Source: Michael Kremer (August 1993): Table 1 and the Author. Kremer's hypothesis signifies that for most of history the rate of population growth should be proportional to the size of the population. To link his model and data with climate change, I started with his estimate of the world's people in 10,000 B.C. and calculated the rate of growth of the population over the next 5,000 years. For each subsequent period, I also computed the rate of increase in numbers of people. Comparing these expected rates with actual growth revealed eras in which the number of humans has expanded faster than predicted and periods during which the world's people has grown more slowly. The chart then shows the centuries in which the growth rate of the globe's populace has exceeded or fallen short of the rate expected under this simple model. As can be seen, warm periods have done considerably better than cold periods in terms of human expansion. The warmest period since the end of the last Ice Age produced the highest rate of population growth compared to what would have been expected in this era agriculture was spreading. Moreover, the Mini Ice Age, which saw the coldest temperatures in the last 10,000 years, underwent the slowest relative population expansion. This chart demonstrates that mankind has prospered in warm periods and the hotter the better! Another measure of the well-being of humans is how long they live. The life of the hunter-gatherer was not as rosy as some have contended. Life was short skeleton remains from before 8000 B.C. show that the average age of death for men was about 33 and that of women 28.19 Death for men was frequently violent, while many women must have died in childbirth. Since women died so young, they had only around thirteen years in which to bear children. Anthropologists have estimated that on average they could have given birth to less than five live babies, assuming that they bore a child every 22 months.20 An infant and childhood mortality rate of about 60 percent would have kept the population stagnant. Table 1 below shows some relevant data. The warmest periods, the Neolithic, Bronze Ages and England in the thirteenth centuries enjoyed the longest life spans of the entire record. The shortening of lives from the late thirteenth to the late fourteenth centuries with the advent of much cooler weather is particularly notable. Moreover, the rise in life expectancies during the warm period could easily explain the population explosion that took place during that period. Good childhood nutrition is reflected in taller adults. Skeleton remains collected over wide areas of Eurasia from the period when roving bands shifted from eating large animals and a few plants to smaller prey and a much wider variety of foods attest to a decline in height for both men and women of about five centimeters (two inches).21 The shorter stature came at the end of the Ice Age when large animals were disappearing. Some archaeologists have found that average age of death for adults also declined during this transitionary period.22 Studies of bone chemistry from Middle Eastern skeletons indicate a reduction in meat consumption. The new diet although more dependent on grains, fruits, and vegetables must have been less nutritious than the old. As large game animals disappeared with the end of the Ice Age, humans widened the variety of plants in their diet, increasingly consuming vegetable matter that they had ignored for thousands of years either because it was less nutritious, more difficult to secure and process, or less tasty. Table 2 on heights, however, signifies that food was more plentiful and better during the medieval Period than during the mini Ice Age. 19Boserup [1981]: 36-37. Table 1 Life Expectancy at Various Periods 31.5 38.2 38 35 Source: Lamb [1977]: 264 from Bergthorsson [1962]. earth was much warmer than today; perhaps 4°F hotter, about the average of the various predictions for global warming after a doubling of CO2.23 Although the climate cooled a bit after 3000 B.C., it stayed relatively warmer than the modern world until sometime after 1000 B.C., when chilly temperatures became more common. During this Climatic Optimum epoch, Europe enjoyed mild winters and warm summers with a storm belt far to the north. Not only was the country less subject to severe storms, but the skies were less cloudy and the days sunnier. Notwithstanding the less stormy weather, rainfall was more than adequate to produce widespread forests. Western Europe, including parts of Iceland and the Highlands of Scotland, was mantled by great woods.24 The timber, until average temperatures dipped temporarily for about 400 years between 3,500 B.C. and 3,000 B.C., consisted of warmth-demanding trees, such as elms and linden in North America and oak and hazel in Europe. Not only did Europe enjoy a benign climate with adequate rainfall, but the Mediterranean littoral, including the Middle East, apparently received considerably more moisture than it does today.25 The Indian subcontinent and China were also much wetter during this Optimal period. 