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Over 70 percent of survey respondents agreed that the Earth’s temperature
has risen steadily during the last century, including the last decade. Global
temperatures indeed rose over the course of the last hundred years, but the
rise was not steady. By most accounts, the Earth’s temperature rose about
0.6 degree Celsius (about 1 degree Fahrenheit) during the twentieth century;[111]
and just as the climate has warmed and cooled throughout recorded history, temperatures
fluctuated during the 1900s. A Science magazine article reports that two distinct
periods of warming—from 1910 to 1945 and again since 1976—were separated
by a period of very gradual cooling.[112] Thus, contrary
to popular opinion, recent warming did not occur steadily. More recently, satellite
data indicate that temperatures have not risen appreciably since 1998 and that
temperatures have actually dropped since 2007.[113]
 Recent
temperature declines are at odds with the warming projected
by various computer models. Such models serve as the basis
for predictions from the United Nations Intergovernmental
Panel on Climate Change (IPCC), whose pronouncements are widely
viewed as authoritative. According to the IPCC, “Warming
of the climate system is unequivocal, as is now evident from
xobservations of increases in global average air and ocean
temperatures, widespread melting of snow and ice and rising
global average sea level.”[114]
Perhaps more widely disseminated than any other projections,
the IPCC’s global temperature estimates indicate a major,
long-term warming trend. These estimates, in turn, confirm
for many that the Earth’s warming is increasing steadily
and inform the belief that recent warming is out of the ordinary.
An important question, then, is whether the temperature swings
of the twentieth century were atypical. Opinions range from
those who feel the twentieth century’s temperature rise
is atypically large to those who feel it was just another
normal phase in a natural climate cycle. In their 2001 synthesis
report, the IPCC stated that “the rate and duration of
warming of the 20th century has been much greater than in
any of the previous nine centuries.”[115]
According to a report from the Australian government’s
Department of the Environment and Heritage, “All reliable
estimates of Northern Hemisphere temperatures over the past
1000 to 2000 years confirm that the 20th century has been
unusually warm.”[116] On the
other hand, certain examinations of the geological record
indicate that recent temperature changes are well within the
range of natural variability.[117]
A September 2007 analysis of peer-reviewed literature reports
evidence that a natural, moderate 1,500-year climate cycle
has produced more than a dozen global warming cycles (similar
to the most recent warming cycle) since the last Ice Age.[118]
A November 2007 paper examined the temperature records of
eighteen locations over a 2,000-year period, concluding that
the Medieval Warm Period (roughly the ninth through thirteenth
centuries) was 0.3 degree Celsius warmer than the twentieth
century.[119]
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Just as the climate
has warmed and cooled throughout
recorded history, temperatures fluctuated during the
1900s.
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To estimate temperatures for the distant past, scientists
extrapolate data from proxies, such as tree rings, ice cores,
boreholes, pollen remains, glacier lengths, ocean sediments,
and changes in the Earth’s orbit.[120]
However, according to a recent National Research Council study,
“very little confidence can be assigned to estimates
of hemisphere average or global average temperature prior
to A.D. 900 due to limited data coverage and challenges in
analyzing older data.”[121] Such
limitations and challenges highlight the difficulties of accurately
determining how much our temperature has changed. Yet one
thing is certain: climate changes and always has. According
to the U.S. Environmental Protection Agency: “From glacial
periods (or ‘ice ages’) where ice covered significant
portions of the Earth to interglacial periods where ice retreated
to the poles or melted entirely—the climate has continuously
changed.”[122]
Our limited knowledge and understanding of the myriad intricacies of the Earth’s
complex climate system make climate-change discussions necessarily inconclusive.123
“While most scientists agree that anthropogenic [man-made] global warming
is a threat, they’re not certain about its scale or its timing or its precise
consequences,” writes John Tierney in the New York Times.124 Until our
collection of climate data becomes more uniform and reliable, and until our
understanding of such data improves, many of our questions about the Earth’s
climate will remain unanswered. Clearly, the Earth has warmed since the late
nineteenth century, but the key is to judge such warming in historical context,
continually refining our interpretation of varying climate data. Moreover, the
important task for policymakers is to proceed with caution, in order to avoid
implementing dramatic public-policy steps based upon an incomplete understanding
of global-climate issues.
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