Despite pronouncements of "scientific consensus," climatologists are still uncertain concerning the likely impact of increased atmospheric greenhouse gas concentrations. Trace gas concentrations, such as CO₂ , have been increasing, largely as a result of fossil fuel emissions. However, concentrations of methane and chlorofluorocarbons (which are also important atmospheric trace gases) have leveled off in recent years. The extent to which fossil fuel emissions have contributed to the rise in global atmospheric air temperatures since about the mid-1800s is still debatable, since much of the warming occurred before significant increases in atmospheric trace gas concentrations, and because there was a dramatic downward trend in air temperatures between the early 1960s and the mid-1970s — after significant increases in atmospheric CO₂ — which led to the short-lived "global cooling" scare. Connections between this recent moderate rise in air temperature and changes in other parts of the climate system are far more tenuous.

"Global warming due to human greenhouse gas emissions is still debatable."

In fact, even among global warming alarmists, projections of global warming for 2100 have decreased significantly since early modeling efforts. The response to a doubling of CO₂ exhibited by the many models used for the Third Assessment Report range between 2.7° and 5.4° F. If such trends continue — and all models except one (the Canadian model) exhibit a gradual, consistent rise over time — global air temperatures should increase by 2.5° F and air temperatures in the United States by about 1° F during the 21^st century. Most of this warming should occur in the coldest winter air masses, while summer rainfall should increase slightly. Such a warming could hardly be called unprecedented or catastrophic. In general, our climate has and will continue to exhibit intricate patterns not reliably reproduced by global climate simulations, thus underscoring their scientific incompleteness — and lack of reliability for prediction of future climate scenarios.

NOTE: Nothing written here should be construed as necessarily reflecting the views of the National Center for Policy Analysis or as an attempt to aid or hinder the passage of any bill before Congress.