Apr. 21, 2013 — Fueled by industrial
greenhouse gas emissions, Earth's climate warmed more between Nature Geoscience
by more than 80 scientists from 24 nations analyzing climate data from
tree rings, pollen, cave formations, ice cores, lake and ocean
sediments, and historical records from around the world.
1971 and
2000 than during any other three-decade interval in the last 1,400
years, according to new regional temperature reconstructions covering
all seven continents. This period of humanmade global warming, which
continues today, reversed a natural cooling trend that lasted several
hundred years, according to results published in the journal
"This paper tells us what we already knew, except in a better, more
comprehensive fashion," said study co-author Edward Cook, a tree-ring
scientist at Lamont-Doherty Earth Observatory who led the Asia
reconstruction.
The study also found that Europe's 2003 heat wave and drought, which
killed an estimated 70,000 people, happened during Europe's hottest
summer of the last 2,000 years. "Summer temperatures were intense that
year and accompanied by a lack of rain and very dry soil conditions over
much of Europe," said study co-author Jason Smerdon, a climate
scientist at Lamont-Doherty and one of the lead contributors to the
Europe reconstruction. Though summer 2003 set a record for Europe,
global warming was only one of the factors that contributed to the
temperature conditions that summer, he said.
The study is the latest to show that the Medieval Warm Period, from
about 950 to 1250, may not have been global, and may not have happened
at the same time in places that did grow warmer. While parts of Europe
and North America were fairly warm between 950 and 1250, South America
stayed relatively cold, the study says. Some people have argued that the
natural warming that occurred during the medieval ages is happening
today, and that humans are not responsible for modern day global
warming. Scientists are nearly unanimous in their disagreement "If we
went into another Medieval Warm Period again that extra warmth would be
added on top of warming from greenhouse gases," said Cook.
Temperatures varied less between continents in the same hemisphere
than between hemispheres. "Distinctive periods, such as the Medieval
Warm Period or the Little Ice Age stand out, but do not show a globally
uniform pattern," said co-author Heinz Wanner, a scientist at the
University of Bern. By 1500, temperatures dropped below the long-term
average everywhere, though colder temperatures emerged several decades
earlier in the Arctic, Europe and Asia.
The most consistent trend across all regions in the last 2,000 years
was a long-term cooling, likely caused by a rise in volcanic activity,
decrease in solar irradiance, changes in land-surface vegetation, and
slow variations in Earth's orbit. With the exception of Antarctica,
cooling tapered off at the end of the 19th century, with the onset of
industrialization. Cooler 30-year periods between 830 and 1910 were
particularly pronounced during weak solar activity and strong tropical
volcanic eruptions. Both phenomena often occurred simultaneously and led
to a drop in the average temperature during five distinct 30- to
90-year intervals between 1251 and 1820. Warming in the 20th century was
on average twice as large in the northern continents as it was in the
Southern Hemisphere. During the past 2000 years, some regions
experienced warmer 30-year intervals than during the late 20th century.
For example, in Europe the years between 21 and 80 AD were likely warmer
than the period 1971-2000.
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