Published online: 9 February 2005; | doi:10.1038/433562a
Past climate comes into focus but warm forecast stays put
A reassessment of past climate records suggests greater fluctuation
than was previously thought.
Munich - Fluctuations in global temperature during the past
millennium may have been larger and more frequent than previously
thought, says a fresh analysis of the climate record.
The analysis is likely to reignite a long-standing controversy over
the cause and extent of natural climate variability, scientists say,
although the unprecedented nature of global warming since the mid-
1980s remains unquestioned. The study was conducted by Anders Moberg
of Stockholm University, Sweden, and his team.
P. D. JONES & M.E. MANN REV. GEOPHYS. 2004
According to an earlier study, which produced the widely
cited 'hockey stick' graph, average Northern Hemisphere temperatures
during the past millennium were relatively stable until the late
nineteenth century, when they began to increase sharply1. In 2001,
this assessment was used to underpin the most recent report of the
Intergovernmental Panel on Climate Change (IPCC) the scientific
branch of the United Nations Framework Convention on Climate Change.
But the Moberg study, which is published just as the Kyoto Protocol
comes into effect, suggests that notable climate changes have
occurred throughout the recent past. If such natural fluctuations
continue in the future, they may "amplify or attenuate anthropogenic
climate change significantly", the authors conclude.
Moberg's group used a combination of different 'proxies' to
reconstruct decadal and centennial temperature changes. Proxies are
climate indicators such as tree rings, pollens and boreholes, and the
researchers used each one at the timescale that it records most
accurately: tree rings are used for reflecting annual variations, for
example, and sediments for longer-term changes. The researchers then
used 'wavelet analysis' to combine the timescales in the optimum
"At timescales longer than 80 years, temperature variability seems to
have been considerably larger than previously thought," says Moberg.
Previously, different scientists had arrived at different curves for
temperature variability over past centuries, depending on the data or
models they used2. The possibility that they generally underestimated
natural climate fluctuations has been one of the main arguments that
sceptics use to reject the notion that human activity is responsible
for current warming.
This argument has hardly any support in the climate community,
however. Many researchers do agree that historic climate changes may
have been underestimated. But the exceptionally strong warming trend
since the mid-1980s cannot be explained by natural variability alone,
they maintain. "Moberg's reconstruction will help to put the record
straight in one of the most contested issues in palaeoclimatology,"
says Hans von Storch, a climate modeller at the GKSS research centre
in Geesthacht, Germany. "But it does not weaken in any way the
hypothesis that recent observed warming is a result mainly of human
"We need to understand the past, but some people become fixated,"
says Phil Jones, a climate researcher at the University of East
Anglia in Norwich, UK. "For projecting the rate of climate change in
the twenty-first century, it is somewhat irrelevant what happened in
medieval times. What really matters is what happened in the twentieth
century and we can expect from that a much warmer climate."
In its 2001 report, the IPCC concluded that "the increase in
temperature in the twentieth century is likely to have been the
largest in any century during the past 1,000 years."
Moberg's reconstruction is consistent with this assessment. But, says
van Storch, the hockey-stick curve, prominently featured in the
IPCC's summary for policy-makers, has become such a powerful icon
that any correction of it will affect the credibility of the IPCC's
work. It could give climate sceptics a boost, despite the fact that
human-driven global warming is not in doubt. The IPCC is likely to
raise the issue in May in Beijing at a closed meeting of its working
group on the physical basis of climate change.
The hockey-stick reconstruction was derived in 1998 by Michael Mann,
a climate researcher now at the University of Virginia in
Charlottesville. A small group of critics, including Stephen
McIntyre, a Toronto-based mineral-exploitation consultant, has since
attempted to prove that the graph is based on insufficient data and
on flawed statistics3. Although McIntyre's work is controversial, a
recent reanalysis by von Storch partly supports his view2. And, in
hindsight, many climate researchers believe that it was premature of
the IPCC to give the visually suggestive curve so much prominence.
"Mann is a pioneer, whose 1998 study was then the best reconstruction
that had ever been done," says Stefan Rahmstorf, a climate researcher
at the Potsdam Institute of Climate Impact Research in Germany. But,
he adds, the controversy it generates is now out of proportion to its
Rahmstorf adds that even if the hockey-stick curve were to be
completely wrong and even if all model simulations of the past
millennium were fundamentally wrong it would hardly touch ideas
about the cause of observed climate change in the twentieth century.
Proxy-based reconstruction of past temperatures are important for
validating the models that researchers use to predict the future
climate. But, he says, "the cause of any particular climate change
must be investigated separately. It would be naive to conclude that
the observed twentieth-century warming must have a natural cause just
because previous warming events have had one."
Meanwhile, Mann concedes that it is plausible that past temperature
variations may have been larger than thought although he insists
that Moberg's reconstruction is not free of methodological and
statistical problems. He says the issue deserves further
investigation and must not be overshadowed by political issues.
"The contrarians would have us believe that the entire argument of
anthropogenic climate change rests on our hockey-stick construction,"
he says. "But in fact some of the most compelling evidence has
absolutely nothing to do with it, and has been around much longer
than our curve."