Writing today in the journal Nature, researchers in the Department of Physics show that there has been a significant change in the Earth's greenhouse effect over the last 30 years, a finding which is consistent with concerns over so-called 'radiative forcing' of the climate (2).

Previous studies in this area have depended on theoretical simulations because of the lack of data. However the Imperial team reached their conclusions after analysing data collected by two different earth-orbiting spacecraft, in 1970 and 1997.

Comparison between the two data sets has unequivocally established that significant changes in greenhouse gas emissions from the Earth have caused the change to the planet's greenhouse effect over this time period.

Professor John Harries, the lead author of the paper says: "These unique satellite spectrometer data collected 27 years apart show for the first time that real spectral differences have been observed and that they can be attributed to changes in green house gases over a long time period."

The team examined the infrared spectrum of long-wave radiation data from a region over the Pacific Ocean, and also over the whole globe. They discovered significant differences in the levels of atmospheric methane, carbon dioxide, ozone and chlorofluorocarbons 11 and 12 between the data, collected in 1970 and 1997.

"The scientists found that by taking the difference between the two sets of data for the same region, they observed the change in the outgoing longwave radiation, and therefore a change in the greenhouse trapping by the atmosphere."

Although the two experiments were flown on separate spacecraft, 27 years apart, the team showed that their comparison of outgoing infrared long-wave radiation spectra is valid. Even allowing for the different spatial and spectral resolutions of the two instruments, there are significant changes in the spectra of the greenhouse gases of the Earth, over this time period.

The team took a number of steps to ensure that their data was reliable. The effects of cloud cover were effectively removed by using a cloud-clearing algorithm. The resulting two datasets were of comparable resolution and representative of clear-sky conditions. To reduce 'noise' in the data, the team selected several regions of the globe and calculated clear-sky average spectra. To avoid seasonal artefacts they used only selected data from the same 3-month period (April - June).

Dr Helen Brindley, second author on the paper says: "Through our modelling studies using independent knowledge of the atmospheric state we have shown that the magnitude of the changes observed can only be explained by long-term changes in the greenhouse gases such as carbon dioxide, methane, and chlorofluorocarbons."

Professor Harries described the next challenges for the team: "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 (3) used in climate change experiments. These use basic knowledge of expected changes in climate forcing (for example changes in greenhouse gas amounts, solar constant) to predict the climate response.

"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.

"We are only at the beginning of making use of these spectral observations. Much more information is locked up in the data that we have. This provides a strong motivation for the launch of similar instruments to monitor the state of our climate."

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