The team looked in particular for 'lag time' effects in the data. For example, the eruption of Mt Pinatubo in the Philippines in 1991 lowered the global temperature by 0.3 C in the following year as fine aerosols of volcanic ash and sulphuric acid caused a cooling effect. However, the team believes that volcanic eruptions have a short lived effect on the long-term temperature. They therefore looked at two possible variables that could adjust climate - greenhouse gases and changes in the sun's output.

Wigley and colleagues ran the two independent computer models four times, each time with a different set of parameters. In the first data set, they created a world in which greenhouse gases remained constant over 100 years and changes in solar output do not affect the climate. In the second set they assumed that the Earth's climate can be affected by solar output - called solar forcing - but greenhouse gases remain constant. The third set had no solar output, but greenhouse gases rise over time. Finally the fourth set had a mixture of both solar forcing and rising levels of greenhouse gases.

They first found that there was wide discrepancies between the output of both computer models and the experimental data in the first data set. This indicated that some mechanism was increasing the global temperature this century. Then they found that if solar heating was solely responsible for climate change, the Earth's climate would have to be six times more sensitive to solar heating than realistic estimates suggest. Wigley, Smith and Santer concluded that unlike a paper published in Physical Review Letters this week (see CERN plans global-warming experiment), the only model that matched both simulation and experimental data sets was the one containing a strong greenhouse gas effect and a small amount of solar heating.

According to Wigley: "These results provide another important piece in the jigsaw puzzle of climate change, strengthening 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."