The principal focus of climate science is certainly atmospheric science and meteorology, but the subject is so complex that it involves many other areas of the earth sciences, as well as different disciplines, such as physics, chemistry, and even biology. This complexity makes climate science both fascinating and controversial. It also undergoes rapid change as new facts and analyses emerge. Yet, public interest in the possibility of climate change due to human activities has become so intense that the subject has to be addressed even before final judgments are possible.

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 IPCC, the UN Intergovernmental Panel on Climate Change. Its first Scientific Assessment Report (1990) concluded that the climate record is “broadly consistent” with what might be expected from the human-enhanced greenhouse (GH) effect, as calculated by General Circulation Models (GCMs). The second scientific assessment, published in 1996, no longer made this claim; instead, it 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” [IPCC, 1996, p. 4]. This ambiguous statement in the Summary for Policymakers does not do justice to the vast compilation of data and model results brought together in the report itself by some hundred climate scientists. Their important work (more than 500 pages, but lacking an index) has been largely ignored by the public, while attention has focused on the politically negotiated (5-page) Summary. Those who are skeptical of the IPCC conclusion have viewed the statement about human influence as trivial and meaningless. On the other hand, the media and many policy experts have welcomed its convenient formula, which they regard as scientific proof of a coming climate catastrophe.

In fact, the IPCC statement is in many ways a truism. There certainly must be a human influence on some features of the climate, locally if not globally. The important question is whether the available evidence supports the results of the model calculations. Unless validated, the predictions of future warming based on GCMs cannot be relied on.

What follows is a personal view of the current state of climate science, how it relates to model results, and what might be expected in the future as human activities continue to raise the level of greenhouse gases in the atmosphere. A disclaimer is in order. Any brief treatment of this complex subject by an individual author inevitably selects certain facts as important and rejects other pieces of evidence as inadequate or unproven. Even so, such a treatment has the advantage of providing a consistent story, compared to a committee report that often dissolves in a mire of uncertainties. It also provides a convenient target for debate and thus may lead, if not to progress, then at least to a sharpening in efforts of data collection and theoretical work.

Can the global warming during the early part of the century, from about 1880 to 1940, “be unequivocally related to human-induced changes in the chemical composition of the atmosphere”? [Jones, 1998].

The IPCC arrived at the ambiguous conclusion that “the balance of evidence suggests there is a discernible human influence on global climate,” based on “fingerprints” in the climate record, i.e., an increasing correlation (with time) between observed and calculated global temperature patterns [IPCC, 1996, Figure 8.10, p.433]. However, this positive trend in correlation depended entirely on the arbitrary choice of the time interval 1940-1990, during most of which temperatures were actually decreasing. A different choice of interval could have produced a zero or even a negative trend. Another piece of evidence cited in the IPCC report to support a human influence depended on showing an increasing temperature trend in the middle troposphere of the Southern Hemisphere [IPCC, 1996, Figure 8.7.c., p.428]. Again, this is related entirely to the particular choice of time interval [Michaels and Knappenberger, 1996]; more complete data sets give a contrary result – a greater warming trend in the Northern Hemisphere.

Following 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 is due to natural causes and represents a recovery from the Little Ice Age. Some would assign a substantial portion to greenhouses gases [Wigley, Jones, and Raper, 1997]. Others claim that most of the temperature increase is caused by solar variability [Soon et al., 1996]. If one applies the “fingerprint” criterion used by the IPCC, then it can be seen from their Figure 8.10 [IPCC, 1996, p.433] that the pattern correlation has a negative trend during the major warming between 1900 and 1940, thereby denying the existence of an appreciable human contribution.

Perhaps the strongest argument against an appreciable human contribution comes from the observed cooling between 1940 and 1975 and the lack of warming since 1979 (in the weather balloon and satellite data).

A Look at the Evidence

The subject of climate change must rest on observations of the climate in all of its aspects; with temperature as the most important and easily measured parameter. On the one hand, we are inundated with data, many of which do not add appreciably to the discussion; on the other hand, we lack crucial information about the past that may never be recovered. For example, individual temperature measurements using thermometers date back for only about 300 years; the record for the Northern Hemisphere dates from about 1860; and it is only since 1979 that weather satellites have been able to cover the complete globe, including the 70% of the surface covered by oceans. Yet we have increasing amounts proxy data from tree rings, ocean sediments, ice cores, and other evidence that tells us about climate in the distant past.

Paleoclimate

To gain perspective on the subject of climate change, one needs to look at the past. While the data are not exactly global and not always of the best quality, certain conclusions can be reached. The Earth’s climate has never been steady; it has either warmed or cooled – without any human intervention. The measured variations have often been large and rapid – larger and more rapid than those predicted by climate models for the year 2100. In the last 3000 years, i.e., during recorded human history, temperatures in the North Atlantic have changed by as much as 3