Global climate - Article 2 1'17<.3'1" i 1"-h.r...

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Unformatted text preview: Article 2 1'17: .'. ;_.<_,.-___- {.3 .;.',.--- .:'- '1". i '. :.-'-. .- 1."-h-.r; . ' Z3" ‘."—."-.‘.-' .~,;.-,_ HOW DID HUMANS FIRST ALTER GLOBAL CLIMATE . A bold new hypothesis suggests, that our ancestors’ farming practices kicked off global warming thousands of years before we started burning coal and driving cars William E. Ruddiman The scientific consensus that human actions first began to have a warming effect on the earth’s climate within the past century has become part of the public perception as well. With the advent of coal-buming factories and power plants, industrial societies began releasing carbon dioxide (C02) and other greenhouse gases into the air. Later, mo- tor vehicles added to such emissions. In this scenario, those of us who have lived during the industrial era are responsible not only for the gas buildup in the atmo- sphere but also for at least part of the accompanying glo- bal warming trend. Now, though, it seems our ancient agrarian ancestors may have begun adding these gases to the atmosphere many millennia ago, thereby altering the earth’s climate long before anyone thought. New evidence suggests that concentrations of C02 started rising about 8,000 years ago, even though natural trends indicate they should have been dropping. Some 3,000 years later the same thing happened to methane, another heat-trapping gas. The consequences of these surprising rises have been profound. Without them, current tempera- tures in northern parts of North America and Europe would be cooler by three to four degrees Celsius—enough to make agriculture difficult. In addition, an incipient ice age— marked by the appearance of small ice caps—would proba- bly have begun several thousand years ago in parts of north- eastern Canada. Instead the earth’s climate has remained relatively warm and stable in recent millennia. Until a few years ago, these anomalous reversals in greenhouse gas trends and their resulting effects on cli- mate had escaped notice. But after studying the problem for some time, I realized that about 8,000 years ago the gas 183 trends stopped following the pattern that would be pre- dicted from their past long-term behavior, which had been marked by regular cycles. 1 concluded that human activities tied to farming—primarily agricultural defores- tation and crop irrigation—must have added the extra C02 and methane to the atmosphere. These activities ex- plained both the reversals in gas trends and the ongoing increases right up to the start of the industrial era. Since then, modern technological innovations have brought about even faster rises in greenhouse gas concentrations. My claim that human contributions have been ALTERING THE EARTH’S CLIMATE FOR MILLENNIA is provocative and controversial. My claim that human contributions have been altering the earth’s climate for millennia is provocative and con- troversial. Other scientists have reacted to this proposal with the mixture of enthusiasm and skepticism that is typical when novel ideas are put forward, and testing of this hypothesis is now under way. The Current View THIS NEW IDEA builds on decades of advances in under- standing long-term climate change. Scientists have known since the 19705 that three predictable variations in the earth's orbit around the sun have exerted the dominant control over long-term global climate for millions of years. ANNUAL EDITIONS Overview/Early Global Warming - A new hypothesis challenges the conventional assumption that greenhouse gases released by human activities have perturbed the earth's delicate climate only within the past 200 years. 0 New evidence suggests instead that our human ancestors began contributing significant quan- tities of greenhouse gases to the atmosphere thousands of years earlier by clearing forests and lrrigatlng fields to grow crops. - Asa result. human beings kept the planet notably warmer than It would have been otherwise—and possibly even averted the start of a new ice age. As a consequence of these orbital cycles (which operate over 100,000, 41,000 and 22,000 years), the amount of solar radiation reaching various parts of the globe during a given season can differ by more than 10 percent. Over the past three million years, these regular changes in the amount of sunlight reaching the planet’s surface have pro- duced a long sequence of ice ages (when great areas of Northem Hemisphere continents were covered with ice) separated by short, warm interglacial periods. Dozens of these climatic sequences occurred over the millions of years when hominids were slowly evolving toward anatomically modern humans. At the end of the most recent glacial period, the ice sheets that had blan- keted northern Europe and North America for the previ- ous 100,000 years shrank and, by 6,000 years ago, had disappeared. Smn after, our ancestors built cities, in- vented writing and founded religions. Many scientists