The Little Ice Age: How Climate Made History 1300-1850 (38 page)

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 Arctic 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.

Another solar phenomenon may also be at play. A group of astronomers and climatologists has studied solar "corona," literally holes in
the sun's outer atmosphere through which streams of charged particles
flow into space to engulf the entire planetary system. They believe this solar wind activity has a direct connection to climatic change on earth:
charged particles hitting the earth's atmosphere affect the properties of
clouds and the percentage of them covering the globe. When numerous
coronae cover the solar surface, the increased solar winds produce greater
terrestrial cloud cover, and average temperatures drop. The importance of
this effect is still unknown.15

Solar radiation is never constant, making it a possible cause of climate
change, and a factor in the current warming. Over the past twenty years,
space-based measurements of solar radiation, the first accurate data on
these fluctuations, have revealed eleven-year cycles that correspond to the
well-known eleven-year cycle of sunspots. Solar intensity is greater at
times of higher sunspot activity. Proxy measurements from tree rings and
ice cores chronicle these cycles and longer term fluctuations in earlier centuries. We know solar activity was high during the twelfth and thirteenth
centuries, the height of the Medieval Warm Period. Present levels of solar
irradiance are higher than periods of unusually low sunspot activity during the Sporer (1425-1575), Maunder (1645-1715), and Dalton Min ima (1790-1820). Solar activity increased steadily during the first half of
the twentieth century but has changed little since 1950, beyond the usual
eleven-year cycles. In computer climate simulations, the surface temperature warming resulting from the known fluctuations in solar radiation between 1600 and the present amount to only 0.45°C. Less than 0.25°C
can be attributed to the period 1900 to 1990, when surface temperatures
rose 0.6°C.16 Changes in solar radiation appear to account for less than
half of twentieth-century warming.