Surface temperatures at the beginning of the last millennium were higher than in later years. However, the difference had nothing to do with the so-called Medievel Warm Period and the Little Ice Age. It was a result of the steadily falling trend.
MEDIEVAL WARM PERIOD AND THE LITTLE ICE AGE
Discussions of global warming bring often argument that the world experienced significant temperature changes in the past. It is claimed that we had the so-called Medieval Warm Period (1000-1300 AD), with temperatures allegedly comparable with the present temperatures, followed by the Little Ice Age (1400-1900) with excessively low temperatures. Such seemingly natural variations are claimed to show that the current global warming is not unusual. To illustrate the point a diagram might be even presented showing two distinct periods in the last 1000 years and thus "proving" that the current elevated mean temperatures are normal.
According to Bradley (2000) it was Hubart Lamb (Lamb 1965) who claimed that there was a Medieval Warm Period. Indeed, we have evidence of warmer climate in the Northern Hemisphere at the beginning of the last Millennium, and cooler conditions in later years. However, did we really have a clear periodic change with a well defined boundary that we could identify as the Medieval Warm Period and the Little Ice Age?
A STEADILY DECREASING TREND IN THE FIRST 900 YEARS
If we look at the surface temperatures of the Northern Hemisphere in the past millennium we can see immediately that there were no such well defined climatic periods. We can see many random fluctuations but no prominent temperature elevation in the first few centuries and no prominent depression in later years (see Figure 1).
Temperature data show a clearly decreasing trend
between 1000 and 1900 AD, so obviously the mean temperatures at the
beginning of this period were higher than the temperature at the end.
Around 1000 AD, the mean surface temperatures were 0.2°C below the
average reference temperature of 1961-1990. The overall decreasing trend
pushed the temperatures to around 0.4°C below the reference temperature
in 1900. However, from that year on, there was an undeniably rapid increase
in the mean surface temperatures (see Figure 2).
MEANINGLESS MANIPULATION OF DATA
By drawing a horizontal line and by adjusting its position, we can divide the steadily decreasing trend into two arbitrary regions. The regions corresponding to earlier years will have temperatures above the horizontal line, while the region for later years will have temperatures below the line. We can then call the first period warm and the second period cold but the division between the two regions will depend on the assumed position of the horizontal line. By moving the horizontal line up or down we can make the warm period longer or shorter. Clearly, any division between such two regions is arbitrary and therefore meaningless. The warmer temperatures in the earlier years and the colder temperatures in later years had nothing to do with the so-called Medieval Warm Period and the Little Ice Age. They were different because the overall trend was steadily falling.
A CLEAR TEMPERATURE INCREASE IN OUR TIME
In contrast with such elaborate but meaningless visual analysis we have no problem to see a clear and undeniably rapid increase in the mean temperature after 1900. The Northern Hemisphere data for the last millennium show a clear net drop of 0.2°C in the mean surface temperature in the first 900 years, followed by an increase of 0.8°C in the last 100 years. Records for the Southern Hemisphere display a similar pattern. Compared with the current steadily increasing temperatures, we have had a long spell of a cold period lasting for at least 900 years. Should we call it the Long Little Ice Age?
TEMPERATURE VARIATIONS IN THE PAST TWO MILLENNIA
Mann and Jones (2003) studied temperature variations during the past two millennia. Their results for the Northern Hemisphere are shown in Figure 3. These data also show a steadily falling trend but now over a longer period of around 1000 years. We see no sign of a clear periodic change that could be identified as a Medieval Warm Period and the Little Ice Age.
Temperatures between 200 and 800 AD were low and nearly constant. The only period in the past 2000 years, or so, was the last 100 years during which we had a rapidly increasing trend leading to temperatures distinctly higher than before. So again, compared with the current temperatures, we had something like a Long Little Ice Age for at least 1700 years.
for the Southern Hemisphere display similar behaviour. The combined
global temperatures show also a steadily decreased trend from around
700 AD to 1900 AD and no sign of the Medieval Warm Period or the Little
Ice Age (Figure 3).
CONCLUSION — NO DISTINCT TWO PERIODS
If we look at the first 900 years of the last millennium we can see that the mean global surface temperature was falling during that time, which of course means that it was higher at the beginning of this period than at the end of it. We can see a number of normal short-term fluctuations but we cannot see any long-term changes that could be interpreted as the Medieval Warm Period and the Little Ice Age. For such a steadily falling trend, any division between two periods, one with slightly higher temperatures and one with slightly lover, is arbitrary and therefore meaningless.
contrast, the last 100 years show a clear and distinct rise in the surface
temperatures reaching now significantly higher values than in the past
two millennia. Thus, the temperature data give no support for the global
Medieval Warm Period and the Little Ice Age.
