For one who lived through the Limits to Growth silliness as a skeptic then, I look at the current global-warming controversy in the same doubting light. There are several questions that need to be answered regarding global warming before the policy decisions can be responsibly made.

1. Is global warming occurring?

There is a large accumulation of evidence that global warming is occurring. Glaciers are melting, and temperatures under various measures seem to be rising. These changes seem to be more than just a warm winter in Siberia or record heat in the South.

Unfortunately, the data are not perfect, primarily because extensive, regular, “real time” measurements have only occurred on a systematic basis for a short period of time—decades for satellite measurements. As a result, data have been pieced together from various sources, such as tree rings and ice-core samples, to create longer time series for analysis than what is available currently.

The evidence that global warming is occurring is strong. A caution, however: the evidence, while large, may not be conclusive. 1) temperature records around urban areas are rising due to the urbanization itself. A new record in northern New Jersey, for example, may be due to global warming, or it may be due to the increasing density of population; 2) there are studies that show a seesaw effect between Northern and Southern Hemisphere temperatures: as one rises the other falls and vice versa; 3) according to some scientists, there are important measures of climate change that have not increased or even have fallen for the last two decades, and 4) the period of time in which accurate and extensive record keeping has taken place is very short and the time period over which data are needed is very long. Widespread and accurate surface temperature data have been recorded for less than two centuries. Satellite data are available for less than a quarter of that time.

Conclusion: Despite the cautions, the evidence seems strong that global warming is occurring.

2. Is the warming abnormal? This is the key question.

To determine abnormal in this instance, one needs to look back over the last several thousand years to see if there have been similar or warmer periods.

We know that from about 950 to about 1250, there were human settlements in Greenland with extensive agriculture. Thus, the ice cap on Greenland must have melted enough—raising ocean levels, presumably—to allow for cultivation of crops.

There is some science to suggest that the melting of Alpine glaciers has occurred maybe a dozen times over the last 15,000 years, so the combination of temperature levels and precipitation that leads to smaller glaciers is normal.

There are observations that the ice cap on Mars is melting as well, which would imply that perhaps sun cycles are the cause of what is happening.

Conclusion: What is occurring today, whether partly caused by human activity or not, is not out of the ordinary when considered over the long horizon. Thus, global-warming advocates need to explain why the past cycles are not applicable to the current situation. I have seen no such explanation.

3. How can we tell the difference between a normal occurrence of a climate-change phenomenon, large as it may be, and something caused by human activity?

If one accepts the fact that the earth has been warmer in the past, then the task is to isolate what is caused by the normal fluctuation of the earth’s temperatures and what the particular effect is of human activity. To illustrate the problem: if the range of high temperatures experienced on August 17 in Ames, Iowa, say, is between 70 degrees and 95 degrees, to make up numbers, and the actual temperature was 85 degrees in 2006, how does one know if there is a trend toward warmer August 17ths or whether the temperature that day is a product of normal fluctuations.

The same problem occurs under an analysis of global warming. Are the retreating glaciers a part of the normal fluctuation of the size of glaciers, or is human activity causing them to melt more quickly than they would have otherwise done?

The answer to this question does not come from measurements of current or past temperatures, because the 20th and 21st Century types of records have not been collected long enough to provide the answers to both the human-activity and long-term change effects on global warming over a few thousand years. The longer series that are constructed from tree rings, carbon measures and such may not be accurate enough to be dispositive of the issue.

The answer to the question really has to come from statistical and systems-analysis models of climate. Unfortunately, the complexity of climate requires extremely complex models. Besides good data, the models need to be able to parse out the difference between climate cycles and human-caused disturbances.

There are two major criteria the models need to meet: a) Do they predict (or backcast) the known wide range of historical climate experience—in particular the Greenland warming and cooling periods that occurred in recorded history at the same time as they are predicting the next century, and b) do the various models exhibit a consistency of results? Neither of these criteria appear to have been met. The Middle Ages warming period reflected in Greenland tends to be ignored—that is, not included in a back cast—and the models, while pointing in the direction of warming, have widely different predicted impacts. As a consequence, the parsing of human-caused versus nature-caused warming is inconclusive at best.

There are two major problems that the models face. 1) The data are not very good. The best-quality data come from recent measurements, but the past data are developed from inference: there are no observations or data points for the year 1217, for example, except what can be gathered from such sources as tree-ring, ice core or carbon analyses.

2) The models come from the autoregressive, operations-research areas of research. Such models can be good or spectacular failures as were the Limits to Growth energy-forecasting models of the 1970s and 1980s. That is, the climate models rely more on mathematics—using complex data patterns—than on the physics that underlies climate—for good reason, because the complex physics of cause and effect is not well understood. If the data patterns are incomplete or omit key impacts of the physics, then the models, though sophisticated, can come to inappropriate conclusions. The Limits to Growth analysis omitted the economics of supply and demand and resulting prices that caused people to consume less and seek more energy supply, so the model-predicted exhaustion of resources did not occur. To the extent that the climate models have omitted key physics results, they may have the same spectacular downfalls.

The models are the weak link in the demonstration that human activity causes global warming.

Conclusion: The models do not provide a solid basis for differentiating between human-caused global warming and that which occurs in nature’s long climate cycles.

OVERALL CONCLUSION OF THE SCIENCE AND DATA: From the data and modeling it is difficult to conclude that the analysis shows in a “slam dunk” fashion that human activity is responsible for global warming.