by Brian Fagan
When an austere and humorless missionary named Samuel Marsden landed at Oihi in New Zealand's Bay of Islands on Christmas Day, 1814, he brought "horses, sheep, cattle, poultry, and the gospel." With a finely developed sense of occasion, he preached to the curious Maori on the text, "Behold I bring you glad tidings of great joy." Within half a century, his tidings had changed Maori society and New Zealand's environment beyond recognition.3
By 1839, Christianity had a strong foothold in the country and tribal wars had virtually ceased. In that year, the missionaries claimed 8,760 Maori as regular churchgoers. A large part of their success came from agriculture. In 1824, a missionary farmer named Davis had founded a model farm at Waimate, where he introduced up-to-date English agriculture and stock-raising practices. His methods were already spreading among the Maori when a flood of European settlers arrived, sponsored by the New Zealand Colonization Company in London. By 1843, 11,500 Europeans lived in New Zealand, most of them in the heavily forested North Island. The settlers pushed inland, cutting down trees and clearing land for intensive European-style farming. The effects on Maori culture were disastrous. Between 1860 and 1875, as more than 4 million hectares of Maori land passed into settler ownership, thousands of hectares of forest and woodland fell before the farmer's axe.
New Zealand was not alone. Mid-nineteenth century immigration brought tens of thousands of land-hungry farmers to Australia, North America, South Africa, and elsewhere. The newcomers felled millions of trees as they cleared their farmlands or provided firewood and lumber for the growth of cities and the spreading Industrial Revolution. The longterm environmental effects for earth were profound.
A standing forest can contain as much as 30,000 metric tons of carbon per square kilometer in its trees, and still more in its undergrowth.4 When the trees are felled, much of this carbon enters the atmosphere. Similarly, virgin grassland soils have an organic content of up to 5,000 metric tons per square kilometer, half of which may be lost within six months when it is cultivated. One estimate has the twenty-year global burst of pioneer agriculture and land modification between 1850 and 1870 raising the carbon dioxide level in the atmosphere by about 10 percent, even allowing for absorption by the world's oceans. Isotopic levels in tree rings from long-living California bristlecone pines chronicle a rise in carbon dioxide levels during these very years, at a time when the discharge from fossil fuel burning was still relatively small.
This change may have been one of the mechanisms that gradually raised global temperatures during the late nineteenth century, ending the Little Ice Age by about 1850. The pioneer agricultural explosion, fueled by large-scale emigration, railroads, and ocean steamships, was the first human activity that genuinely altered the global environment. The second came from coal, already a significant air polluter in large cities.
"In the third week of November, in the year 1895, a dense yellow fog settled down upon London. From the Monday to the Thursday I doubt if it were ever possible from our windows in Baker Street to see the loom of the opposite houses." Thus Arthur Conan Doyle keeps Sherlock Holmes pacing restlessly at home, impatient for action, watching "the greasy, heavy brown swirl still drifting past us and condensing to oily drops on the window panes."5 The choking "pea soupers" of the great detective's London came from factory chimneys and millions of coal fires pouring smoke into still, cold air. I remember a day in the early 1960s when you could not see your hand in front of your face, and everyone wore masks to protect their lungs. The advent of smokeless fuels has mercifully consigned the pea souper to historical oblivion.
By the sixteenth century, England's forests had shrunk drastically in the face of rising rural populations and an insatiable demand for construction lumber and firewood. Londoners turned to coal instead and choked in clouds of coal smoke hovering over streets and crowded roofs. During a cold spell in January 1684, diarist John Evelyn complained of the "fuliginous steame of the Sea Coal," which filled Londoners' lungs with "grosse particles." King Charles II contemplated ways of abating London's smog, which was later made worse by the Industrial Revolution with its coal-powered steam engines, railroads, and factories.6 You can see the nineteenth century's air pollution in the work of London artists. Sailing ships, tugs, and freighters work their way against the Thames tides in a yellowy or pink-gray light; sunsets over Saint Paul's Cathedral glow with a hazy red that was unknown in earlier centuries.?
Global temperature anomalies since 1860. Note the pronounced warming trend, which accelerates after the I970s.
