Radical environmentalists characterize global warming/climate change as a potentially cataclysmic phenomenon that is caused largely by human industrial activity and its polluting by-products and has the potential for ending life as we know it.
The most widely quoted attempt to address climate changes of the past -- and to speculate about the future -- is the series of reports produced by the UN Intergovernmental Panel on Climate Change (IPCC). Its first Scientific Assessment Report (1990) concluded that the climate record is "broadly consistent" with what might be expected from the human-enhanced greenhouse effect, as calculated by General Circulation Models (GCMs).
The second UN Scientific Assessment Report, published in 1996 and citied by radical environmentalists as scientific proof of a coming catastrophe, found it necessary to introduce a previously overlooked factor -- human-caused atmospheric sulfate aerosols -- to reach the conclusion that "the balance of evidence suggests there is a discernible human influence on global climate."
But the IPCC arrived at its ambiguous conclusion based on "fingerprints" in the climate record; i.e., an increasing correlation (over time) between observed and calculated global temperature patterns. And this positive trend in correlation depended entirely on the arbitrary choice of the time interval 1940-1990, during which temperatures were, for the most part, actually decreasing. A different choice of interval could have produced a zero or even a negative trend.
Since the publication of the IPCC report in 1996, an increasing number of researchers have adopted the view that much or most of the pre-1940 warming was due to natural causes and represented a recovery from the Little Ice Age of 1300-1850. Some would assign a substantial portion of that warming to greenhouses gases. Others claim that most of the temperature increase was caused by solar variability.
Perhaps the strongest argument against an appreciable human contribution to climate patterns comes from the observed cooling that took place between 1940 and 1975, and the lack of warming since 1979 (in the weather balloon and satellite data).
The Earth's climate has never been steady; it has either warmed or cooled -- without any human intervention -- since the dawn of time. The measured variations have often been larger and more rapid than those currently predicted by climate models for the year 2100. In the last 3,000 years, temperatures in the North Atlantic have changed by as much as 3°C within a few decades. During the most recent Ice Age, the variability was even greater.
The frequent ice ages of the last few million years appear to be linked to changes in the absorbed incident solar radiation, which in turn is affected by orbit changes of the Earth -- the so-called astronomical theory. Longer-term climate changes seem to be linked to continental drift and other tectonic events. Shorter variations, on the time-scale of decades, appear to be caused by atmosphere-ocean interactions and changes in ocean circulation. Alternatively, they could be due to external causes, such as variations in solar irradiance or in solar activity.
None of the climate models incorporate the effects of a variable sun. It has always been assumed that solar variability is simply too small, but this view is now changing. Evidence shows that solar winds and sunspots can affect the earth's ozone layer and influence atmospheric circulation or cloudiness -- which in turn can cause significant climate changes.
As for the association of climate change with atmospheric greenhouse gases, on the time-scale of hundreds of millions of years, carbon dioxide (CO2) has sharply declined; its concentration was as much as 20 times the present value at the beginning of the Cambrian Period, 600 million years ago. Moreover, glaciations have occurred throughout geologic time even when CO2 concentrations were high.
Radical environmentalists and high government officials have declared repeatedly that climate science is "settled" and "compelling." The clear implication is that we know enough to act; that any further research findings would be "policy-irrelevant" and not important to the international deliberations of the parties to any climate treaty. But in fact, as of mid-2008, no fewer than 31,000 U.S. scientists had signed a petition rejecting the assumption that the human production of greenhouse gases was damaging Earth's climate.
In late November 2009, the so-called "Climategate" scandal cast grave doubt on the intellectual integrity of those leading the effort to spread fear about the alleged dangers of global warming. At the heart of the controversy was the discovery that a number of leading American and British climatologists who held that mankind's industrial activity was causing a dangerous warming trend in the earth's atmosphere, had intentionally manipulated the evidence in order to provide "proof" that their warnings were justified. The scientists' deceptions were found out when hundreds of their private email messages and documents were obtained and publicized by computer hackers.