By Donella Meadows

–April 26, 1990–

With an aha! gotcha! flourish a skeptical reader sent me a recent news release about global warming. “Scientists who analyzed 10 years of weather satellite data found no evidence that the earth has warmed from the so-called greenhouse effect,” it said. “The northern hemisphere goes up slightly during those 10 years and the southern hemisphere goes down slightly…. The data cannot be used to say we’ve got an enhanced greenhouse effect.”

My correspondent fumed, “Why don’t you recognize this in your weekly diatribe?”

I’m happy to recognize it, but in fairness I need to recognize all the information the scientific community is putting forth to prove or disprove that the planet is running a fever. It doesn’t add up to an easy gotcha! for either greenhouse alarmists or skeptics. It mainly shows how hard it is to take the temperature of a planet.

For instance:

Temperature measurements made at the earth’s surface show a slight cooling from 1940 to 1965 and an increase since then, with a distinct rise in the 1980s. By these records the five hottest years of the past century were (in increasing order) 1980, 1983, 1987, 1981, 1988. Looks like a greenhouse effect, doesn’t it?

The trouble is, those measurements come from 2000 meteorological stations not at all evenly distributed over the earth’s surface (most are in the northern hemisphere). Their number and position change over time. They could be influenced by the local warming of urban heat islands. On the other hand the satellite data quoted by my correspondent come from a series of changing satellites, whose instruments are not so easily calibrated as instruments on the ground. And they only cover 10 years, not nearly enough time to spot a climate change.

The best place to look for the greenhouse effect should be near the poles, which are expected to warm much more than the equator. Sure enough, permafrost in the Alaskan Arctic seems to be warming rapidly — as much as five degrees F. in last 50 or so years. Tree ring measurements of northern forests also indicate warming.

But Alaska is too small a sample of the Arctic to be significant. And tree rings are affected by other things than temperature, such as moisture and acid rain.

Ice melting should be a clue to global warming. Sea ice off Antarctica decreased alarmingly in 1970s. But then it began to bounce back in the 1980s.

The greenhouse effect should warm the lower atmosphere but cool the high stratosphere. That checks out — there are clear measurements of stratospheric cooling.

A warmer ocean should expand and raise the sea level. Sea level is indeed going up, but only at the rate it has been for a century or more. There is no sign of the acceleration that would be expected from greenhouse warming.

Just to complicate matters, scientists who monitor the varying output of the sun say that since 1979 incoming solar energy has been on a slight downturn — the decrease has been just about exactly the amount that would offset the predicted warming from greenhouse gases.

With all this contradictory evidence, you might expect scientists to be undecided about the greenhouse effect. Most of them aren’t, however. A recent poll (conducted by the journal Climatic Change) of 22 leading U.S. scientists who study the earth’s climate showed that 18 of the 22 believe strongly in the ultimate reality of the greenhouse effect. Only two indicated disbelief.

Are the scientists being unscientific? No, they are just looking at causes as well as effects. Measurable warming is an effect, and a long-delayed one at that. Its cause is the buildup of heat-trapping gases in the atmosphere. Those gases are easily measured. There is no doubt whatsoever that they are increasing — because we release them when we burn fossil fuels, destroy forests, and generate various kinds of air pollutants.

Will the buildup of heat-trapping gases warm the planet? It might not, if other cooling factors overwhelm it, or if there are self-correcting features of the global climate system we don’t understand. The warming might have already started, but our ways of measuring might be too crude to detect it. It might still be ahead of us, because of the thermal drag of the oceans. But its cause is there. Rationally you’d have to say the effect is very likely.

To hold up preventive action, waiting for an unmistakable measurement of actual warming, is like smoking an increasing number of cigarettes, refusing to stop until we can measure the lung cancer. By some lucky miracle we might escape the cancer. It could already be there but not yet detectable. For sure, by the time it’s unmistakable, its treatment will be painful and perhaps too late.

Are ten years of satellite data enough to disprove the greenhouse effect? No.

Are the hot years of the 1980s enough to prove the greenhouse effect? No.

Does that mean we should do nothing about the greenhouse effect? No.

Copyright Sustainability Institute 1990