Note: See also the Addendum below
You may use the information contained in this article as long as you refer to it as Nielsen, R. 2005, 'Medieval Warm Period and the Little Ice Age Myths', http://home.iprimus.com.au/nielsens/.
For a discussion of all critical global trends shaping our future see The Little Green Handbook.
Bradley, R. 2000, '1000 Years of Climate Change', Science 288:1353-1354.
Houghton, J. T., Ding, Y., Griggs, D. J., Noguer, M., Linden, P. J. van der, and Xiaosu, D. (eds) 2001, Climate Change 2001: The Scientific Basis, IPCC, Cambridge University Press, Cambridge, UK.
Lamb, H. H.1965, ‘The Early Medieval Warm Epoch and Its Sequel’, Palaeogeogr., Palaeoclimatol., Palaoecol. 1:13–37.
Mann, M. E. and Jones, F. D. 2003, 'Global Surface Temperatures over the Past Two Millennia' Geophysical Research Letters 30(15):1-4.
You might be interested in looking at the following figure and read its caption. Source: Mann, M. E. 2002, "Little Ice Age" in Ted Munn, Editor-in-Chief, Encyclopaedia of Global Environmental Change, John Wiley & Sons, Ltd, Chichester.
To appreciate global changes of surface temperature we have to be aware of a few important points:
1. Global v. regional or local. The average global change should not be confused with the average regional change. Furthermore, the average regional change is not the same as the average local change. For instance, the projected increase of surface temperature during the current century is about 2°C in New Zealand but about 6°C in Canada (see The Little Green Handbook). The average increase in the last century in Australia was 0.7°C but in some places the average increase was up to 2.5°C. In two places the average decreased by about 1°C.
2. Regional. The so-called Medieval Warm Period and the Little Ice Age were not global but regional. (It appears that they were confined to some regions in the Northern Hemisphere.)
3. Three important features of temperature change. In discussing temperature changes it is important to consider the range, duration, and gradient.
4. Short-term fluctuations are characterised by their duration and amplitude. They can be considered as normal if their duration and amplitude are similar to the generally observed changes.
5. Two distinct periods. In the graph below, we can distinguish two abnormal periods in the Central England: one with elevated temperatures at the beginning of the last millennium and one with depressed temperatures in the second half. We can label these two periods and the Medieval Warm and the Little Ice Age.
6. The range of temperature change. We can see that the maximum regional change during any of these two periods was less than 0.4°C. In contrast, if we look at the Figure 1 above, we can see that the current global change is already 0.8°C, that is, about twice as high.
7. The gradient. We should also compare the rate of change, i.e. look at the gradient of the temperature change. We can compare the steepest and average gradients. We shall find that the regional variations in the past were much slower than the current global change. The approximate estimate indicates that the steepest gradient in the Little Ice Age was about 30 times lower than the current gradient. It is like the difference between walking a slightly inclined street or climbing a steep mountain. The average gradient (measured on the way down or on the way up) during the Medieval Warm Period and the Little Ice Age in the Central England was about 60 times lower than the gradient of global temperature change towards the end of the last millennium.
8. Duration. The Mediaeval Warm Period lasted for about 400 years counting from the time the temperature started to rise to the time when it reached the original starting reference level. Even with a smaller increase (a maximum of 0.4°C above the reference level) longer exposure of certain areas in the Northern Hemisphere resulted in significant changes. The global temperature increase in the 20th century (with a maximum of 0.6°C occurred over a period of only about 30 years. Just try to imagine what would happen to global environment during the next 400 years with this relatively small increase. However, global mean temperature continues to rise.
9. Exaggerated claims. (See an article by Scott A. Mandia, Professor of Physical Sciences.) Greenland was not as green and not as warm as it is sometimes claimed. Greenland was not settled by Vikings because some intrepid explorers discovered a paradise on Earth. In 960 AD, a man living in Norway killed another man and had to flee his native country. He settled in Iceland. In 982, his son also killed a man and had to look for a place to hide. He discovered Greenland. He called it Greenland because he "believed more people would go thither if the country had a beautiful name." Greenland was a cold country. Farming was done only is selected sheltered places and for a short period each year.
Rather than to deny the ongoing global warming and climate change we should take steps to mitigate them. The future of our children is at stake and there is no room for squabbling and for playing smart.