The Hottest Century?

The Hottest Century?

Where the 20th century ranks in terms of temperature is a critical point in the ongoing debate over global warming.

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“Reconstructing Climatic and Environmental
Changes of the Past 1,000 Years: A Reappraisal” by Willie Soon et
al., in
Energy & Environment class="text14"> (Mar. 2003), 5 Wates Way, Brentwood Essex CM15 9TB,
United Kingdom.


The world has just put a long, hot century behind it,
and now the question of where the era stands in the history of the
world’s climate has become an item in the debate over global warming.
One influential recent study of global temperature changes over the past
millennium found that, for the Northern Hemisphere at least, the 20th
century was the warmest century, the 1990s the warmest decade, and 1998 the
warmest year. These conclusions lend more weight to the argument that
anthropogenic (human-generated) greenhouse gases have produced anomalously
high temperatures. (Many other, though narrower, studies point toward this
reading of climate history.) Soon, a physicist at the Harvard-Smithsonian
Center for Astro­physics, in Cambridge, Massa­chusetts, and his
colleagues, taking a different approach, have concluded that the 20th
century was probably “
not class="text44"> the warmest” of the millennium.


In the earlier study, Michael E. Mann, an
environmental scientist at the Univer­sity of Virginia, and his
colleagues attempted an ambitious mathematical reconstruction of global
temperature changes over the past thousand years based on various
“proxy” data, such as ice core samples. Besides selecting
winners (or losers) in the “warm­est” category, they
dismissed the conventional wisdom among climatologists that there were two
previous periods of great divergence from the climate norm: the so-called
Little Ice Age (1300–1900) and the Medieval Warm Period
(800–1300). The elimination of those two epochs would cast the 20th
century as even more of an anomaly.


Soon and his coauthors, taking “a
non-quantitative and very ‘low-tech
class="text46">” approach to the problem, examined a multitude of
research results obtained from local and regional climate indicators, such
as coral and tree ring growth, lake fossils, ice cores, glaciers, and
seafloor sediments. The results cannot be combined into a simple
hemispheric or global numerical composite, the authors say, but still are
revealing. “The picture emerges from many localities” that the
Little Ice Age and the Medieval Warm Period were indeed
“widespread” phenomena, even if not “precisely timed or
synchronous.”


As for the rising thermometer readings of the 20th
century, say Soon and his colleagues, they appear in historical perspective
“neither unusual nor unprecedented.” Tree ring chronologies in
one study “show that the Medieval Warm Period [was] as warm as, or
possibly even warmer than, the 20th century,” at least for a region
of the Northern Hemisphere.


The authors agree that human activity has had a
significant impact on some local environments, but just how big a
role humans have played in heating the atmosphere in recent decades
remains up in the air.


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