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

By Robert Bradley Jr. -- June 21, 2023

“As for using proxy data to detect a man-made greenhouse effect, I don’t think we’re ever going to get to the point where we’re going to be totally convincing.” – Thomas Wigley, National Center for Atmospheric Research, (April 28, 1998)

When did the “greenhouse signal” become recognized and “settled science”? Despite the 35th anniversary of James Hansen’s June 1988 testimony to a Senate subcommittee, the historical record should be clear that detection was not in 1988. Or 1991. Or 1995. Or 1998.

And “Exxon Knew“?


Here is some history from the used-to-be newspaper of record, the New York Times, and its global warming scribe, William K. Stevens.

In early 1991, Stevens 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 “Global Warming Experts Call Human Role Likely.” 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.”

And “Exxon Knew”?


  1. Russell Seitz  

    Robert, what did Exxon know in 1990 that Roger Revelle didn’t know in 1965, when he chaired the group that wrote the White House report on fossil fuels, CO2 and the environment?

    The subject surfaced the year Abraham Lincoln was elected , and President Taft’s staff doubtless read Popular Mechanics update in 1912.

    The same task fell to me in 1990, and in the decades since Moore’s Law has advanced the predictive power of global circulation and radiative forcing models to the point of making those who persevere in obfuscating matters on behalf of API and its clients the laughingstocks they deserve to be.

    As you can still read today:

    I meant what I said in 1990 about the scientific necessity of changing your mind if the evidence changes —


    • rbradley  

      Russell: The global warming issue has not been about a positive qualitative relationship between temperature and GHG forcing, it has been about the magnitude and distribution of the warming. Pat Michaels argued this from the 1980s until he died. I have followed his lead.

      The good news is that the warming is “global lukewarming” with many positives that climate economists have documented (Robert Mendelsohn). Warmer winters are an obvious plus. And concerning CO2 benefits, what could be better than photosynthesis to make Planet Earth greener and more productive.

      Also, the “greenhouse signal” is more winter and night and cold regions, quite benign. So the sign of the ‘externality’ is in debate, rightly so.

      Exxon’s here-and-gone scientists did not know much in the 1970s and early 1980s (before they were laid off), and the search for the “greenhouse signal” in the 1990s was a very troubled one.



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