`Greenhouse effect' on climate is a cloudy issue. Recent studies suggest that warming trend may be natural fluctuation

SCIENTISTS trying to understand humanity's impact on climate are frustrated by not knowing what happens to climate naturally. Possible warming by carbon dioxide (CO2) pollution from burning coal, oil, and natural gas is a case in point.

Theory and computer simulations predict that a doubling of the concentration of this heat-trapping gas could warm our planet by several degrees Fahrenheit over the next half century. Some analyses suggest that this so-called CO2 ``greenhouse effect'' may already be evident. Climate data do reflect a warming trend. But it's different in character from what the computer models project. It may well be only a natural fluctuation.

In a comprehensive review of this problem, researchers at the Lawrence Livermore National Laboratory reached ``the clearly unsatisfactory conclusion that, while we are witnessing a warming of the terrestrial climate, we cannot identify its cause.''

They add: ``Even if it is of anthropogenic origin, it need not be due only to increased CO2. But if it's due predominately to CO2, then our present climate models require work to reconcile them with observational data on patterns of surface temperature change.''

The review team includes Hugh W. Ellsaesser, Michael C. MacCracken, John J. Walton, and Stanley L. Grotch. Their report appears in the current (November) issue of Reviews of Geophysics, a quarterly journal devoted to overviews of important research fields.

The climatic record is complex, patchy, and often tricky to interpret. Geological evidence, inferences from old agricultural records, and other non-instrumental data can be an unreliable guide. Even the instrumental records of the past century are marred by differences in observing standards in different regions and by changes of standards over time. Just moving a thermometer a few hundred feet can give a false indication of ``warming.''

Also, there's what the Livermore team calls ``the trap into which we fell because ... collection and archiving of worldwide weather records began in 1881, near the apparent [natural] temperature minimum of 1883.'' The team adds that, if information now available on that temperature trend had been known earlier, ``it is unlikely that there would now be the present degree of concern over the climatic effect of CO2.''

There seems little doubt that the planet has been warming. The Livermore scientists conclude that ``the global mean surface temperature has indeed been increasing, albeit erratically'' since about 1750.

The researchers note that the surface temperature changes in the climatic record have a patchwork distribution over the planet that is significantly different from the distribution predicted by computer simulations. The team says this suggests ``either that our climate models are inadequate or that there is a fundamental difference in character between climate changes of the past and those to be expected from changes in CO2....'' If there is such a difference between CO2-induced warming and natural climate change, no one knows what it is.

The team notes that, because the rise in CO2 and the warming trend have developed together and because other warming mechanisms aren't obvious, there's ``a strong inclination'' to blame CO2 pollution. ``Pending supporting information,'' the team warns, ``this inclination should be resisted.''

That doesn't mean the greenhouse effect can be ignored. Planetary scientists have only to look at Venus to know what a CO2 blanket can do. With a surface hot enough to melt lead (roughly 900 degrees F.), any oceans Venus may have had were boiled away.

Some scientists have wondered why Earth escaped a similar evolution. It has at least two-thirds as much CO2 as does Venus. Most of this inventory now is locked up in carbonate rocks. But a substantial part of it probably was in the atmosphere when Earth was young. The Sun was 25 to 30 percent dimmer then. Yet there's no sign of glaciation.

Large amounts of CO2 in the primitive air probably kept the planet warm. This raises the question of why Earth, too, didn't undergo a runaway greenhouse effect.

James F. Kasting and Thomas P. Ackerman of the NASA Ames Research Center have simulated what happened on a computer. They find that, unlike Venus, Earth's atmosphere could have had thousands of times as much CO2 as it does today without heating the planet enough to vaporize its ocean.

Unlike Venus, our planet seems to have been stable against a runaway CO2 greenhouse effect. While that's an intriguing finding, it doesn't mean that doubling atmospheric CO2 through pollution won't significantly affect climate. To pin down that possibilty, we first have to understand what's happening to climate today.

A Tuesday column. Robert C. Cowen is the Monitor's natural science editor.