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Fluctuations in global temperatures throughout history have been accompanied by changes in sea levels and altered weather patterns, both of which have been linked to mass migrations, famines leading to disease, the collapse of some civilizations, and the growth of others. These cycles of warm and cold are affected by energy exchanges between oceans and the atmosphere, fossil-fuel emissions, and solar energy.
Many world historians explain changing Earth history in terms of its changing climate. Scientists however focus more directly on the causes for long-term climate change. They are: the exchange of energy by the oceans and atmosphere, fossil-fuel emissions, and solar energy. With global temperatures on the rise since 1860, climatologists predict that they may continue to increase by as much as 2°C during this century. Evidence of global warming appears in a melting Arctic ice cap—a reduction in volume of 3 to 4 percent each decade since the 1970s. As a result, sea levels have been rising since 1900 with the rate of change accelerating in the last half century. Contracting ice sheets and rising sea levels submerge coastal areas that affect the natural migration of some plants, animals, and microbes. According to the United Nations’ Intergovernmental Panel on Climate Change (IPCC) report titled Climate Change 2007, “Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global mean sea level.” During warming phases in the northern latitudes, migrating forests replace northern tundra and formerly marginal lands may become suitable for cultivation and food production. The costs to millions and possibly billions of people who live near the rising seas, slightly above, at, or below sea level would be devastating as rising temperatures and volatile weather in the tropics and middle latitudes would displace them from their homelands.
Climate fluctuations from warm, temperate, and interglacial to cold, arctic, and glacial can occur rapidly within a century and without much warning. The final collapse of the last Ice Age occurred about 9500 BCE and marked the beginning of the current global warming period. This rapid atmospheric warming, possibly the most significant climate event in the last forty thousand years, caused abrupt rises in global sea levels. Warming and glacial melt at about 7500 BCE flooded the Black Sea basin. The biblical “flood” may have been a reference to this natural catastrophe. Although many gaps exist in our knowledge about climate warming and cooling, efforts to unravel the complexities of the global climate have focused on specific events that trigger changes in the weather.
The Atlantic Circulation Energy Exchange
The world’s oceans serve as a heat transportation system absorbing much of the heat from the solar energy penetrating the atmosphere. As rising temperatures increase freshwater snowmelt, the salinity of the oceans decreases, affecting their circulation. Decreasing the volume of heavy salt water disrupts the great Atlantic Deep Water Circulation, which brings warm tropical water across the equator toward the North Pole. These warm waters become the “gulf stream” that warms the New England coast and brings moisture and warmth to the British Isles. Without it, these coastal regions would become several degrees colder, turning fertile soil into permafrost.
Since the oceans transport heat, abrupt small increases in temperature or increases in fresh glacial melt lower the density of the water, namely its capacity to sink, slowing down and in severe instances stopping the circulation. According to some climatic models, turning off or slowing down this current, often called “a huge heat pump,” has cooled down the northern temperate regions and been responsible for many of the abrupt climate oscillations during the last 100,000 years.
El Nino’s Impact on Climate Change
For the past forty years the salt content of the North Atlantic has declined continuously. But recent failures to simulate the relationship of these rapid climate changes to the Atlantic circulation have led scientists to search for another climatic event that may contribute to either hemispheric or global climate change. They have concluded that the tropical Pacific Ocean and its El Nino episodes in combination with the Atlantic circulation may provide an answer.
As the world’s largest ocean, the Pacific covers about 181 million square kilometers, with the widest band along the tropics, where solar energy is converted mostly into heat. Ocean currents and wind velocity serve to distribute this heat. During some years, atmospheric and sea temperatures are abnormally warm for reasons that are not completely understood. These reasons may include sunspot activity, the effects of the Atlantic Deep Water Circulation, and the fact that the world’s oceans now consume a larger carbon load caused by fossil-fuel emissions.
Some climatologists believe that these anomalies may trigger El Nino oceanic conditions, meaning that the flow of cold water from South America to the warm Pacific pool decreases or ceases entirely. In the absence of the easterly wind shears that drive the surface cold water to Asia, westerly winds push the warm Pacific pool toward the Americas. Hot and humid air travels with this warm pool, soaking previously arid equatorial islands and the coastal regions of the Americas from Peru to the west coast of the United States with torrential rains. With accelerating westerly winds, precipitation extends into the Western Hemisphere from the Americas and Eurasia to the Russian plains. Drought strikes India, China, Indonesia, and Africa.
