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Twentieth century 'warmest in 500 years'
Source: BBC
17th Feb 2000
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Drilling down into subsurface rocks shows
temperature changes over centuries
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Studies of temperature records preserved deep in underground rocks
show that the Earth has been gradually warming over at least the last
500 years. And the studies, by scientists in the US and Canada, show
that the trend accelerated markedly during the 20th Century, which was
the warmest of the past five centuries.
Since 1500, the Earth's temperature has increased by about one degree
Celsius, with half of that increase occurring in the last century.
Trend picks up
The warming trend is speeding up. Almost 80% of the net temperature
increase observed occurred in the 19th and 20th centuries. In the Northern
Hemisphere, the five-century change has been 1.1 degrees, with 0.6 recorded
since 1900. The studies, reported in the science journal Nature, are
based on analysis of borehole temperatures from 616 sites on every continent
except Antarctica. The scientists lowered sensitive thermometers into
holes drilled down from ground level to discover how surface temperature
altered in the past. A typical borehole was measured at 10-metre depth
intervals down to as far as 600 m.
Records preserved
The technique is possible because of heat conduction, which means that
temperature changes at the surface generate "signals" that penetrate
subterranean rocks.
The signals from short-term daily or seasonal variations penetrate only
a few metres and are rapidly lost. But changes over centuries are preserved
in deeper rock, although the signals travel very slowly, penetrating
only about 500 metres in 1,000 years.
One of the team, Professor Henry Pollack of the University of Michigan,
said: "The upper 500 metres is an archive. Like any historical archive,
there are of course missing pages, and the ink has run in a few places.
"But in principle, if you drilled a borehole anywhere on a continent,
you could observe a temperature profile and be able to reconstruct what
had happened at that location."
The team's work involved calculating averages from all the boreholes
investigated, and built on a previous analysis of borehole temperature
data from 358 sites.
The scientists also compared their results with those obtained from
other methods of estimating past temperature change, including studies
of tree rings, ice cores, lake sediments and coral growth.
"All the methods generally show a very unusual 20th Century, and ours
does too," said Professor Pollack. "It is the warmest century of the
last five, and the one which is most rapidly changing."
Forecast re-inforced
"What we show that is somewhat different is that the total temperature
change over the past five centuries has been greater than some of the
other methods are showing."
In an accompanying article in Nature, Jonathan Overpeck, of the University
of Arizona, Tucson, says the team's results re-inforce the forecast
for this century: continued warming ahead.
"But they also provide unsettling indications that human alteration
of the climate system over the past century will make the reliable prediction
of climate change an even tougher business than expected.
"Their analysis is the latest of several to indicate that late 20th
Century warming is without precedent in the past 400 to 1,000 years.
"We do not know of any combination of natural mechanisms that can explain
this phenomenon. So we are left with the likelihood that human-induced
global warming is under way."
And he adds a warning. "The results show yet again that the 20th Century
record of climate variability is too short and cloaked with human-induced
influences to provide a clear indication of natural climate variability.
"Earlier studies may have underestimated the full amplitude of natural
decade-to century-scale climate variability."
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