credit much of the progress of civilization to this natu- rally warm gap between less favorable glacial intervals, but in my opinion this view is far from the full story. In recent years, cores of ice drilled in the Antarctic and Greenland ice sheets have provided extremely valuable evidence about the earth's past climate, including changes in the concentrations of the greenhouse gases. A three-kilometer-long ice core retrieved from Vostok Sta- tion in Antarctica during the 19905 contained trapped bubbles of ancient air that revealed the composition of the atmosphere (and the gases) at the time the ice layers formed. The Vostok ice confirmed that concentrations of C02 and methane rose and fell in a regular pattern during virtually all of the past 400,000 years. Particularly noteworthy was that these increases and decreases in greenhouse gases occurred at the same inter- vals as variations in the intensity of solar radiation and the size of the ice sheets. For example, methane concen- trations fluctuate mainly at the 22,000-year tempo of an orbital cycle called precession. As the earth spins on its rotation axis, it wobbles like a top, slowly swinging the Northern Hemisphere closer to and then farther from the sun. When this precessional wobble brings the northern continents nearest the sun during the summertime, the at- 184 mosphere gets a notable boost of methane from its pri— mary natural source—the decomposition of plant matter in wetlands. After wetland vegetation flourishes in late summer, it then dies, decays and emits carbon in the form of meth~ ane, sometimes called swamp gas. Periods of maximum summertime heating enhance methane production in two primary ways: In southern Asia, the warmth draws addi- tional moisture-laden air in from the Indian Ocean, driv- ing strong tropical monsoons that flood regions that might otherwise stay dry. In far northern Asia and Eu- rope, hot summers thaw boreal wetlands for longer peri- ods of the year. Both processes enable more vegetation to grow, decompose and emit methane every 22,000 years. When the Northern Hemisphere veers farther from the sun, methane emissions start to decline. They bottom out 11,000 years later—the point in the cycle when Northern Hemisphere summers receive the least solar radiation. Unexpected Reversals EXAMiNlNC RECORDS from the Vostok ice core closely, I spotted something odd about the recent part of the record. Early in previous interglacial intervals, the meth- ane concentration typically reached a peak of almost 700 parts per billion (ppb) as precession brought summer ra- diation to a maximum. The same thing happened 11,000 years ago, just as the current interglacial period began. Also in agreement with prior cycles, the methane concen- tration then declined by 100 ppb as summer sunshine subsequently waned. Had the recent trend continued to mimic older interglacial intervals, it would have fallen to a value near 450 ppb during the current minimum in summer heating. Instead the trend reversed direction 5,000 years ago and rose gradually back to almost 700 ppb just before the start of the industrial era. In short, the methane concentration rose when it should have fallen, and it ended up 250 ppb higher than the equivalent point in earlier cycles. Like methane, C02 has behaved unexpectedly over the past several thousand years. Although a complex combi- nation of all three orbital cycles controls C02 variations, the trends during previous interglacial intervals were all surprisingly similar to one another. Concentrations peaked at 275 to 300 parts per million (ppm) early in each warm period, even before the last remnants of the great ice sheets finished melting. The C02 levels then fell steadily over the next 15,000 years to an average of about 245 ppm. During the current interglacial interval, C02 concentrations reached the expected peak around 10,500 years ago and, just as anticipated, began a similar decline. But instead of continuing to drop steadily through mod- ern times, the trend reversed direction 8,000 years ago. By the start of the industrial era, the concentration had risen to 285 ppm—roughly 40 ppm higher than expected from the earlier behavior. Article 24. HOW' DID HUMANS FIRST ALTER GLOBAL CLIMATE? What could explain these unexpected reversals in the natural trends of both methane and C02? Other investi- gators suggested that natural factors in the climate sys tern provided the answer. The methane increase has been ascribed to expansion of wetlands in Arctic regions and the C02 rise to natural losses of carbon-rich vegetation on the continents, as well as to changes in the chemistry of the ocean. Yet it struck me that these explanations were doomed to fail for a simple reason. During the four pre- ceding interglaciations, the major factors thought to influ- ence greenhouse gas concentrations in the atmosphere were nearly the same as in recent millennia. The northern ice sheets had melted, northern forests had reoccupied the land uncovered by ice, meltwater from