(Part of the How to Talk to a Global Warming Skeptic guide) Objection: Climate is an inherently chaotic system, and as such its behavior can not be predicted. Answer: Firstly, let's make sure we define climate: an average of weather patterns over some meaningful time period. We may thus discount the chaotic annual fluctuations of global mean temperature. That's weather, and one or two anomalous years does not represent a climate shift. Quite a few people believe that climate is a chaotic system, and maybe on some large-scale level it is. But it is not chaotic on anything approaching the time scales of which humans need to be mindful.
(Part of the How to Talk to a Global Warming Skeptic guide) Objection: Scientists can't even predict the weather next week, so why should we believe what some climate model tells us about 100 years from now? Answer: Climate and weather are very different things, and the level of predictability is comparably different.
(Part of the How to Talk to a Global Warming Skeptic guide) Objection: Scientists claim that global warming from greenhouse gases is being countered somewhat by global dimming from aerosol pollution. They even claim that aerosol pollution caused the cooling in the mid-century. But GHGs are evenly mixed around the globe, while aerosols are disproportionately concentrated in the Northern Hemisphere. It follows that warming should be greater in the Southern Hemisphere -- but that's the opposite of what is happening. Clearly climate scientists do not know what is really going on.
(Part of the How to Talk to a Global Warming Skeptic guide) Objection: Why should we trust a bunch of contrived computer models that have never had a prediction confirmed? Talk to me in 100 years. Answer: Given the absence of a few duplicate planets and some large time machines, we can't test a 100-year temperature projection. Does that mean the models can't be validated without waiting 100 years? No.
(Part of the How to Talk to a Global Warming Skeptic guide) Objection: Clouds are a large negative feedback that will stop any drastic warming. The climate models don't even take cloud effects into account. Answer: All of the atmospheric global climate models used for the kind of climate projections synthesized by the IPCC take the effects of clouds into account. You can read a discussion about cloud processes and feedbacks in the IPCC TAR.
(Part of the How to Talk to a Global Warming Skeptic guide) Objection: Sure, Oreskes found no one bucking the consensus, but her paper was refuted by Benny Peiser, who did the exact same survey and found very different results. Answer: True, Benny Peiser did attempt a similar study and submitted it as a letter to Science responding to the Oreskes study. But for very good reasons, it was not published. Peiser claimed to find 34 articles in his "reject or doubt the consensus view" category. That's 3 percent of the total, so even taken at face value it doesn't cast much doubt on the consensus. But it is greater than the 0 percent Oreskes found, and serves as ammunition for the "there is no consensus" crowd.
(Part of the How to Talk to a Global Warming Skeptic guide) Objection: More and more, climate models share all the same assumptions -- so of course they all agree! And every year, fewer scientists dare speak out against the findings of the IPCC, thanks to the pressure to conform.
(Part of the How to Talk to a Global Warming Skeptic guide) Objection: All those institutional position statements are fine, but by their very nature they paper over debate and obscure the variety of individual positions. The real debate is in the scientific journals. Answer: This is a fair point. Group position statements are designed to present a united front. The best indicator of what individual scientists think is in the current scientific literature, where new and different is the paramount value and scientists are free to express their own ideas, as long as they're supported by data and logic. What does the literature look like in terms of the climate debate? Sounds like a good topic for research.
(Part of the How to Talk to a Global Warming Skeptic guide) Objection: Climate is complicated and there are lots of competing theories and unsolved mysteries. Until this is all worked out, one can't claim there is consensus on global warming theory. Until there is, we should not take any action. This is similar to the "global warming is a hoax" article, but at least here we can narrow down just what the consensus is about. Answer: Sure there are plenty of unsolved problems and active debates in climate science. But if you look at the research papers coming out these days, the debates are about things like why model predictions of outgoing longwave radiation at the top of the atmosphere in tropical latitudes differ from satellite readings, or how the size of ice crystals in cirrus clouds affect the amount of incoming shortwave reflected back into space, or precisely how much stratospheric cooling can be attributed to ozone depletion rather than an enhanced greenhouse effect. No one in the climate science community is debating whether or not changes in atmospheric CO2 concentrations alter the greenhouse effect, or if the current warming trend is outside of the range of natural variability, or if sea levels have risen over the last century. This is where there is a consensus.