Sunday, August 22, 2010

Little Ice Ages

The 500-year-long Little Ice Age prevailed from about 1350 to 1880, throughout the entire Earth, with temperatures averaging 1° lower than today’s. The Baltic Sea could be traversed by sleigh from Poland to Sweden, staying overnight in taverns built on the ice! The paintings by Pieter Breughel and Hendrick Avercamp illustrate the period. Here, Breughel’s “The Hunters.”

At the end of the 19th Century, the amount of CO2 discharged into the atmosphere by world industry was 13 times smaller than now. But the climate at that time had warmed up, as a result of natural causes, emerging from the 500-yearlong Little Ice Age, which prevailed approximately from 1350 to 1880. This was not a regional European phenomenon, but extended throughout the whole Earth19, 20 During this epoch, the average global temperature was 1°C lower than now. Festivals were organized on the frozen Thames River, and people travelled from Poland to Sweden, crossing the Baltic Sea on sleighs and staying overnight in a tavern build on ice.

This epoch is well illustrated by the paintings by Pieter Breughel and Hendrick Avercamp. In the mountains of Scotland, the snowline stretched down 300 to 400 meters lower than today. In the vicinity of Iceland and Greenland, the sea ice was so extensive that the access to a Greenland Viking colony, established in 985, was completely cut off; the colony was finally smashed by the Little Ice Age.

All this was preceded by the Middle Ages Warming, which lasted for more than 300 years (900 to 1100), and during which the temperature reached its maximum (1.5°C more than today) around the year 990. Both the Little Ice Age and the Middle Ages Warming, were not regional phenomena as implied by Mann and his co-authors, but were global and were observed around the North Atlantic Ocean, in Europe, Asia, South America, Australia, and Antarctica. During the Medieval Warming, the forest boundary in Canada reached 130 kilometers farther north than today, and in Poland, England, and Scotland vineyards for altar wine production flourished—only to be destroyed by the Little Ice Age. Still earlier, 3,500 to 6,000 years ago, a long-lasting Holocene Warming took place, when the average air temperature exceeded the current one by 2°C (Figure 5).

The Little Ice Age is not yet completely behind us. Stenothermal (warm-loving) diatom species, which reigned in the Baltic Sea during the Medieval Warming, have not yet returned.35 Diatom assemblages obtained from sediment core from the seabed of the north Icelandic shelf indicate that during the past 4,600 years the warmest summer sea-surface temperatures, about 8.1°C, occurred at 4,400 years before the present. Thereafter the climate cooled, with a warmer interlude of about 1°C near 850 years before the present. This was followed again by a cold span of the Little Ice Age, which brought mean summer sea-surface temperatures down by about 2.2°C. Today’s temperature of only 6.3°C still has not reached the Holocene warming level of 8.1°C.

The fastest temperature growth occurred in the early 20th Century, and the maximum was reached around 1940. It was then that the mountain and Arctic glaciers were shrinking violently, but their retreat from the record sizes (during the coldest part of Little Ice Age) had started 200 years earlier, around 1750, when no one even dreamed of industrial CO2 emissions. An illustration of this process is a map of glacier front changes between 1750 and 1961, at what is probably the best studied Storbreen Glacier in Norway, in which the first measurements of CO2 in ice were performed in 1956 (Figure 6). The attack of glaciers on Swiss villages in the 17th and 18th centuries—sometimes the velocity of ice movement reached 20 meters annually, destroying homes and fields—was perceived as a calamity. Yet, the withdrawal of glaciers in the 20th Century has been deemed, somewhat foolishly, to be a disaster.

Since the exceptionally hot 1940s, until 1975, the Earth’s climate cooled down by about 0.3°C, despite a more than three-fold increase of annual industrial CO2 emission during this period. After 1975, meteorological station measurements indicated that the average global temperature started to rise again, despite the decline in “human” CO2 emissions. However, it turns out that it was probably a measuring artifact, brought about by the growth of the cities and resulting “urban heat island” effect. Meteorological stations, which used to be sited outside of urban centers, have been absorbed by the cities, where the temperature is higher than in the countryside.


Disclaimer: Global Warming position of this blog is undecided.

No comments: