http://www.eurekalert.org/releases/icstm-fdo031601.html
First, direct observational evidence of a change in the Earth's greenhouse
effect between 1970 and 1997
Scientists from Imperial College, London, have produced the first direct
observational evidence that the earth’s greenhouse effect increased between
1970 and 1997.
Writing today in the journal Nature (1), 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."
