(Part of the How to Talk to a Global Warming Skeptic guide) Objection: Climate scientists never talk about water vapor -- the strongest greenhouse gas -- because it undermines their CO2 theory. Answer: Not a single climate model or climate textbook fails to discuss the role water vapor plays in the greenhouse effect. It is the strongest greenhouse gas, contributing 36% to 66% to the overall effect for vapor alone, 66% to 85% when you include clouds. It is however, not considered a climate "forcing," because the amount of H2O in the air basically varies as a function of temperature.
(Part of the How to Talk to a Global Warming Skeptic guide) Objection: Today's warming is just a recovery from the Little Ice Age. Answer: This argument relies on an implicit assumption that there is a particular climatic baseline to which the earth inexorably returns -- and thus that a period of globally lower temperatures will inevitably be followed by a rise in temperatures. What is the scientific basis for that assumption?
(Part of the How to Talk to a Global Warming Skeptic guide) Objection: It's clear from ice cores and other geological history that CO2 fluctuates naturally. It is bogus to assume today's rise is caused by humans. Answer: We emit billions of tons of CO2 into the air and, lo and behold, there is more CO2 in the air. Surely it is not so difficult to believe that the CO2 rise is our fault. But if simple common sense is not enough, there is more to the case. (It is worth noting that investigation of this issue by the climate science community is a good indication that they are not taking things for granted or making any assumptions -- not even the reasonable ones!)
(Part of the How to Talk to a Global Warming Skeptic guide) Objection: According to the IPCC, 150 billion tonnes of carbon go into the atmosphere from natural processes every year. This is almost 30 times the amount of carbon humans emit. What difference can we make? Answer: It's true that natural fluxes in the carbon cycle are much larger than anthropogenic emissions. But for roughly the last 10,000 years, until the industrial revolution, every gigatonne of carbon going into the atmosphere was balanced by one coming out. What humans have done is alter one side of this cycle. We put approximately 6 gigatonnes of carbon into the air but, unlike nature, we are not taking any out.
(Part of the How to Talk to a Global Warming Skeptic guide) Objection: Climate has always changed. Why are we worried now, and why does it have to be humans' fault? Answer: Yes, climate has varied in the past, for many different reasons, some better understood than others. Present-day climate change is well understood, and different. Noting that something happened before without humans does not demonstrate that humans are not causing it today.
(Part of the How to Talk to a Global Warming Skeptic guide) Objection: Natural variability is the null hypothesis; there must be compelling evidence of an anthropogenic CO2 warming effect before we take it seriously. Answer: The null hypothesis is a statistical test, and might be a reasonable approach if we were looking only for statistical correlation between increasing CO2 and increasing temperature. But we're not -- there are known mechanisms involved whose effects can be predicted and measured. These effects are the result of simple laws of physics, even if their interactions are quite complex.
(Part of the How to Talk to a Global Warming Skeptic guide) Objection: One decent-sized volcanic eruption puts more CO2 in the atmosphere than a decade of human emissions. It's ridiculous to think reducing human CO2 emissions will have any effect. Answer: Not only is this false, it couldn't possibly be true given the CO2 record from any of the dozens of sampling stations around the globe. If it were true that individual volcanic eruptions dominated human emissions and were causing the rise in CO2 concentrations, then these CO2 records would be full of spikes -- one for each eruption. Instead, such records show a smooth and regular trend. (image from Global Warming Art)
(Part of the How to Talk to a Global Warming Skeptic guide) Objection: Global warming is happening on Mars and Pluto as well. Since there are no SUVs on Mars, CO2 can't be causing global warming. Answer: Warming on another planet would be an interesting coincidence, but it would not necessarily be driven by the same causes. The only relevant factor the earth and Mars share is the sun, so if the warming were real and related, that would be the logical place to look. As it happens, the sun is being watched and measured carefully back here on earth, and it is not the primary cause of current climate change.
(Part of the How to Talk to a Global Warming Skeptic guide) Objection: Current warming is just part of a natural cycle. Answer: While it is undoubtedly true that there are natural cycles and variations in global climate, those who insist that current warming is purely natural -- or even mostly natural -- have two challenges. First, they need to identify the mechanism behind this alleged natural cycle. Absent a forcing of some sort, there will be no change in global energy balance. The balance is changing, so natural or otherwise, we need to find this mysterious cause. Second, they need to come up with an explanation for why a 35% increase in the second most important greenhouse gas does not affect the global temperature. Theory predicts temperature will rise given an enhanced greenhouse effect, so how or why is it not happening?