26 Compared to cooler periods in the last few thousand years, the Sahara was much wetter and more fertile during the Climatic Optimum.27 Cave paintings from the epoch depict hippopotamuses, elephants, crocodiles, antelopes and even canoes.28 The water level in Lake Chad about 14° north of the equator in central Africa was some 30 to 40 meters, that is, 90 to 125 feet higher, than it is today, indicating much greater 23 Lamb (1988): 22. 24Giles, [1990]: 133. 26Lamb (1982): 120. precipitation. Ruins of ancient irrigation channels in Arabia, probably from the warmest millennia, derived their water from sources well above current water supplies, indicating a wetter climate.29 With the cooling that started after 3000 B.C., North Africa dried up and the abundance of life disappeared. As already mentioned, the invention of agriculture coincided with the end of the last Ice Age and the melting of the glaciers. Archaeologists have found the earliest evidence for husbandry and farming in Mesopotamia around 9000 B.C.30 The domestication of plants appears to have occurred around the world at about the same time: from 10,000 YBP to 7,500 YBP.31 Planting of wheat and barley began in southwest Asia between 8000 B.C. and 7000 B.C. Food production in China extends back at least into the sixth millennium B.C. In the Americas domestication of some grains and chili peppers dates from between 7000 B.C. and 6000 B.C.; anthropologists have documented maize in the Tehuacan Valley by 5700 B.C. and production may have started earlier. In South America the evidence suggests that domestication of two species of beans and chili pepper as well in the Andean highlands arose 8,500 years ago. Domestication of cattle occurred in the Sahara about 8,000 years before the present.32 The development of agriculture and the establishment of fixed communities led to a population explosion and the founding of cities. Agricultural societies produce enough surplus to support such urban developments, including the evolution of trades and new occupations. A large community could afford to have specialists who made farm tools, crafted pots, and traded within the village and between the locals and outsiders. The people in today's Palestine established the first known city, Jericho, and thus the first step towards specialization which lies at the heart of economic advancement — around 8000 B.C.33 In Europe, the Optimum period produced an expansion of civilization with the construction of cities and a technological revolution. The Bronze Age replaced the New Stone Age.34 The more benign climate with less severe storms encouraged travel by sea. Trade flourished during this warm period. People from ancient Denmark shipped amber along the Atlantic coast to the Mediterranean. As early as 2000 B.C., the Celts apparently were sailing from Cornwall and Brittany to both Scandinavia and southern Italy. Astrological monuments built around this time, such as Stonehenge, indicate that the skies were less cloudy than now.35 With the glaciers in the Alps during the late Bronze Age being only about 20 percent of the size of the ice in the nineteenth century, merchants made their way through the Brenner Pass, the dominant link between northern and southern Europe. Northern Europeans exchanged tin for manufactured bronze from the south. Alpine people mined gold and traded it for goods crafted around the Mediterranean. Prior to around 2500 to 1750 B.C., northwestern India, which is now very dry, enjoyed greater rainfall than it does in the twentieth century.36 In the Indus Valley, the Harappas created a thriving civilization that reached its apogee during the warmest and wettest periods, when their farmers were growing cereals in what is now a desert.37 The area was well watered with many lakes. This civilization disappeared around 1500 B.C. at a time when the climate became distinctly drier.38 Virtually all change can make some worse off and the warming after the last Ice Age is no exception. As the ice sheets melted, the sea level rose sharply and probably peaked around 2000 B.C.39 Although as the population explosion indicates most humans benefited, the growing warmth harmed some people, especially those who lived near the coast or who had earned their living hunting large animals. During the many centuries in which the waters mounted, storms often led to ocean flooding of coastal communities. Cooler, More Varied, and Stormy Times From the end of the Optimum period of sustained warmth until around 800 A.D. to 900 A.D., what we know of the world's climate and, in particular, the European varied between periods of warmth and cold. Based on the height of the upper tree lines in middle latitudes' mountains, the temperature record following the peak warm period around 5000 B.C. demonstrates a more or less steady decline right up to the 20th century.40 Tree ring data for New Zealand indicate that after temperatures reached a maximum around 6000 to 8000 B.C., the climate subsequently cooled in that part of the world. After 1000 B.C. the climate in Europe and the Mediterranean cooled sharply and by 500 B.C. had reached modern average temperatures.41 The period from 500 B.C. to 600 A.D. was one of varied warmth, although cooler on average than the previous 4,500 years. However, the climate became more clement and somewhat more stable from 100 35 Lamb [1977]: 254. 36Lamb [1977]: 251. 37Lamb [1977]: 389. 38Claiborne [1970]: 295. 39Lamb [1977]: note 1, p. 257. 40Lamb [1982]: Fig. 43, p. 118. 41Lamb [1988]: 22. |