the ice had re- turned sea level to its high interglacial position, and solar radiation driven by the earth’s orbit had increased and then began to decrease in the same way. Why, then, would the gas concentrations have fallen during the last four interglaciations yet risen only during the current one? I concluded that something new to the natural workings of the climate system must have been operating during the past several thousand years. The Human Connection THE MOST PLAUSIBLE “new factor” operating in the climate system during the present interglaciation is farming. The basic timeline of agricultural innovations is well known. Agriculture originated in the Fertile Crescent region of the eastern Mediterranean around 11,000 years ago, shortly thereafter in northern China, and several thou- sand years later in the Americas. Through subsequent millennia it spread to other regions and increased in so- phistication. By 2,000 years ago, every crop food eaten to- day was being cultivated somewhere in the world. Several farming activities generate methane. Rice pad- dies flooded by irrigation generate methane for the same reason that natural wetlands do—vegetation decomposes in the stagnant standing water. Methane is also released as farmers burn grasslands to attract game and promote growth of berries. In addition, people and their domesti- cated animals emit methane with feces and belches. All these factors probably contributed to a gradual rise in methane as human populations grew slowly, but only one process seems likely to have accounted for the abruptness of the reversal from a natural methane decline to an unexpected rise around 5,000 years ago—the onset of rice irrigation in southern Asia. Farmers began flooding lowlands near rivers to grow wet-adapted strains of rice around 5,000 years ago in the south of China. With extensive floodplains lying within easy reach of several large rivers, it makes sense that broad swaths of land could have been flooded soon after the tech- nique was discovered, thus explaining the quick shift in the methane trend. Historical records also indicate a steady expansion in rice irrigation throughout the interval when methane values were rising. By 3,000 years ago the tech- nique had spread south into Indochina and west to the Ganges River Valley in India, further increasing methane emissions. After 2,000 years, farmers began to construct rice paddies on the steep hillsides of Southeast Asia. Future research may provide quantitative estimates of the amount of land irrigated and methane generated through this 5,000-year interval. Such estimates will be probably difficult to come by, however, because repeated ir— rigation of the same areas into modern times has probably disturbed much of the earlier evidence. For now, my case rests mainly on the basic fact that the methane trend went the ”wrong way" and that farmers began to irrigate wet- lands at just the right time to explain this wrong-way trend. Another common practice tied to fanning—deforestation— provides a plausible explanation for the start of the anom- alous C02 trend. Growing crops in naturally forested ar- eas requires cutting trees, and farmers began to clear forests for this purpose in Europe and China by 8,000 years ago, initially with axes made of stone and later from bronze and then iron. Whether the fallen trees were burned or left to rot, their carbon would have soon oxi- dized and ended up in the atmosphere as C02. Scientists have precisely dated evidence that Europe- ans began growing nonindigenous crop plants such as wheat, barley and peas in naturally forested areas just as the C02 trend reversed 8,000 years ago. Remains of these plants, initially cultivated in the Near East, first appear in lake sediments in southeastern Europe and then spread to the west and north over the next several thousand years. During this interval, silt and clay began to wash into riv- ers and lakes from denuded hillsides at increasing rates, further attesting to ongoing forest clearance. The most unequivocal evidence of early and extensive deforestation lies in a unique historical document—the Doomsday Book. This survey of England, ordered by Wil- liam the Conqueror, reported that 90 percent of the natural forest in lowland, agricultural regions was cleared as of AD. 1086. The survey also counted 1.5 million people liv- ing in England at the time, indicating that an average den- sity of 10 people per square kilometer was sufficient to eliminate the forests. Because the advanced civilizations of the major river valleys of China and India had reached much higher population densities several thousand years prior, many historical ecologists have concluded that these regions were heavily deforested some two or even three thousand years ago. In summary, Europe and southern Asia had been heavily deforested long before the start of the industrial era, and the clearance process was well un- der way throughout the time of the unusual C02 rise. An Ice Age Prevented? [F FARMERS