The impact of El Nino on the distribution of heat and precipitation around the world is well known. What remains unknown is the relationship between Atlantic and Pacific oceanic events on the global climate. Ocean temperatures and salinity influence circulation. In fact, some North Atlantic deep water may enter the equatorial Pacific and cool the water temperature and the atmosphere as it does in the Atlantic Ocean and in this way defuse the catastrophic climatic effects of El Nino. If the cold water circulation of the Atlantic slows or stops, however, then there exists no known constraint on El Nino’s harmful effects.
El Nino’s Impact on World History
Prior to the Spanish conquest of the Incas in 1500 CE, advanced civilizations of native farmers and fishermen spread out along the northern Peruvian coast. These Native Americans, called the Moche, built pyramids, crafted pottery, and made gold ornaments to celebrate their spiritual and material achievements. The remains of their irrigated canals and mud-brick buildings attest to their advanced civilization. The sedimentary evidence found in riverbeds, in coastal lagoons, and by examining the fossil remains in these sediments reveals that repeated episodes of El Nino disrupted Moche civilization. Floods and droughts forced the Moche to abandon established sites for newer ones. With more energy from the overheated oceans, mega-El Ninos activated water and wind circulation, raised atmospheric temperatures, and created volatile weather.
By 1100 CE the warm period was giving way to the little Ice Age and mega-El Ninos were in decline. With less energy transfer to activate the tropical oceans, one last major El Nino reached landfall in northern Peru in 700 CE. These pre-Spanish Conquest El Ninos, similar to other major climatic events in world history, caused major political and cultural dislocations in the lives of native people. Floods in coastal areas and drought east of the Andes Mountains forced populations to relocate, rebuild, and adapt to the volatile weather systems that visited South America throughout this warming period.
El Nino in Recent History
Research into the history of the El Nino weather phenomenon stems from the discovery that the El Ninos of the 1980s had worldwide effects. Because of drought conditions during that decade, peasants were forced to leave northeastern Brazil, political instability occurred in some sub-Saharan countries, and food shortages became commonplace in India, China, and Japan. El Nino weather systems prove unmistakably the connectedness of the global climate system and its specific effects on biological entities, historical developments, and local and regional weather patterns.
The El Nino events of 1982–1983 reinforce theories and knowledge about the energy exchanges of the world’s cold and warm waters, especially in the Indian, Pacific, and Atlantic, by absorbing solar energy and releasing heat into the atmosphere. The destructive forces unleashed by El Nino episodes suggest that the world’s oceans reach a point of energy overload and need to discharge their accumulated heat. In this way an unpredictable global climate system is brought back into delicate balance.
As we have seen, scientists have identified three basic causes of global climate change—the exchange of energy by the oceans and the atmosphere, fossil-fuel emissions, and solar energy. Much remains to be discovered about its causes and effects, however. Solar energy output has periodic and sometimes irregular patterns of intensity. Periods of high intensity occur during eleven-year cycles and those of low intensity take place about every three and one-half years. Although the similarities in the pattern are not conclusive, some scientists argue that El Nino returns at intervals of high intensity. Finally, no substantial body of evidence points to a relationship between the warming of the Pacific waters in recent decades, a trigger for El Nino, and atmospheric pollution. Since much is unknown about El Nino, it remains an area of great interest for scientists and historians because of its potential effects on the global climate and global population.
The Role of Fossil-Fuel Emissions
Rising global temperatures translate into increased atmospheric water vapor, a greenhouse gas, as more of the world’s warming ocean water evaporates, causing more precipitation. Releasing the energy sequestered for millions of years in fossilized plants and animals by burning coal, oil, and gas elevates concentrations of another greenhouse gas, carbon dioxide (CO2), in the atmosphere. Most of these emissions come from three key sectors: electricity generation, transportation, and the heating and cooling of residential, commercial, and public buildings. Electric power generation globally contributes 41 percent of all CO2 emissions. During periods of industralization, the burning of fossil fuels and the process of deforestation have increased the carbon dioxide load in the atmosphere by about 25 percent. Within the last hundred years, 40–50 percent of the world’s pioneer forests and uninhabited lands that change CO2 into oxygen by the process known as photosynthesis have been transformed into agricultural production and commercial and residential construction. Also caused by the burning of fossil fuels, other fast-growing greenhouse gases such as methane (CH4) and chlorofluorocarbons (CFCs) with greater heat-absorbing qualities than CO2 have affected atmospheric temperatures. From 1850 to 2000, the human contribution to the increased concentration of CO2 by burning fossil fuels, deforestation, and agriculture was about 1.7 trillion tons. About 40 percent of this CO2 remains in the atmosphere and it is increasing at a rate of about 0.5 percent per year. The life span of CO2 molecules in the atmosphere is one hundred years, meaning that the emissions from the first Model T automobile that became available to consumers in 1927 and every vehicle built since remains a part of the human-induced global carbon load.