Industrial and domestic coal burning not only choked passers by, it released enormous concentrations of atmospheric carbon dioxide. In the early twentieth century, the mass-produced automobile and a shift from coal to oil and gas poured even more carbon dioxide into the atmosphere. Antarctic and Greenland ice cores have trapped air bubbles that date back long before the Industrial Revolution and show that carbon dioxide levels in the atmosphere have risen sharply since 1850. Other greenhouse gases, such as methane, have increased at the same time, as global human populations rise and engage in ever more intense rice paddy agriculture and cattle herding. It is surely no coincidence that global temperatures have gradually and inexorably risen over the past 150 years.
The first half of the twentieth century was not unusual by Little Ice Age standards. At first, the warming matched that between the late 1600s and the 1730s, when full twentieth-century warmth was attained for a decade. The eighteenth-century warm spell ended with the harsh winter of 1739/40. Unpredictable climatic shifts continued for the next century and three-quarters, with no significant longer term trends except for cycles of cold caused by volcanic activity. Europe also enjoyed twenty years of warmth in the 1820s and 1830s, but again nothing like the sustained warming that began between 1890 and 1900 and continues, with one short interruption, to this day.
The period 1900 to 1939 saw a high incidence of westerly winds and mild winters, characteristic of a high North Atlantic Oscillation condition. The strong pressure gradient between the Azores high and the Icelandic low helped maintain the westerlies. Air temperatures over the globe reached a peak in the early 1940s, after decades of strong atmospheric circulation. Locations near the edge of the Arctic like Iceland and Spitzbergen experienced warming even more extreme than that in Europe. The area of the north covered by pack ice shrank between 10 and 20 percent. Snow levels rose on northern mountains. Ships could now visit Spitzbergen for over seven months a year, compared with three months before 1920. The distribution and variability of rainfall altered over much of the world as well. Northern and western Europe experienced more rainfall, as mud-bound troops on the Western Front in 1916 found to their cost. My father, who fought on the Western Front, complained in his diary of "constant rain, grey skies, and mud everywhere. We sink into it up to our knees, sloppy dreadful stuff that rots our feet. No one fights in either side, we just suffer silently in the wet."s
The additional precipitation continued into the 1920s and 1930s as subpolar cyclones became larger and spread their wind circulation further into the Arctic. The warming increased the length of the growing season in western Europe by as much as two weeks compared with the mid-nineteenth century, as the last spring frost came earlier and the first autumn freeze later. After 1925, Alpine glaciers disappeared from valley floors up into the mountains. Equivalent stronger westerlies over the Pacific extended the arid wind shadow of the Rocky Mountains far eastward, bringing the disastrous droughts of the Oklahoma Dust Bowl era in the 1930s. Changes in atmospheric circulation brought much more reliable monsoon winds to India. There were only two partial monsoon failures in the thirty-six years between 1925 and 1960, a dramatic contrast to the catastrophic failures of the late nineteenth century, when millions of Indian villagers perished in terrible famines.9
By the 1940s, scientists were talking of the persistent warming trend, which had begun to transcend the familiar climatic swings. At first they focused their attention on the most obvious sign of global warming, retreating Arct
ic sea ice. What would happen if the ice pack vanished by the end of the twentieth century? Would it be possible to grow food even further north, to settle lands closer to the Arctic than even the valleys and hill slopes farmed during the Medieval Warm Period? The climatologists had few research tools to draw on, working as they were before an era of computer modeling, satellites, and global weather tracking. Their researches were also hampered by the sheer variability of rainfall and temperature, which tended to obscure longer term trends, and by a lack of properly organized long-term meteorological data.
