Climate may be defined as the total variety of weather experienced in a place over some specified period of time, usually some decades. Climate is never static, but is constantly changing over several timescales. Important timescales for climate change include:
Any climate change has the potential to disrupt human activities. Consideration of human impact on climate, and its effects therefore must begin with an understanding of natural climate variability, and its effects on humans through history. In this lecture, we examine climate changes over the last 1,000 years, and look at the way in which climate change has impacted on human activities.
Humans have been vulnerable to changes in climate at least since the introduction of permanent settlements. Climate change affects the availability of water, communications, the length of the growing season, the wellbeing of livestock, and many other factors, and has been a decisive factor in the survival of communities throughout history. Here, we give a few examples to illustrate the vulnerability of human activity to climate change.
Prior to c. 1200 AD, climate in Europe was warmer than today: this is known as the Medieval Warm Period. At this time, vineyards thrived in England, and agricultural productivity was high.
The Vikings settled Iceland in the 860s and Greenland in the 980s, aided by a long period when sea-ice lay far to the north, and severe storms comparatively rare (Iceland had been settled at least 100 years earlier by Irish monks). During the heyday of Viking settlement in Greenland, the coastal waters were at least as warm as in the warmest years of the 20th Century, and possibly rather warmer. However, by c. 1200 a general cooling of the arctic began, resulting in the expansion of sea-ice, thus restricting fishing and communications. On land, colder conditions meant that agriculture became more difficult. At this time, the Inuit (Eskimos) began to move south, apparently also due to deteriorating climate, and the first contact between them an Vikings in Greenland occurred around 1200-1250. Around 1350, one of the two main Norse settlements was wiped out, either by conflict or disease. Regular trade between Greenland and Europe ceased in 1370, due to increasing severity of the weather and extensive sea-ice. The remaining Norse settlement fell into decline, and died out around 1500. In Iceland, Norse society underwent a severe decline from around 1200, at least partly due to climatic causes. Worsening winters meant that there was less fodder for sheep, and crops failed. Pollen evidence indicates that oats ceased to be cultivated in southern Iceland around 1200, and later the hardier barley was also abandoned. Sea-ice expanded throughout the 13th century, restricting fisheries. After a respite in the 1400s, climate began to worsen again, in the late 1500s. The arctic pack ice frequently reached the coast of Iceland, and in 1695 it completely surrounded the island for several weeks, so that no ships could enter or leave. Arctic sea-ice reached the coast of Iceland as late as 1756. In the 1600s, the beginning of the Little Ice Age, glaciers began to expand, and overran several farms. As a result of the hardship brought on by this climatic change, the Icelanders were reduced to a wretched state by the mid 1700s, and the island was one of the poorest and most backward areas of Europe.
The period 1500-1800 was one of climatic severity throughout Europe, with glaciers advancing in the Alps and Scandinavia. Farms were destroyed by ice advance in many areas of Norway. It was also a time of extreme storminess, with several gales apparently exceeding the severity of modern events. Some of these storms exerted an influence on history: the Spanish Armada (1588) had to contend with desperate weather as well as Francis Drake: gales prevented the return of the fleet westward through the English Channel, and forced a voyage around the north of Scotland, resulting in many shipwrecks and huge loss of life.
The severity of winters is indicated by the freezing of the Thames in London - 11 times in the 1600s, and c. 20 times between 1564 and 1814, allowing ice fairs to be held in mid-river. These hard winters had their hardest impact in the north of Europe, including Scotland.
Winters in Scotland. The Little Ice Age was a time of severe winters in Scotland. Annual temperatures were c. 2¡ cooler than today, with prolonged winters and much snow. The weather was particularly severe between 1690 and 1700, when harvests failed in seven of these years, causing widespread famine. Parish records from Rothiemurchus (Speyside) reported: "the poorer sort of people frequented the churchyard to pull a mass of nettles, and frequently fought over... and greedily fell upon it". Some families were reported to have sold their children into slavery. In parishes all over the country, between one- and two-thirds of the people died, a greater diasater in many areas than the Black Death. Some whole villages were depopulated at that time. Perhaps 20% of the population of Scotland was reduced to begging.
