been observed by satellite since 1979 and shows an increasing
trend. The melt zone, where summer warmth turns snow and
ice around the edges of the ice sheet into slush and ponds of
meltwater, has been expanding inland and to record high
elevations in recent years (source: Arctic Impacts of Arctic
Warming, Cambridge Press, 2004).
by Steve Connor
Published on Friday, February 10, 2006 by the Independent / UK
The entire northern hemisphere is experiencing a sustained period of warming that
is unprecedented in the past millennium, a study has found.
A review of a range of temperature records, from tree rings and ice cores to
historical documents, has found that at no time since the 9th century have
temperatures been so consistently high. The study, published in the journal
Science, found that the late 20th century was the warmest period for the northern
hemisphere since at least 800AD, eclipsing the well-known medieval warm period
when vines were cultivated successfully in northern Europe and the Vikings
exploited the ice-free seas to colonise Greenland.
Seasonal surface melt extent on the Greenland Ice Sheet has been observed by
satellite since 1979 and shows an increasing trend. The melt zone, where summer
warmth turns snow and ice around the edges of the ice sheet into slush and ponds
of meltwater, has been expanding inland and to record high elevations in recent
years (source: Arctic Impacts of Arctic Warming, Cambridge Press, 2004).
Timothy Osborn and Keith Briffa, climate scientists from the University of East
Anglia in Norwich, analysed 14 sets of temperature records from America, Europe
and East Asia. Each record covered a relatively wide region, such as northern
Sweden or the low countries of the Netherlands and Belgium, and extended back at
least several centuries.
Ten of the 14 records were based on tree-ring data, which went back as far as
800AD, one measured ice cores from Greenland, one involved historical documents
from Europe and one covered the chemical composition of sea shells on the east
coast of the US. The final set of records came from China and Japan and used a
variety of records, from ice cores to historical documents.
"Our results show that, during the late 20th century, warming affected the entire
northern hemisphere and that at no point in the past 1,000 years has the northern
hemisphere experienced the same widespread warming," Dr Osborn said.
The study showed that the medieval warm period ran from about 890 to 1170 and
that this was later followed by a significant period of cooling between 1580 and
1850, which included the period known as the "little ice age" when frost fairs were
held on the River Thames.
"The key conclusion was that the 20th century stands out as having unusually
widespread warmth, compared to all of the natural warming and cooling episodes
during the past 1,200 years," Dr Osborn said.
Climate scientists have in the past found evidence to suggest that the late 20th
century was warmer than at any time in the past millennium but this study is the
first to look at a variety of temperature records from across the entire northern
hemisphere.
