period and Little Ice Age In South America

  • Date: 24/05/11

It is often said that the Medieval Warm Period (or Medieval Climatic Anomaly if you want to portray it as something unusual) is largely confined to Europe.

New evidence that it was a wider effect than was thought comes from a paper by Bird et al looking at how the South American monsoon has varied over the past two thousand years.

Laguna Pumacocha is a remarkable lake some 4,300 metres high in the Eastern Peruvian Andes. It is 23.5 metres deep and has a large volume of water despite its small surface area. It is the uppermost lake in a topographically isolated catchment area meaning that its water balance is dominated by rainfall and not water accumulation from other catchment areas.

This makes it a good site to obtain a sediment core to look at the accumulation of calcite from organic matter laid down over the past 2,000 years. It has been described as “the most detailed geochemical record of tropical climate fluctuations yet uncovered.”

To create a climate record from the sediment core, the team looked at the ratio of the oxygen isotope delta-O-18 in each annual sediment layer of mud. Levels of delta-O-18 are low during the wetter seasons and high when monsoon rain is light. The team found that the rainfall history produced by the lake core matched that established by delta-O-18 analyses from Cascayunga Cave in the Peruvian lowlands and the Quelccaya Ice Cap located high in the Andes.

The Pumacocha core matched the data obtained by these other sources between the years 980 and 2006, but provided much more detail.

The researchers found that the oxygen peaked between 900 – 1100 AD when the monsoon weakened during the time known as the Medieval Warm Period elsewhere in the world. The climate was warmer and drier. The oxygen had a low point between 1400 – 1820 when the climate was colder and wetter during the period known as the Little Ice Age.

The data suggests that the MWP and LIA had a considerable effect on South America.

The authors go further and state that the recent warm spell of the late 20th century shows the greatest shift seen in their data since records began in 300 BC, “the sediment record illustrated that rainfall during the South American summer monsoon has dropped sharply since 1900 —exhibiting the greatest shift in precipitation since around 300 BCE—while the Northern Hemisphere has experienced warmer temperatures.”

The press release associated with this research says something similar; “A sediment core from a South American lake revealed a steady, sharp drop in crucial monsoon rainfall since 1900, leading to the driest conditions in 1,000 years as of 2007 and threatening tropical populations with water shortages, a team from Pitt, Union College, and SUNY-Albany reports.”

Here is their data (click on image to enlarge.)





Perhaps, like me, you are not convinced that the data support this claim. What seems to be evident is that the current warm period’s extent is comparable to the MWP, and has been rising in a fairly steady fashion since the LIA, or about 1700 as seen in the graph.

The rapid increase seen post 1900 is there but only if you see it in isolation from the data preceding it. To me it looks like an increase sine 1700 with a few shorter-term variations. Only if you choose the start point at the bottom of one of these cycles do you come up with the statement that the increase seen since 1900 is the greatest seen in the data.

The viewpoint that the Medieval Warm Period is confined to Europe and the North Atlantic cannot be sustained not only because of this research but because of others as well. One example is the work described by Oppo et al as detailed in a previous report by the GWPF.

They measured the Indo Pacific Warm Pool which is the largest reservoir of warm surface water on the Earth and the main source of heat for the global atmosphere. They found that sea surface temperature for the MWP was comparable with current values.

Regarding the extent of the MWP and the LIA one would not expect it to be uniform across the globe, there will be warm and cool regions, but it seems they were certainly more global in extent than just Europe and the North Atlantic.



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