Global warming occurs more rapidly in frigid regions rather than in the temperate and tropical areas because arctic air lacks water vapor. This characteristic makes CO2 a more important greenhouse gas where the air is cold and dry. In warmer, humid air, water vapor is a more important as a transporter of heat than CO2. Because warming is unevenly distributed across the planet, what exactly CO2 contributes to global warming remains debatable.
During the current warming phase one can expect the physical properties of CO2 to contribute to more rainfall, higher atmospheric and oceanic temperatures, more clouds, and higher wind velocity. The biological effects of CO2 are noteworthy, contributing to longer growing seasons in the temperate and tropical climates. Arid and semiarid lands mainly unavailable for agriculture may receive sufficient moisture to increase food stocks for a global population whose growth rate will stabilize at between 9 and 12 billion people by 2050. The specific impacts of human population growth on the global climate system remain unknown, however.
The Role of Solar Energy
Two additional forces drive the global climate system. One cited often in the scientific literature but recently challenged is the Milankovitch explanation. The Serbian astronomer M. M. Milankovitch argued that the eccentric orbit of the Earth established its major global climatic cycle of 100,000 years. During that time the planet goes through a full interglacial/ glacial cycle. Within this longer pattern, another 41,000-year cycle controls the amount of solar energy reaching the Earth’s higher latitudes. It is caused by the tilt of the Earth on its axis.
A much shorter cycle caused by the “wobble” of the Earth on its axis occurs either at 23,000- or 19,000-year intervals and affects the amount of radiation striking the low latitudes and the equator. Milankovitch argued that during the last 800,000 years, Earth experienced eight complete glacial/ interglacial cycles. The Ice Ages lasted for 90,000 years followed by 10,000-year periods of warming. Accordingly, the current interglacial phase should be coming to an end.
Since the Milankovitch explanation accounts for only 0.1 percent change in the total solar energy reaching the Earth, however, some climatologists have looked elsewhere for a more coherent driving force behind climate change. They argue that fluctuations in solar energy follow a cyclical pattern of sunspot activity. Using this pattern, they have identified a pattern of eight cycles during the last 720,000 years of the Earth’s history. They are ninety thousand years in length, from full glacial with –0.3 percent of solar energy output to full warming with +0.3 percent. Given the dynamic changes in the Earth’s history and the gaps in our knowledge about its physical and biological properties however, predictions about future global climate changes remain illusive, despite the existence of this cyclical pattern.
Impact of Climate Changes on World History
A warming phase during the last major ice age from 33,000 to 26,000 BCE may have eased the migration of anatomically modern humans from Africa and southwestern Asia into Europe, replacing the resident Neanderthals. Before rising global sea levels eliminated the passage from Siberia to North America, this warming phase allowed human hunters to cross the frozen Bering Straits. In successive waves, possibly beginning as early as 32,000 BCE but no later than 11,000 BCE, they followed the hunt and populated the Americas.
As the last glacial maximum was ending about 13,000 BCE, a time when rising temperatures on Greenland approximated current ones, the retreat of the glaciers was interrupted by two little ice ages, the Older Dryas at 12,100 BCE and the Younger Dryas at 10,800 BCE. (A dryas is an Arctic flower that grew in Europe during the last Ice Age.) Evidence that the warm water circulation failed to reach the northern hemisphere around 12,700 BCE and inaugurated the Younger Dryas suggests that glacial melt entering the North Atlantic at the end of the last glacial maximum either slowed or stopped the Deep Water Circulation. It transformed the northern temperate regions into a little ice age for the next 1,300 years. The landscape of the British Isles became permafrost, with summer temperatures dropping below 32°C and winter ones below –10°C. Icebergs floated to the Iberian coast and long periods of drought affected Asia, Africa, and the midcontinent of North America. The slowdown of the Atlantic circulation may have been responsible for the changing hemispheric climate.
Impact of Climate Change on Indo-European Civilizations
Little ice ages have punctuated world history during these warming phases. As a result of cooling, the once fertile pastoral civilization of the Sahara collapsed, forcing the migration of its inhabitants to the Nile River valley about 5500 BCE. This settlement along the Nile coincided with the millennia-long rise of the ancient Egyptian civilization. Between 5000 and 4500 BCE, the Egyptians established their first empire and within centuries built the great pyramids at Giza. The Harappa civilization in the Indus Valley flourished as well, constructing public buildings and private dwellings of mud and fired bricks and using geometric plans to organize its cities.
