Global Warming: A Verdict in Search of Evidence

In response to a request by the Bush administration, the National Academy of Sciences (NAS) released a report on global warming on June 6. Environmental special interests, members of the media, certain politicians, and international bureaucrats trumpeted selected portions of the report to claim that the science of global warming was now settled, that human activities were responsible, and that the Bush administration was to blame.

Obviously they did not read the entire document.

The NAS report makes several points of uncertainty abundantly clear:

there is much we do not know about how climate works,

there remains a great deal of uncertainty what effect — if any — human activity is having on climate,

computer models used to predict future climate are unreliable, and

we don’t know what factors have caused climate change in the past or will cause it in the future.

Contrary to claims of global warming alarmists, the science is not “settled.”

“Climate projections will always be far from perfect. Confidence limits and probabilistic information, with their basis, should always be considered as an integral part of the information that climate scientists provide to policy-and decision-makers. Without them, the [Intergovernmental Panel on Climate Change Summary for Policy Managers] could give an impression that the science of global warming is ‘settled,’ even though many uncertainties remain.” (emphasis added) (NAS Report, p. 22)

Sounding the Alarm

The first paragraph of the NAS report states that human activities are causing temperatures to rise.

“Greenhouse gases are accumulating in Earth’s atmosphere as a result of human activities, causing surface air temperatures and subsurface ocean temperatures to rise. Temperatures are, in fact, rising.” (NAS Report, p. 1)

This is the line that got the media’s attention. However, after stating that humans are responsible, the report concludes that maybe this really isn’t the case.

“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.” (emphasis added) (NAS Report, p. 17)

Future Climate is Anybody’s Guess

Despite claims that we will see dramatic warming in the future, the NAS report concludes that we really have no idea whether temperatures will go up or down — or by how much.

“Because there is considerable uncertainty in current understanding of how the climate system varies naturally and reacts to emissions of greenhouse gases and aerosols, current estimates of the magnitude of future warming should be regarded as tentative and subject to future adjustments (either upward or downward). (emphasis added) (NAS Report, p. 1)

Unnatural Changes?

Surface temperature over the past century appear to have increased slightly, by between 0.7 to 1.5 degrees Fahrenheit. Global warming theory says that temperatures should have increased in unison with a buildup of carbon dioxide (CO2). However, the NAS panel points out that this has not occurred. Instead, temperatures rose until 1940, then fell again, then rose once more over the past two decades.

“The observed warming has not proceeded at a uniform rate. Virtually all the 20th century warming in global surface air temperature occurred between the early 1900s and the 1940s and during the past few decades.” (emphasis added) (NAS Report, p. 3)

Moreover, this slight rise in temperature is well within the range of what has occurred naturally throughout Earth’s history.

“The range of natural climate variability is known to be quite large (in excess of several degrees Celsius) on local and regional spatial scales over periods as short as a decade.” (emphasis added) (NAS Report, p. 2)

In fact, the NAS report admits that temperature changes could be the result of natural causes. Perhaps these changes are caused by the sun…

“It is not implausible that solar irradiance has been a significant driver of climate during part of the industrial era, as suggested by several modeling studies.” (emphasis added) (NAS Report, p. 14)

…or perhaps from oceanic circulation.

“It is also possible that at least part of the rapid warming of the Northern Hemisphere during the first part of the 20th century and the subsequent cooling were of natural origin — a remote response to changes in the oceanic circulation at subarctic latitudes in the Atlantic sector, as evidenced by the large local temperature trends over this region.” (emphasis added) (NAS Report, p. 16)

Satellite Data Differs

In addition to the fact that surface temperatures have not behaved quite as global warming theory predicts, neither have temperatures in the atmosphere. Satellite temperature measurements in the area of Earth’s troposphere show very little warming (the most recent numbers actually show a slight cooling).

“Although warming at the Earth’s surface has been quite pronounced during the past few decades, satellite measurements beginning in 1979 indicate relatively little warming of the air temperature in the troposphere.” (emphasis added) (NAS Report, p. 17)

This difference shows again just how little we really know about how climate works.

“The finding that surface and troposphere temperature trends have been as different as observed over intervals as long as a decade or two is difficult to reconcile with our current understanding of the processes that control the vertical distribution of temperature in the atmosphere.” (emphasis added) (NAS Report, p. 17)

Climate Models Just a Guess

Predictions of global warming are based entirely on climate models, which are computer representations of how climate works. Of course, these computer models can’t accurately portray climate, as the NAS report concludes. There simply isn’t enough information, computers are still not sophisticated enough, and we aren’t sure how to read the results the models produce.

“However, climate models are imperfect. Their simulation skill is limited by uncertainties in their formulation, the limited size of their calculations, and the difficulty of interpreting their answers that exhibit almost as much complexity as in nature.” (emphasis added) (NAS Report, p. 15)

We don’t know if the models can predict past climate. There just isn’t enough information about temperatures.

“A major limitation of these model forecasts for use around the world is the paucity of data available to evaluate the ability of coupled models to simulate important aspects of past climate.” (emphasis added) (NAS Report, p. 24)

Even information about our current climate is sketchy.

“In addition, the observing system available today is a composite of observations that neither provide the information nor the continuity in the data needed to support measurements of climate variables.” (emphasis added) (NAS Report, p. 24)

Water Vapor: The Great Unknown

Global warming alarmists claim that eliminating emissions of CO2 is the only way to prevent catastrophic warming. However, a much more important greenhouse gas than CO2 is water vapor.

“Water vapor and cloud droplets are in fact the dominant atmospheric absorbers, and how these substances respond to climate forcings is a principal determinant of climate sensitivity…” (emphasis added) (NAS Report, p. 9)

And, as the NAS report points out, we really don’t know much about how water vapor and clouds affect temperatures. Once again, we are forced to recognize that we have only scratched the surface of understanding how our climate works.

“The nature and magnitude of these hydrological feedbacks give rise to the largest source of uncertainty about climate sensitivity, and they are an area of continuing research.” (emphasis added) (NAS Report, p. 7)

No Cause For Alarm

Despite the shrill cries of doom, the NAS report is not cause for alarm. It introduces no new science to the debate, and actually reinforces the fact that there is a great deal we don’t know about climate — especially whether or not human activities are having any impact.

Environmental special interests, the media, certain politicians, and international bureaucrats all want to distort perceptions of global warming to serve their own agendas.

The next time you hear claims that global warming has been ‘proven,’ be sure to read the fine print.