Historic Climates II - The Medieval Warm Period (900-1300)

[WYorksWeather]

Given a bit more interest in my first post recently, and having a bit more time recently, I thought I'd continue the series. As a reminder, this series is working backwards through history, looking at major climatic changes, their impacts, causes if known, and what conclusions can be drawn. Today, I'm taking a look at the Medieval Warm Period (MWP), probably one of the most well-studied but also quite controversial eras, and the subject of much debate and criticism.

Date for the MWP

First, a quick one on the date. This is always somewhat subjective, but the most commonly cited date is around 950-1250 AD. I've extended this a little to 900AD-1300AD, because a few studies quote those sorts of time ranges, but anything that considers the MWP in a longer context and goes back to near the start of the Common Era has to be considered separately, for example if I did a piece on the Roman Warm Period, it would fit there.

What was the MWP?

Views on the MWP based on the established science have changed quite notably compared to what was thought in the early years of climate science. Here's a quote from the IPCC First Assessment Report in 1990 (see page 9 of the PDF), accompanied by the corresponding diagram:

The late tenth to early thirteenth centuries (about AD 950-1250) appear to have been exceptionally warm in western Europe, Iceland and Greenland (Alexandre 1987, Lamb, 1988) This period is known as the Medieval Climatic Optimum China was, however, cold at this time (mainly in winter) but South Japan was warm (Yoshino, 1978) This period of widespread warmth is notable in that there is no evidence that it was accompanied by an increase of greenhouse gases.

By 2001, however, in the IPCC Third Assessment Report (see page 40 of the PDF), accompanied by the corresponding diagram:

It is likely that the rate and duration of the warming of the 20th century is larger than any other time during the last 1,000 years. The 1990s are likely to have been the warmest decade of the millennium in the Northern Hemisphere, and 1998 is likely to have been the warmest year. There has been a considerable advance in understanding of temperature change that occurred over the last millennium, especially from the synthesis of individual temperature reconstructions. This new detailed temperature record for the Northern Hemisphere is shown in Figure 5. The data show a relatively warm period associated with the 11th to 14th centuries and a relatively cool period associated with the 15th to 19th centuries in the Northern Hemisphere. However, evidence does not support these “Medieval Warm Period” and “Little Ice Age” periods, respectively, as being globally synchronous. As Figure 5 indicates, the rate and duration of warming of the Northern Hemisphere in the 20th century appears to have been unprecedented during the millennium, and it cannot simply be considered as a recovery from the “Little Ice Age” of the 15th to 19th centuries. These analyses are complemented by sensitivity analysis of the spatial representativeness of available palaeoclimatic data, indicating that the warmth of the recent decade is outside the 95% confidence interval of temperature uncertainty, even during the warmest periods of the last millennium.

Summary

So, in general, as with the Little Ice Age, the understanding, as more paleoclimate samples have been taken outside the traditional observing countries in Europe and North America, we find a broadly similar trend to that seen for the Little Ice Age, that there has been a move from understanding the MWP as an event that had a major impact on global temperature, to one that had large effects, but primarily at the regional level, and at slightly different times.

If we now look at the more modern reconstructions I used for the Medieval Warm Period, you can see that the PAGES2k consensus for global temperature shows little trend, or if anything, a slight fall during the years 900-1300 (black vertical lines added by me).

 

However, this does not in any way mean that the MWP isn't of interest in a more regional sense. Those of you who remember my previous post will remember Ljungqvist's 2010 reconstruction of Northern Hemisphere extratropical temperatures for the last 2000 years, which still shows the classic pattern, with a distinct MWP peaking around 1000AD. 

 

Another point of confusion tends to be around how warm the MWP was. At the time when the first IPCC Assessment Report was being produced in 1990, there was a certain amount of evidence to suggest that the MWP was warmer than the then-current 1961-1990 average, at least across the Northern Hemisphere.

The two major changes since then are that firstly, the global temperature has increased, so confidence that temperatures today are warmer than those during the MWP has risen, and secondly that a greater range of observations shows that the MWP was a large scale regional phenomenon, but not truly global. However, at the regional level, particularly at very high latitudes (Greenland, Iceland, Northern Canada), there are still a few areas that may well have been warmer around 1000AD than they are now.

For the regional impacts, the following map from a 2009 study is useful (see page 3 of the PDF), which includes both MWP and LIA. Hatching indicates statistical significance, and the shapes on the right hand side show the proxies that were used, sized according to their weightings.

 

Selected notable events

• Southern Greenland likely saw temperatures close to those seen today, allowing the establishment of Norse colonies, which were later abandoned
• Early Norse settlers kept cattle, but their diet shifted more towards seafood as the climate became colder, and the growing season shortened
• Temporary Norse colonisation of Newfoundland coincided with the MWP
• Some glaciers in Southern Europe retreated, though not to present levels
• Towards the later part of the period, some of the impacts moved to parts of the Southern Hemisphere
• Only a very small number of areas were sufficiently warm that they would be warmer than modern baseline climates, primarily in the Arctic
• There were some impacts closer to the equator, but generally more on precipitation patterns than temperature
• Warming in some regions of China
• Warming in Japan (see the cherry blossom records below)

A useful proxy for Japanese temperatures is the cherry blossom peak, which has been recorded for over a millennium. It can be clearly seen that the cherry blossom peak was generally earlier during the time of the MWP than in the time period normally associated with the LIA, but clearly much later than in the modern warm period.

Causes

Due to it being further into the past than the LIA, less is known about potential causes of the MWP than the LIA. Any or all of the following may have been responsible:

• Solar / orbital changes
• Decreased volcanic activity
• Changes in ocean circulation, particularly the North Atlantic Oscillation (NAO), which may have been locked into a persistently positive phase (though this is still controversial!)

Summary

Like the LIA, we see another example where evidence has gradually shifted, and the modern understanding is that the global impact of the MWP was small. Nevertheless, it is considered a part of the cultural history of much of the high-latitude Northern Hemisphere, and with good reason, as changes here were quite dramatic. Many areas were as warm during the MWP as they were in the latter half of the 20th century, but only a very small number of areas were as warm during the MWP as they are today.

Next up

Next in my series is likely to be the Roman Warm Period. If you've got this far, thanks very much for reading. As always, happy to discuss further based on any replies.