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Past climate comes into focus but warm forecast stays put

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  • Mike Neuman
    Nature Published online: 9 February 2005; | doi:10.1038/433562a Past climate comes into focus but warm forecast stays put Quirin Schiermeier A reassessment of
    Message 1 of 1 , Feb 14, 2005
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      Published online: 9 February 2005; | doi:10.1038/433562a
      Past climate comes into focus but warm forecast stays put
      Quirin Schiermeier

      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

      Moving on

      "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
      scientific significance.

      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."
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