A team of UK-based scientists have published evidence which they say proves unequivocally that global warming is real.

Comparing data obtained from two satellites which orbited the Earth 27 years apart, they found that significantly less radiation is now escaping into space than was previously the case.

What we are seeing can only be due to the increase in the gases

Dr John Harries

Earlier studies saying that global warming was happening have been based on computer models. The scientists say their new findings are unambiguous, and they are certain that the greenhouse effect is at work.

The team analysed data in the form of spectra of Earth's outgoing longwave radiation, which measures the escape of heat to space and bears the imprint of the gases believed to be causing global warming.

They compared two sets of data. The more recent was collected over a nine-month period from October 1996 by the Interferometric Monitor of Greenhouse Gases instrument on the Japanese ADEOS satellite. The earlier data set were collected by Nasa's Infrared Interferometric Spectrometer on the Nimbus 4 spacecraft from April 1970 to January 1971.

Special algorithm

The researchers say they found differences which showed a statistically significant increase in the characteristic spectral bands of carbon dioxide (CO2), methane, ozone, and two chlorofluorocarbons, refrigerants blamed for ozone depletion.

Their findings, reported in the journal Nature, show that less radiation was escaping from Earth to space in 1997 than in 1970. It means the gases are being kept in the atmosphere, and are trapping the Sun's heat.

The team took several steps to ensure that their data were reliable, including effectively removing the influence of cloud cover by using a cloud-clearing algorithm.

They used only selected data from the same three-month period, April to June, and to reduce "noise" in the data they chose several regions of the globe and calculated clear-sky average spectra.

The scientists do not produce evidence to show the Earth's surface temperature is rising. The team leader, Dr John Harries, said the greenhouse effect in the atmosphere could produce more clouds that reflected solar radiation and prevented the Sun's heat from reaching the surface.

Cloud feedback

But he is in no doubt that global warming is real. He said: "The results presented here provide to our knowledge the first experimental observation of changes in the Earth's outgoing longwave radiation spectrum, and therefore the greenhouse effect.

"We're absolutely sure, there's no ambiguity. What we are seeing can only be due to the increase in the gases."

Dr Harries was president of the UK's Royal Meteorological Society from 1996 to 1997, and is a former director of projects and technology at the British National Space Centre.

He said: "The next step is to assess whether these data can provide information about changes in not only the greenhouse gas forcing, but the cloud feedback, which is a response of the cloud field to that forcing.

"We must also work to test agreement with the general circulation models used in climate change experiments. Since these are the models used to predict future climate and influence policy decisions, it is imperative that they can accurately simulate measurements of what is considered to be the driving mechanism behind climate change.

"Much more information is locked up in the data that we have."