Earth Climate Changes -
It's the Sun and Us
BBC News Sci/Tech
By Alex Kirby
Environment Correspondent
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.
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.
Their findings are reported in the 27 November issue of Science magazine.
Sorting the signal from the noise
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.
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.
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.
What they found was that the correlations were far stronger for the actual temperature data than for the simulations taken from the two models.
The models were designed deliberately to omit any effect of this century's increase in greenhouse gases, by holding them constant.
That means the models replicated only the natural year-to-year variability of the climate system.
The implication of their results, the authors say, is that this century's warming trend has overpowered that natural variability.
They also looked at the influence on the climate of volcanoes and changes in the sun's output.
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.
Sun not acting alone
But they say variations in solar output over the last century could have been large enough to affect some long-term trends.
Global temperature rose sharply from about 1900 to 1940, and then levelled off until the 1970s.
Then it began another warming spell, which has accelerated in this decade.
To establish the sun's role, the authors did a separate experiment with a third, simpler climate model.
They subtracted estimates of the possible influence of greenhouse gas levels, and of solar input, from the actual temperature record.
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.
They conclude: "Solar forcing alone is insufficient to explain the behaviour of the observed temperature data."
But by contrast, they found that combining solar input with changes in greenhouse gas levels produced a more credible result.
Confidence in predictions
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.
The authors say their results "imply that both anthropogenic [human-induced] activities and solar forcing have significantly affected global climate".
Tom Wigley says the results "strengthen yet further our confidence that there has been a discernible human influence on climate.
"Furthermore, they provide additional evidence that the models used to make projections of future climate change are realistic."