In Search of the “Greenhouse Signal” in the 1990s (and when did they know?)

In early 1991, William K. Stevens, global warming writer at the New York Times, reported that scientists were not ready to pronounce evidence of man-made global warming: 

… most scientists are far from ready to announce that greenhouse warming has arrived, since the warming recorded over the last decade could also be part of a natural climatic change. Instead, they are struggling to answer a crucial question: how can a greenhouse warming of the climate be recognized and distinguished from natural warming? They are focusing their detective efforts on various subtle changes that a greenhouse warming would be expected to induce. These signs are known collectively as the greenhouse “fingerprint.”

The task, climatologists say, is by no means as easy and straightforward as it might seem. Stevens added: But the greenhouse “signal,” if in fact it is there now, is still so small on a global scale that it is obscured by the “noise” of the many other factors that influence climate. These other factors, the climatologists say, could well be the cause of the overall global warming observed in the last decade.

Or, equally possibly, they could have produced an overall cooling that partly offset an even larger greenhouse warming than the rise in average global temperature might suggest. In 1995, Stevens reported that “Climate Experts Warm Up to Blaming Greenhouse Effect.” And in 1998:

It is clear, climatologists say, that the earth’s surface has warmed since the start of the Industrial Revolution. But is the warmth out of the ordinary? And did people cause it? Scientists have been unable to provide a definitive answer to these questions because they do not know how much the global climate has varied on its own in the comparatively recent past — say, the last thousand years —which offers the best basis for comparison with today’s climate. Temperature records based on thermometer readings go back only about 150 years.

Some scientists were saying ‘yes,’ but others no. Stevens quoted

climatologist Thomas Wigley of the National Center for Atmospheric Research in Boulder, “regarded as a leading expert on the issue of detecting the greenhouse signal”:

”They’re making progress, and there is a lot of hard work involved, and I hold them in the highest regard,” Dr. Tom Wigley, a climatologist at the National Center for Atmospheric Research in Boulder, Colo., said of Dr. Mann and his colleagues. ”But I think there’s a limit to how far you can ever go.” As for using proxy data to detect a man-made greenhouse effect, he said, ”I don’t think we’re ever going to get to the point where we’re going to be totally convincing.”

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