NAS summary distorted

Published August 1, 2001

The very first sentence of the Summary to the National Academy of Sciences’ recently issued report on global climate change (NAS/S) makes clear that the politics of climate change continues to take precedence over the science of climate change, even among scientists.

That sentence states unambiguously: “Greenhouse gases are accumulating in Earth’s atmosphere as a result of human activities, causing surface air temperatures and subsurface ocean temperatures to rise.” Only near the end of the report itself (page 17 of 24 pages) do we learn of the considerable uncertainties that could offset the clear and unequivocally stated conclusion of the first paragraph:

Because of the large and still uncertain level of natural variability inherent in the climate record and the uncertainties in the time histories of the various forcing agents (and particularly aerosols), a causal linkage between the buildup of greenhouse gases in the atmosphere and the observed climate changes during the 20th century cannot be unequivocally established. The fact that the magnitude of the observed warming is large in comparison to natural variability as simulated in climate models is suggestive of such a linkage, but it does not constitute proof of one because the model simulations could be deficient in natural variability on the decadal to century time scale.

Even this backing-off is wrong in several respects.

General comments on NAS/S

  • The claimed temperature rise of the past few decades is based entirely on surface data from poorly controlled stations and sea-surface measurements (of water, not air, temperatures); those data are badly suspect. The NAS/S ignores the observed climate cooling that took place between 1940 and 1975, so obviously at variance with the increase in greenhouse gases. It completely ignores the data from weather satellites and radiosondes, which show no appreciable warming trend since 1979. In addition, none of the many proxy measurements (tree rings, ice cores, etc.) shows any warming trend after 1940.
  • Even if one were to accept the claim that the climate has warmed in the past 50 years, there is no evidence (in the form of “fingerprints,” for example) that such a warming is human-related. On the contrary, the available evidence directly contradicts the idea that humans have made and are making a substantial contribution to temperature changes. The past century’s trends can best be explained in terms of natural variability, most likely caused by solar variability.
  • Furthermore, if one were to assign such a warming entirely to an increase of greenhouse gases, the “climate sensitivity” thus obtained would be well below even the lowest value quoted by the IPCC, a rise of 1.5 C for a doubling of greenhouse gases.
  • The observed warming trend of the deep ocean is best explained as a delayed consequence of the pre-1940 warming. There is also an observed cooling trend of the ocean, ignored in the NAS/S, again a reflection of a previous surface cooling (Singer in Eos, American Geophysical Union, Spring meeting 2000).
  • The first paragraph of the NAS/S mentions “associated sea-level rise” as if sea-level rise were a necessary consequence of an anthropogenic climate warming. Geologic evidence confirms that sea-level rise has been ongoing for the past 18,000 years and has resulted in a total rise of 120 meters [400 feet]. The current sea-level rise is mostly due to the slow melting of Antarctic ice sheets, which will continue for several more millennia.

Specific comments on NAS/S

  • NAS/S understates the natural variability of climate by at least a factor of 100. The geologic record shows variability of a few degrees C in decades, not millennia.
  • The NAS/S estimate of the “removal time” (lifetime) of carbon dioxide (CO2), as greater than 100 years, is overstated by a large factor.
  • The NAS/S sidesteps the fact the atmosphere has not been warming since 1979 (according to satellite data): “The troposphere warmed much more during the 1970s than during the two subsequent decades.” Of course; there was a major sudden warming between 1975 and 1978, unconnected to any human activity.
  • The NAS/S constantly refers to “observed warming of the last 50 years” in spite of overwhelming evidence against such an observation.
  • The NAS/S gives credence to extreme scenarios used by the IPCC and leaves the impression that a future warming of 5.8 C is as likely as a lower value. But it admits later that the IPCC scenarios have already been proven wrong by actual observations of CO2 growth rates.
  • In discussing weather extremes on a regional basis, the NAS/S states, “Some models project an increased tendency towards drought,” but fails to mention that other models predict the opposite–extreme precipitation–for the same regions.
  • Finally, the NAS/S manages to sidestep the fact that the IPCC Summary, a political document, quotes the IPCC report selectively and exaggerates disasters while downplaying uncertainties. As the NAS/S puts it artfully: “The [IPCC] Summary for Policymakers reflects less emphasis on communicating the basis for uncertainty and a stronger emphasis on areas of major concern. . . . This change in emphasis appears to be the result of a summary process in which scientists work with policymakers on the document.” Yes indeed.

Concluding thoughts

The NAS report stands or falls principally on whether the climate warmed in the past 50 years, and especially since 1980. The overwhelming bulk of data from different independent sources shows no such warming trend. We are not talking just about science, but about evidence. A full-scale open debate is in order to settle this matter.

S. Fred Singer is professor emeritus of environmental sciences at the University of Virginia and president of the Science & Environmental Policy Project. Singer’s “The Week that Was” columns can found on the Internet at