From 4500 to 3800 BCE, a “global chill” interrupted human progress with seemingly endless periods of drought. The global climate may have been colder than at any time since the Younger Dryas. As happened during earlier periods of cold and arid conditions, the human population migrated south, escaping from the most extreme climatic conditions. Farming populations, the descendants of Indo- Europeans who in progressive migrations had brought farming technology into western and northern Europe from Southwest Asia many thousands of years before, were forced by the cold climate to retreat southward. They retreated to warmer regions along the Mediterranean and southeastward to Ukraine, to southwest Asia, India, and into northwest China.
Another protracted cold period brought drought to the irrigation-dependent “fertile crescent” civilizations of the Tigris, Euphrates, and Indus valleys between 3250 and 2750 BCE. In fact, some archaeologists suggest that the lush, naturally irrigated landscape in modern southern Iraq may have been the location of the biblical “Garden of Eden.” Recent archaeological research has verified that the collapse of the great agricultural Akkadian Empire in northern Mesopotamia (3200 to 2900 BCE) coincided with a major volcanic eruption and a subsequent climate shift from moist and cool to dry and hot that lasted for more than a century. These concurrent events forced this ancient population to leave the north and migrate into southern Mesopotamia (modern Iraq).
Climate Change and the Mayan Civilization
Another global cold spell that lasted from 2060 BCE to 1400 CE had beneficial effects turning tropical and subtropical regions into cooler and dryer climates. In Central America, the Mayan civilization expanded its agricultural productivity northward into the Yucatan, now part of Mexico, and built pyramids and cities in areas formerly thick with tropical vegetation and malaria-bearing mosquitoes. They remained there for about a thousand years.
Years without rainfall caused a series of collapses in Mayan agricultural productivity. Sedimentary records suggest that droughts began 1200 CE and revisited the region for the next five hundred years. They abandoned some cities in 1240 CE and the remaining ones in 1190 CE, when another severe dry period hit the area. Other causes may have contributed to the demise of the Mayans but the relationship of climate change to the collapse is a compelling one. After this particular global chill ended, the hydrological cycle gained strength as the climate warmed. The tropical forest returned along with the mosquitoes and forced the remaining Mayans to abandon their homes and to migrate southward. The fact that Mayan ruins are discovered now in the dense tropical rain forests of Central America is evidence of more recent global warming.
With the stronger hydrological cycle of the last 150 years, pioneers have cleared forests for agriculture, growing seasons have expanded, and more food has become available for a growing population. The relationships between climate change, the migration of human and animal populations, and the rise and decline of civilizations will require more detailed study before global climate change becomes more accepted as a causal factor in world history.
The Little Ice Age
Evidence from sediments and ice cores reveal that a little ice age (shorter than the typical 80,000 to 90,000 years) of long duration from approximately 1300 to 1850 CE swept across the northern hemisphere. Viking outposts in Greenland populated during the Medieval Warm Period (1000–1300 CE) succumbed to the freeze between 1200 and 1300. Food production plummeted in preindustrial Europe. Even in the best of times, where diets consisted mostly of bread and potatoes, daily food consumption seldom exceeded 2,000 calories. Widespread malnutrition was followed by famine and the outbreak of infectious diseases.
The bubonic plague followed the great European famine in 1400. Between 1100 and 1800, France experienced frequent famines, twenty-six in the twelfth century and sixteen in the nineteenth century. Increasing cold temperatures shortened the growing season by at least one month in northern European countries and the elevation for growing crops retreated about 18 meters. Not all populations suffered equally during this ice age, however. For those living along major rivers and coastal areas, fishing provided the animal protein lacking in the diets of the majority. In New England, 1815 was called “the year without a summer.” After 1850 and without warning this little ice age came to an end. Increased solar energy, the impact of industrialization on the atmospheric concentrations of greenhouse gases and changes in the Atlantic Deep Water Circulation have been identified as causes, either alone or in combination.
Climate Change: The Future
Examining climate events in history and reviewing the scientific findings of the present suggest that no single cause can adequately explain significant climate oscillations. The convergence of many changes in the world’s oceans, atmosphere, and land causes the disruptions and outbursts that we identify as significant climate events. These events possess such complex and unpredictable characteristics that to date the most advanced computers and global climate models (GCMs) have been unable to predict future climate events.
Despite our fragmentary knowledge of past climate events, knowing what to do in the future presents us with a great challenge. Population growth into coastal areas and onto marginal lands makes catastrophes more likely during periods of abrupt change. Increases in material consumption and energy use will continue to place stress on global ecosystems. The goal of sustainable growth in the developed world and the expectations for the same in the developing world remain elusive. In the words of Vaclav Smil (1990, 23), “If concerns about planetary warming will help to bring some sanity into the craven pursuit of economic growth and personal affluence throughout the rich world, and if they will aid in promoting control of population growth and responsible development policies in the poor world, then a warming trend might actually be an effective catalyst of desirable changes.”
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