- A report by US scientists says a new
analysis of more than a century of global temperature records supports
the belief that humans are influencing the climate.
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- The report is by three scientists from
the National Center for Atmospheric Research (NCAR), the University of
North Carolina, and the Lawrence Livermore National Laboratory.
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- Their findings are reported in the 27
November issue of Science magazine.
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- Sorting the signal from the noise
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- The authors, led by Tom Wigley of NCAR,
subjected 115 years of global temperature data to statistical analysis,
and compared the result with the output from two leading computer climate
models.
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- Their study is the first to examine each
year's average temperatures for the northern and southern hemispheres by
correlating them with readings taken up to 20 years earlier or later.
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- If the numbers rise and fall randomly
over time, the authors say, then the correlations are weaker than if there
is a consistent long-term trend.
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- What they found was that the correlations
were far stronger for the actual temperature data than for the simulations
taken from the two models.
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- The models were designed deliberately
to omit any effect of this century's increase in greenhouse gases, by holding
them constant.
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- That means the models replicated only
the natural year-to-year variability of the climate system.
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- The implication of their results, the
authors say, is that this century's warming trend has overpowered that
natural variability.
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- They also looked at the influence on
the climate of volcanoes and changes in the sun's output.
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- They concluded that volcanic eruptions
are so infrequent and their effects so short-lived that they can be rejected
as an explanation for the differences between the data and the models.
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- Sun not acting alone
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- But they say variations in solar output
over the last century could have been large enough to affect some long-term
trends.
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- Global temperature rose sharply from
about 1900 to 1940, and then levelled off until the 1970s.
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- Then it began another warming spell,
which has accelerated in this decade.
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- To establish the sun's role, the authors
did a separate experiment with a third, simpler climate model.
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- They subtracted estimates of the possible
influence of greenhouse gas levels, and of solar input, from the actual
temperature record.
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- In order to explain their results using
solar effects alone, the authors found that their model had to be about
six times more sensitive to changes in solar input than they thought realistic.
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- They conclude: "Solar forcing alone
is insufficient to explain the behaviour of the observed temperature data."
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- But by contrast, they found that combining
solar input with changes in greenhouse gas levels produced a more credible
result.
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- Confidence in predictions
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- The model then showed a sensitivity in
keeping with current understanding of the climate system which was enough
to reconcile the correlations in actual and simulated temperature data.
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- The authors say their results "imply
that both anthropogenic [human-induced] activities and solar forcing have
significantly affected global climate".
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- Tom Wigley says the results "strengthen
yet further our confidence that there has been a discernible human influence
on climate.
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- "Furthermore, they provide additional
evidence that the models used to make projections of future climate change
are realistic."
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