Just as climatologists were pondering a half-century or more of gradual warming, changes in the atmospheric circulation regime in the 1950s brought a lowering of the global average temperature to about the level of 1900-1920. This cooling endured longer than any other downward temperature trend since 1739-70. The NAO was now in a low phase, with the westerlies weakened and shifted southward. Western Europe became colder and generally drier during the winter months. The winter of 1962/63 was the coldest in England since 1740. I remember skating on the River Cam near Cambridge for kilometers, after having rowed on it a few days earlier in water so cold that the spray froze to our oars. The Baltic Sea was completely ice covered in 1965/66. In 1968, Arctic sea ice surrounded Iceland for the first time since the exceptional winter of 1888. The effects of the circulation changes were also felt elsewhere. Between 1968 and 1973, prolonged drought in the Sahel zone, on the southern margins of the Sahara, killed thousands of people and decimated cattle herds. In 1971/72, eastern Europe and Turkey experienced their coldest winter in 200 years. The Tigris River froze over for the only time in the century. Record low winter temperatures in 1977 over the Midwest and eastern United States convinced many people that another Ice Age was imminent. Time magazine ran a story on the repetitive cycles of Ice Ages. Cold was fashionable again.
Then, suddenly, the NAO index flipped to high. The warming resumed and seemed to accelerate. In 1973/74, the Baltic was virtually icefree. England enjoyed its warmest summer since 1834. Record heat waves baked England, the Low Countries, and Denmark in 1975/76. More weather extremes, a higher incidence of hurricanes, and drought after drought: the world's climate seemed very different from that of a century (or even a decade) earlier.
Few scientists were actively studying long-term climatic change at the time. They worked far from the limelight until June 1988, when a searing two-month heat wave settled over the Midwest and eastern United States. Weeks of dry, record heat reduced long stretches of the Mississippi River to shallow streams. Barges ran aground on mud banks and were stranded for weeks. At least half the barley, oats, and wheat crops on the northern Great Plains were lost. The drought ignited destructive wild fires over 2.5 million hectares of the west, engulfing much of Yellowstone National Park. The dry spell was caused by a relatively common meteorological event: a blocking high that kept heat hovering over the Midwest and east. But a single Senate hearing in which everyone sweltered was sufficient to turn global warming from an obscure scientific concern into a public policy issue.10
On June 23, 1988, climatologist James Hansen testified before a hearing of the Senate Energy and Natural Resources Committee on a day when the temperature in Washington D.C. reached a sweltering 38°C. The heat wave was an appropriate backdrop for some startling climatic testimony. Hansen had impressive data from 2,000 weather stations around the world, which documented not only a century-long warming trend but a sharp resumption of warming after the early 1970s. Four of the warmest years of the past 130 had occurred in the 1980s. The first five months of 1988 had brought the highest temperatures yet. Hansen flatly proclaimed that the earth was warming on a permanent basis because of humanity's promiscuous use of fossil fuels. Furthermore, the world could expect a much higher frequency of heat waves, droughts, and other extreme climatic events. His predictions thrust global warming into the public arena almost overnight.
We live on a benign planet, protected by the heat absorbing abilities of the atmosphere, the so-called "greenhouse effect." Energy from the sun heats the surface of the earth and so drives world climate. The earth, in turn, radiates energy back into space.'' Like the glass windows of a greenhouse, atmospheric gases such as water vapor and carbon dioxide trap some of this heat and re-radiate it downward. Without this natural greenhouse effect, earth's temperature would be about -18°C instead of the present comfortable +14°. But the effect is no longer purely natural. Atmospheric concentrations of carbon dioxide have now increased nearly 30 percent since the beginning of the Industrial Revolution; methane levels have more than doubled; and nitrous oxide concentrations have risen by about 15 percent. These increases have enhanced the heat-trapping capabilities of the atmosphere.
Ozone depletion is another consequence of human industrial activity. The earth's atmosphere is divided into several layers, the troposphere extending from the surface to about ten kilometers, the stratosphere from ten to about fifty kilometers. Most airline traffic travels in the lower part of the stratosphere. Most atmospheric ozone is concentrated in a layer in the stratosphere about fifteen to fifty kilometers, where it absorbs a portion of solar radiation, preventing it from reaching the earth's surface. Most importantly, it absorbs ultraviolet light that causes harm to some crops, some forms of marine life, and causes skin cancers and other med ical complications in humans. Ozone molecules are constantly formed and destroyed in the stratosphere, but the total ozone level stays relatively stable and has been measured over several decades. Over the past halfcentury, intensive use of chlorofluorocarbons (CFC) as refrigerants and for many other applications has damaged the protective ozone layer, resulting in ozone depletion, reflected in the famous annual ozone "hole" over Antarctica and a fall in ozone levels of up to 10 percent over many parts of the world.
