Useful notes on Climatic changes during the Christian era

Since the beginning of the Christian era there has been an increasingly detailed and accurately dated knowledge of climatic variations. The level of the Caspian Sea provided an important clue to the past climatic conditions in Asia.

Historical records in China and India also proved much helpful. In Europe scientists studied the peat hogs, changing lake levels, and advances and retreats of glaciers as evidenced by the marks of glaciations on the earth’s surface.

Other valuable sources for the reconstruction of past climatic conditions were the location of settlements, literary records and old weather journals. In north­east Africa the fluctuations in the levels of the Nile and the history of oases could speak volumes for the climatic conditions of the past years.

In the western part of North America the study of the width of the annual tree rings could help in ascertaining accurately the amount of rainfall. It is interesting to note the general agreement in a wide variety of evidences from the North Temperate Zone. However, there are minor differences of detail.

On the basis of the varied lines of evidence, it has been established that the climatic conditions at the beginning of the Christian era were much the same as at present. It north-east Africa the climate was somewhat rainier.

There began a period of relative warmth and dryness at about 400 A.D. The Caspian Sea was much below its present level. The peak of this dry period was reached at about 700 A.D., when the Alpine passes began to be greatly used by increased traffic.

The Atlantic and the North Sea were storm free so that there was much sea traffic off Western Europe. It was the period when there were lowland settlements in the Saxon lands, and Celtic and Northumbrian cultures flourished.

Civilization of Ireland reached new heights, and missions from the Celtic Church were sent to foreign lands. It was also the time of great Viking voyages.

Iceland and Greenland being much warmer than at present were largely settled. During the same dry period, China had a number of severe droughts. There is evidence of low Nile levels. Because of increased dryness, the North African oases settlements were adversely affected.

The decline set in again. The period 1200-1400 A.D. witnessed remarkable climatic insta­bility in Western Europe with great floods and droughts. Sometimes winters were very severe, sometimes notably mild.

Polar ice was more extensive in Greenland and Iceland waters than ever before. Most of the previous colonies in Greenland died out. Links between Greenland and the outside world were severed. Even the height of people of the Norse stock declined.

After 1200 A.D. there was a period of great storminess in Western Europe. North Sea coasts of England were inundated many times. The ground was permanently frozen at about 1400 A.D., and the colonies disappeared.

The next centuries were generally rainy. All over the world the glaciers re-advanced. The period from 1550 to about 1850 A.D. has been called the Little Ice Age. By 1780-1800 the ice extended more than half way from Greenland to Norway.

The glaciers which began to advance during the 13th century stopped completely during the second half of the 16th century. From about 1600 to 1650 the advance of the glaciers became very rapid. In the Alps and Iceland there was a temporary halt.

But at about 1750 which marks the second maximum-the glaciers reached their maximum extension. Since the end of the Pleistocene Ice Age, the glaciers had never advanced so far. Since then there has been a general retreat.

The present era of instrumental records brings us to the last phase of the climatic changes that have been described so far. The barometer and the thermometer were invented in 1643, and the use of rain gauges began between 1676 and 1690s.

In the eighteenth century various types of barometers, thermometers and rain gauges were made available. Despite the fact the different countries used different units, now we have homogeneous records of temperature and other) climatic elements for a number of places in Europe and elsewhere over periods going back into the eighteenth century.

On the basis of instrumental records and weather diaries it may be concluded that during the past 100-200 years there have been many changes in temperature, precipitation and other climatic elements.

The most significant aspect of these changes has been the world-wide climatic amelioration from the 1830s to the 1930s. This is the first climatic change that can be measured and plotted on maps.

The present climatic phase resembles the warmer periods in the middle ages. Investigations have shown that the intensification of the general circulation has continued from about 1800 to the 1920s in the northern and the southern hemispheres. This has led to the most rapid shrinkage of the Arctic ice.

The pronounced effect of the intensified atmospheric circulation has been that the extent of the oceanic influence has increased, particularly during the winter months. Very low surface temperatures over the continents became rare.

Thus, in winter the gradients of surface temperature showed a declining trend. On the contrary, the temperature gradient in summer increased.

After 1850, winters, according to meteorological records, became progressively warmer in all polar and temperate regions. However, a reversal of this tendency is indicated by the colder winters beginning from the middle of the twentieth century.

Since the beginning of the Christian era there have been a number of climatic fluctuations that are well documented. There has been a fluctuation in the level of the Caspian Sea to the tune of 15 meters. Glaciers from the Alps have spread out and retreated several times. Tree rings also bear testimony to the fluctuations in precipitation.