A bizarre occurrence - possibly due to the increase in pack ice extent - was the appearance on several occasions of solitary Innuit hunters in kayaks in the Orkneys, and once in the River Don at Aberdeen, between 1690 and 1728. Severe winters continued in the 1700s. A few examples serve to illustrate the situation. January 1794: a particularly severe snowstorm hit the south-west of Scotland, lasting several days. The storm became known as the Gonial Blast, due to the large numbers of sheep that died (ÔgonialÕ being the name for the mutton of sheep found dead, and turned into smoked ÔhamsÕ). James Hogg, shepherd and author, wrote in 1829 about the aftermath of the subsequent floods:
"there is a place called the Beds of Esk, where the tide throws out and leaves whatever is carried into it by the rivers. When the flood after the storm subsided, there was found on that place and shores adjacent, one thousand eight hundred and forty sheep, nine black cattle, three horses, two men, one woman, forty-five dogs and one hundred and eighty hares, beside a number of meaner animals"
The following winter of 1794/95 was also very severe, and prolonged. Severe weather began in November, and persisted well into the new year. April brought floods, and May more snow. On 15th May, snow lay 30cm deep in Aberdeenshire, and thick layers of ice covered the rivers.
Another example of human activities being disrupted by climate change concerns the introduction of agriculture to the Great Plains of the USA. The Great Plains (stretching from the Dakotas to Texas) had been crossed by settlers en route to Oregon and California in the1840s, but few permanent settlements were established, partly due to the resistance of the local tribes, but largely because the Plains were regarded as too arid for agriculture. Indeed, the region was known as the Great American Desert. The 1870s and 1880s were wet years, and the Plains were settled by farmers, encouraged by the Homestead Act and promoters, who encouraged the belief that the climate had permanently changed for the better. This was popularly supposed to be the result of Divine Providence, opening up the region for Christian settlement. Some scientists suggested physical reasons for why agriculture should increase rainfall, and land speculators eagerly spread the idea that Õrain follows the plowÕ.
The cultivated area was increased dramatically during the First World War, when wheat prices were high and rainfall abundant. However, the expansion of agriculture on the plains ignored the fact that the region is subject to periodic drought, apparently every 20 years or so. A particularly severe and prolonged drought occurred during 1930s: successive summers between 1932 and 1937 were hot and dry, parching crops and the soil. The native grasses of the prairie could withstand drought and bound the soil with their matted roots, but the new crops died, exposing the soil to the arid winds. Severe dust storms turned the region into a Dust Bowl, making life unbearable and causing thousands to abandon the area, as described in John SteinbeckÕs great novel The Grapes of Wrath. On occasion, the dust reached as far as New York, filtering the sun and causing half-light conditions at mid-day.
Cattle ranching on the Plains was also hit by unexpected weather events. Ranching had expanded dramatically in the Northern Plains in the 1880s, as new railroads opened up eastern markets and with the eradication of the native peoples and buffalo which had hitherto occupied the area. By the summer of 1886, overgrazing had seriously depleted fodder and weakened herds, then the winter of 1886/1887 was more severe than any in recent years. Snow began in November 1886, and did not stop for a month, followed by severe frosts and more blizzards. The cattle were severly hit, and when the thaw eventually came hundreds of thousands lay dead across the land, and the winter became known as ÔThe Great Die UpÕ.
Despite the belief that 'rain follows the plow", the climate changes prior to the 20th Century form are very unlikely to have been due to human activities. Instead, the Medieval Warm Period, the Little Ice Age, and the early part of the subsequent warming are believed to be part of a natural climatic cycle, probably related to variations in the activity of the sun. The role of the sun, and other factors in natural climate cycles are discussed in Lecture 2. Whatever the causes of climate change, it is clear that it has the ability to severely disrupt human activities. Today, people are more insulated against nature by warm housing, electricity, and other technologies. But snowfalls of comparatively minor depth still manage to disrupt traffic, communications; winter storms cause flooding and disrupt power supplies; and summer droughts threaten water supplies. How would the people of today cope with the Little Ice Age?
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