WERE responsible for greenhouse gas anoma- lies this large—250 ppb for methane and 40 ppm for C02 by the 1700s—the effect of their practices on the earth’s climate would have been substantial. Based on the aver- age sensitivity shown by a range of climate models, the ANNUAL EDITIONS Human Disease and Global Cooling Concentrations of CO; in the atmosphere have been climbing since about 8,000 years ago. During the past two millennia, how- ever, that steady increase at times reversed direction, and the C02 levels fell for decades or more. Scientists usually attribute such C02 drops—and the accompanying dips in global temperaturk to natural reductions in the sun's energy output or to volcanic eruptions. These factors have been regarded as major drivers of climate change over decades or centuries, but for the C02 pat- terns, such explanations fall short—which implies that an addi- tional factor forced C02 levels downward. Because [had already concluded that our human ancestors had caused the slow rise in C02 for thousands of years by clearing forests for agriculture [see min article}, this new finding made me wonder whether some kind of reversal of the ongoing clearance could explain the brief C02 drops. The most likely root cause turns out to be disease—the massive human mortality accompanying pandemies. Two severe outbreaks of bubonic plague, the single most devastating killer in human his- tory, correlate well with large C02 drops at approximately AD. 540 and 1350 [graph]. Plague first erupted during the Roman era, with the most virulent pandemic, the Plague of Justinian, in AD. 540 to 542. The infamous ”Black Death” struck between 1347 and 1352, fol- lowed by lesser outbreaks for more than a century. Each of these pandemics killed some 25 to 40 percent of the population of Europe. An even worse catastrophe followed in the Americas after 1492 when Europeans introduced smallpox and a host of other diseases that killed around 50 million people, or about 90 percent of the pre- Columbian population. The American pandemic coincides with the largest C02 drop of all, from 1550 to 1800. Observers at the time noted that the massive mortality rates produced by these pandemiss caused widespread abandonment of rural villages and farms, leaving untended farmland to revert to the wild. Ecologists have shown that forests will reoccupy abandoned land in just 50 years. Coupled with estimates of hu- man population and the acreage cultivated by each farmer, calcu- lations of forest regrowth in pandemic-stricken regions indicate that renewed forest's could have sequestered enough carbon to re- duce concentrations of C02 in the atmosphere by the amounts ob- served. Global climate would have cooled as a result, until each pandemic passed and rebounding populations began cutting and burning forests anew. WWMth,mmmmmmn-s manmuwmmn-mmmm Mummmumwmwmi combined effect from these anomalies would have been an average warming of almost 0.8 degree C just before the industrial era. That amount is larger than the 0.6 degree C warming measured during the past century—implying that the effect of early farming on climate rivals or even 186 exceeds the combined changes registered during the time of rapid industrialization. How did this dramatic warming effect escape recogni~ tion for so long? The main reason is that it was masked by natural climatic changes in the opposite direction. The earth’s orbital cycles were driving a simultaneous natural cooling trend, especially at high northern latitudes. The net temperature change was a gradual summer cooling trend lasting until the 18005. Had greenhouse gases been allowed to follow their natural tendency to decline, the resulting cooling would have augmented the one being driven by the drop in summer radiation, and this planet would have become considerably cooler than it is now. To explore this possi- bility, [joined with Stephen J. Vavrus and John E. Kutz- bach of the University of Wisconsin-Madison to use a climate model to predict modern-day temperature in the absence of all human-generated greenhouse gases. The model simulates the average state of the earth’s climate—— including temperature and precipitation—in response to different initial conditions. For our experiment, we reduced the greenhouse gas levels in the atmosphere to the values they would have reached today without early farming or industrial emis- sions. The resulting simulation showed that our planet would be almost two degrees C cooler than it is now—a significant difference. In comparison, the global mean temperature at the last glacial maximum 20,000 years ago was only five to six degrees C colder than it is today. In effect, current temperatures would be well on the way to- ward typical glacial temperatures had it not been for the greenhouse gas contributions from early farming prac- tices and later industrialization. . I had also initially proposed that new ice sheets might have begun to form in the far north if ...
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