Evidence of an increasing greenhouse effect
(March 2001)

The middle of March saw the world's most reputable scientists united in outrage at the action of US president George W. Bush, who elected not to cap the carbon dioxide emissions of US power generation facilities, citing the claim that the world's scientists were yet to agree that carbon dioxide emissions are linked to global warming. The end of March saw the world's more sensible politicians joining in as the US president decided that the US economy could not take the strain of adherence to the Kyoto protocol. Unlike the scientists, who preferred to remain dispassionate, European politicians in particular were scathing of Bush and his decision.

As we pointed out in Global warming - too early to judge, August1997, the problem then was that we could not at that time be certain that carbon dioxide was the major cause of global warming, but that even then, faced with the common political stance that we have seen no problems so far, some scientists were already suggesting that this was a bit like the person who jumps from the twentieth storey of a building, and reports no problems after completing 95% of the fall.

We also explained in Former Nobel laureates call for Greenhouse cuts, October 1997, how more than 1500 of the world's leading senior scientists, including the majority of Nobel laureates in science, had signed a landmark consensus declaration urging leaders world wide to act immediately to prevent the potentially devastating consequences of human induced global warming.

In The Evaporation Paradox, November 1997, we pointed out that scientists have no shortage of data in support of a trend to global warming: temperature, precipitation, stream flow and cloud cover records all indicate that warmer, rainier weather is now more common in many regions of the world. In that article, we showed how one apparent problem in the theory, the 'evaporation paradox' had been overcome, and by that time, the tide of opinion was turning.

In Climate Change and Greenhouse Gases, September 1999, we recalled that the nature of science and the way science is done means that nothing is ever totally absolutely proved to be true. Even so, there are many things that all scientists believe and accept, because to believe otherwise seems ludicrous. This was in the context of noting that the vast majority of scientists were now inclined to favour the view that carbon dioxide and other greenhouse gases were at the base of our problems. In that story, we described how the American Geophysical Union had published a thorough, documented analysis of the peer reviewed literature, itself rigorously peer-reviewed. The document included a bibliography of 189 authoritative research studies, and the analysis was based on those studies. From that time on, anybody who claimed the scientists were in disagreement was grasping at straws.

The question of global warming, of course, is aside from the risk of coral bleaching, described in Threat to tropical coral reefs, April 1999, where there is no room for doubt that carbon dioxide alone is causing the problem by changing the acidity of the  oceans. Nonetheless, there remain 'greenhouse deniers', people who will argue that carbon dioxide is not to blame, and some of those may be reputable (if elderly) scientists who prefer to cling to an old paradigm, like Wilhelm Ostwald, who refused to accept the reality of atoms until 1915.

But for any politician to act on the assessment that 'the scientists cannot agree' is doubly irresponsible; the scientists who are in the best position to know are in agreement, and even if they were wrong, there is a clear and present danger, and the precautionary principle tells us that we must act as though there was no shred of doubt in the mind of any scientist anywhere. Instead, president George W. Bush has elected to listen to people whose idea of scientific proof is to say that carbon dioxide is only 0.03% of the atmosphere and what effect a low concentration like that have?

One slightly emotive response heard around the Internet recently is to note that an atmosphere containing 0.03% hydrogen cyanide is quite different from cyanide-free air, and to invite these people to test the difference. This is admittedly a little childish, though it has the merit of being no more so than the argument that there is not enough carbon dioxide to have an effect, but still the 'greenhouse deniers' ask where the smoking gun is, the evidence that nails CO2 as the source of our worries?

We need look no further. Scientists from Imperial College, London, have produced the first direct observational evidence that the Earth's greenhouse effect increased between 1970 and 1997. Note: they have measured the greenhouse effect, not any form of global warming

Writing in Nature for March 15, the researchers from the College's 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.

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

In the careful language of science, the scientists observe that'' . . . a strong link between increases in surface temperatures and greenhouse gases has been established. But this relationship is complicated by several feedback processes - most importantly the hydrological cycle - that are not well understood.''

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. 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 greenhouse gases over a long time period.''

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.

Comparison data based on different instruments are always open to question, so the team took a number of steps to ensure that its data were 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, it used only selected data from the same three-month period (April-June).

The next step, according to Harries, 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. But in the meantime, the research shows clearly that greenhouse gases are involved in the observable and entirely undeniable global warming.

©WebsterWorld Pty Ltd/contributors 2002

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