June 22, 2006
Last 25 years warmest on Earth since 1600
By Deborah Zabarenko
WASHINGTON (Reuters) - The last few decades were the
warmest on Earth in the past 400 years, and may well have been
warmer than any comparable period since the Middle Ages, U.S.
scientists reported on Thursday.
for much of the hurricane-fueling rise in temperatures in the
North Atlantic last year, when there were a number of
devastating hurricanes, including Katrina.
In a new report by the National Research Council,
researchers said they were highly confident the mean global
surface temperature was higher in the past 25 years than any
comparable period during the previous four centuries.
They had less confidence the past quarter-century was
hotter than any comparable period in the years from 900 to
1600, but found that plausible. For the years before 900, the
scientists said they had very little confidence about what the
Earth's mean surface temperatures were.
They did not dispute multiple measurements that showed the
world warmed up by about 1 degree F (0.6 C) over the course of
the 20th century, a quick rise compared with previous
The scientists also noted that temperature reconstructions
for periods before the Industrial Revolution -- when levels of
climate-warming greenhouse gases were much lower -- supported
the notion the current global climate change was caused by
human activities, rather than natural variations in climate.
"Natural climate variability is something that we'd like to
know about," said Kurt Cuffey of the University of
California-Berkeley, who served on the council's committee and
spoke at a Webcast about the report.
"But if we did know for example that the climate was as
warm at 1000 AD as it is now, it would have no essential impact
on our understanding of climate change in the 20th century, the
role of humans in causing it and the need to think seriously
about how that may evolve in the next few centuries," he said.
The human causes of global warming have been under dispute,
especially by a skeptical Bush administration, but are
generally accepted by scientists as a key factor in climate
Figuring out global temperatures over the past 150 years is
relatively simple, since reliable records exist. But for the
years and centuries before that, researchers must read clues
left by the growth rings on trees, the retreat of glaciers and
even old paintings and diaries that document climate.
Such clues are called proxies, and scientists began using
them in sophisticated ways in the 1990s to estimate Earth's
surface temperature in past eras.
The council's report was prompted by a request from the
U.S. Congress, spurred by a controversial 1998 report in the
journal Nature that used a number of sources, including
proxies, to estimate temperatures in the Northern Hemisphere
over the last 1,000 years.
That report concluded the hemisphere was warmer during the
late 20th century than at any other time in the past
millennium, and that the 1990s were the warmest decade and 1998
was the warmest year during that whole period.
In another report on climate change, a new analysis blamed
global warming for about half of the extra hurricane-fueling
warmth in the waters of the tropical North Atlantic in 2005.
Natural cycles were only a minor factor, according to research
by Kevin Trenberth and Dennis Shea of the National Center for