Global mean surface temperatures have risen between 0.4 and 0.8°C since 1860 and about 0.2 to 0.3°C since 1900 in some parts of the world. If the present levels of emission release continue, carbon dioxide concentrations may be 30 to 150 percent higher than today's levels by the year 2100. Some estimates place the potential temperature rise between 1.6 and 5.0°C in different parts of the world, an unprecedented climb by post-Ice Age standards. With this rise would come major environmental changes: decreased pack ice and snow cover in the Arctic and Northern Hemisphere, further climbs in sea level, beyond the ten- to twenty-fivecentimeter rise of the past century (the largest over the past 6,000 years), which would threaten many coastlines and low-lying nations like the Bahamas and many Pacific islands; perhaps a higher frequency of exceptional storms and extreme weather events; and severe droughts in places like tropical Africa. Many of these environmental changes carry potentially catastrophic political and social consequences.
Since Hansen's 1988 testimony, temperatures have risen to their highest levels since at least A.D. 1400 and show no signs of cooling off. The 150year warmup is now the most prolonged in 1,000 years. One record after another has toppled. January to September 1998 was the second warmest period on record in North America, exceeded only by 1934. September 1998 was the warmest September ever globally for over a century, more than 0.6°C above the long-term mean for 1880-1997. Blistering heat enveloped much of the American south during spring and summer that year. Del Rio, Texas, suffered through a record sixty-nine days with temperatures above 38°C.
No less than 67 percent of all winters since 1980 have been warmer than the long-term average. The winter of 1999/2000 was by a huge margin the warmest in the United States in 105 years of record keeping, 0.3°C higher than the previous record, set in 1998/99. Europe has also seen a series of unusually mild winters. Over the Northern Hemisphere as a whole, both land and ocean winter temperatures were the sixth warmest on record, only slightly cooler than the record years of 1997/98 and 1998/99.12 Summer temperatures are now equal to the means of the Medieval Warm Period. Globally, minimum temperatures have been increasing about twice the rate of maximum temperatures since the 1950s, which is lengthening frost-free seasons over much
of the Northern Hemisphere.
Are the record temperatures of the 1990s simply part of the endless cycling of cooler and warmer climate that has gone on since the end of the Ice Age? Or do they result, at least in part, from unwitting human interference with global climate? On the face of it, the rising temperatures of the past decade seem to bear out James Hansen's predictions. But computer models have their limitations. Long-term climatic projections require models of mind-boggling complexity, based on as complete data as possible from every corner of the world. While these models improve from year to year, they are no better than the technology and software that ran them, or the data fed into them. They are obviously statistical estimates based on geographically incomplete information.
Still, they show some disturbing trends. For example, the North Atlantic Oscillation's current high mode has endured for several decades longer than usual, and brought significant winter warming of nearby Northern Hemisphere land masses. Numerical models of the climate system show that the NAO's stability from the 1960s to early 1990s is outside the range of normal variation.13 Does this mean that recent temperature changes are the result of human-generated greenhouse gases? The statistical odds that they are rank in the 90th percentile, but we will not have anything approaching a definitive answer to this question for another three decades.
Part of the reason we don't know to what extent our present climate change is natural has to do with the sun. The sun has always been a dynamic player in global climate change, but the extent of its influence is still a mystery. Research has hardly begun. A helioseismograph based on an orbiting observatory named SOHO 1.6 million kilometers in space sends sound waves toward the sun, which bounce back from the layers that form its interior. The waves acquire high-quality observations without the interference caused by atmospheric "noise" and have located two parallel gas layers about 225,000 kilometers beneath the solar surface that speed up and slow down in a synchronous pattern in regular twelve- to sixteen-month cycles. This "tachcline" is where the turbulent outer region of the sun meets the orderly, interior radiative core. The tachcline may be the source of powerful magnetic fields that produce solar flares and solar winds, and create the eleven-year cycle of sunspots.14 The effects of these cycles on global climate are still unknown.
