Given my strong libertarian leanings, it would certainly be ideologically convenient if the evidence for a human contribution to climate change were less strong. Alas, I believe the preponderance of evidence strongly supports the claim that anthropogenic emissions are having an effect on the global climate, and that effect will increase as greenhouse gases accumulate in the atmosphere. While I reject most apocalyptic scenarios as unfounded or unduly speculative, I am convinced that the human contribution to climate change will cause or exacerbate significant problems in at least some parts of the world. For instance, even a relatively modest warming over the coming decades is very likely to have a meaningful effect on the timing and distribution of precipitation and evaporation rates, which will, in turn, have a substantial impact on freshwater supplies. That we do not know with any precision the when, where, and how much does not change the fact that we are quite certain that such changes will occur.
It might be that it … just so happens that natural climate variability has boosted global temperatures and the trends discussed above are occurring coincidentally at the same time the concentrations of carbon dioxide are 30 percent above their highest levels in the past 800,000 years. Correlation does not imply causation. The data cited (and uncited) do not prove beyond a reasonable doubt that man-made climate change is real. However, in my best judgment the preponderance of the evidence suggests that the greenhouse gases produced by humanity are warming the climate and that it could be a significant issue later in this century.
Personally, I find that the most objectionable feature of the conservative attitude is its propensity to reject well-substantiated new knowledge because it dislikes some of the consequences which seem to follow from it—or, to put it bluntly, its obscurantism. I will not deny that scientists as much as others are given to fads and fashions and that we have much reason to be cautious in accepting the conclusions that they draw from their latest theories. But the reasons for our reluctance must be rational and must be kept separate from our regret that the new theories upset our cherished beliefs.
To restate: The existence of man-made warming does not mandate any particular policies. So back to the headline question: If generally rising temperatures, decreasing diurnal temperature differences, melting glacial and sea ice, smaller snow extent, stronger rainstorms, and warming oceans are not enough to persuade you that man-made climate is occurring, what evidence would be?