My latest Mind and Matter column in the Wall
Street Journal is on the Medieval Warm Period:
A flurry of recent scientific papers has tried to measure the
warmth of the "Medieval Warm Period" (MWP) of about 1,000 years
ago. Scientists have long debated whether it was cooler or warmer
than today, and whether the warmth was global or regional. The
point for nonscientists: If recent warming has precedents, some
might find it less alarming.
Until the late 1990s, researchers generally agreed that the MWP
was warmer than today and that the "Little Ice Age" of 1500-1800
was colder. Then in 2001 the Intergovernmental Panel on Climate
Change adopted the "hockey stick" graph devised by Michael Mann at
the University of Virginia and colleagues.
Using temperature indicators such as tree rings and lake
sediments, the graph rewrote history by showing little warmth in
the 11th century and little cold in the 17th, but a sharp spike in
late-20th-century temperatures. That graph helped to persuade many
people (such as me) that recent temperature rises were
unprecedented in scale and speed in at least 1,400 years.
But critics of the graph pointed out that it used a statistical
technique that overemphasized hockey-stick shaped data from
unreliable indicators, such as tree rings in bristlecone pine trees
and Scandinavian lake sediments influenced by 20th-century land-use
changes. Four recent studies have now rehabilitated the MWP as a
period of unusual warmth, though they disagree on whether it was as
warm or warmer than today.
Jan Esper of the University of Mainz and his colleagues looked
at pine wood densities from Sweden and Finland and found "evidence for substantial warmth during
Roman and medieval times, larger in extent and longer in duration
than 20th-century warmth." Bo Christiansen of the Danish
Meteorological Institute and Fredrik Ljungqvist of Stockholm
University looked at 32 indicators across the Northern Hemisphere
and found the level of warmth during the peak of
the MWP "in the second half of the 10th century equaling or
slightly exceeding the mid-20th century warming."
Thomas Melvin of the University of East Anglia and colleagues
reanalyzed one of the tree samples from Sweden used in the "hockey
stick" and concluded: "We can infer the existence of
generally warm summers in the 10th and 11th centuries, similar to
the level of those in the 20th century."
A fourth study of creatures called diatoms in Chinese lake
sediments found that the period "between ca. A.D. 1150
and 1200 was the warmest interval of the past 1,000 years."
Taken together, these studies cast doubt on the IPCC's conclusion in 2007 that "the evidence is not
sufficient to support a conclusion that [Northern] hemispheric mean
temperatures were as warm, or the extent of warm regions as
expansive, as those in the 20th century as a whole, during any
period in medieval times."
But was the medieval warm period confined to the Northern
I consulted a database of papers collated by the
climate-skeptic website CO2Science.org, run by the Center for the
Study of Carbon Dioxide and Global Change, a nonprofit research
center in Tempe, Ariz. The database contains numerous published
studies of isotopes and other indicators in caves, lake sediments
and other samples from Chile, New Zealand, South Africa and
Antarctica that find the MWP warmer than today. Two Antarctic
studies, for instance, concluded that current warming "is not yet as
extreme in nature as the MWP" and that "the present state of reduced ice on
the western Antarctic Peninsula is not unprecedented." A far
smaller number of studies, such as one from Lake Tanganyika, found
the MWP cooler than today.
It remains possible that today's warming is different from that
of the Middle Ages. For example, while summers might have been
warmer then, winters might be warmer today (if today's warming is
caused by carbon dioxide, that should be true). And of course, it
is the future, not the past, that scientists expect to be
dangerous. Nonetheless, the evidence increasingly vindicates the
scientists who first discovered the Medieval Warm Period.