(Part of the How to Talk to a Global Warming Skeptic guide) Objection: Why should the U.S. join Kyoto while India and China haven't? Answer: The U.S. puts out more CO2 than any other nation on earth, including China and India, by a large margin. Considering the relative populations (a billion-plus each for China and India versus 300 million in the U.S.), per capita emissions in the U.S. are many times larger. This has been true for the past 100-plus years of CO2 pollution. For the U.S. to refuse to take any steps until India and China do the same is like the fattest man at the table, upon realizing the food is running out, demanding that the hungry people who just sat down cut back just as much as him, at the same time.
(Part of the How to Talk to a Global Warming Skeptic guide) Objection: The Kyoto treaty, even if fully implemented, would only save us about a tenth of a degree of future temperature rise many decades from now. What a waste of effort! You can see for yourself here at the Junk Science website. Answer: There are three big problems with this claim.
(Part of the How to Talk to a Global Warming Skeptic guide) Objection: The earth has had much warmer climates in the past. What's so special about the current climate? Anyway, it seems like a generally warmer world will be better. Answer: I don't know if there is a meaningful way to define an "optimum" average temperature for planet earth. Surely it is better now for all of us than it was 20,000 years ago when so much land was trapped beneath ice sheets. Perhaps any point between the recent climate and the extreme one we may be heading for, with tropical forests inside the arctic circle, is as good as any other. Maybe it's even better with no ice caps anywhere. It doesn't matter. The critical issue is not what the temperature is, or may be, or will be. The critical issue is how fast it is moving.
(Part of the How to Talk to a Global Warming Skeptic guide) Objection: The sun is the source of warmth on earth. Any increase in temperature is likely due to changes in solar radiation. Answer: It's true that the earth is warmed, for all practical purposes, entirely by solar radiation, so if the temperature is going up or down, the sun is a reasonable place to seek the cause.
(Part of the How to Talk to a Global Warming Skeptic guide) Objection: In the geological record, it is clear that CO2 does not trigger climate changes. Why should it be any different now? Answer: Given the fact that human industrialization is unique in the history of planet earth, do we really need historical precedent for CO2-triggered climate change before we accept what we observe today? Surely it is not far-fetched that unprecedented consequences would follow from unprecedented events. But putting this crucial point aside, history does indeed provide some relevant insights and dire warnings.
(Part of the How to Talk to a Global Warming Skeptic guide) Objection: Over the last 600 million years, there hasn't been much correlation between temperatures and CO2 levels. Clearly CO2 is not a climate driver. Answer: While there are poorly understood ancient climates and controversial climate changes in earth's long geological history, there are no clear contradictions to greenhouse theory to be found.
(Part of the How to Talk to a Global Warming Skeptic guide) Objection: In glacial-interglacial cycles, CO2 concentration lags behind temperature by centuries. Clearly, CO2 does not cause temperatures to rise; temperatures cause CO2 to rise. Answer: When viewed coarsely, historical CO2 levels and temperature show a tight correlation. However, a closer examination of the CH4, CO2, and temperature fluctuations recorded in the Antarctic ice core records reveals that, yes, temperature moved first. Nevertheless, it is misleading to say that temperature rose and then, hundreds of years later, CO2 rose. These warming periods lasted for 5,000 to 10,000 years (the cooling periods lasted more like 100,000 years!), so for the majority of that time (90% and more), temperature and CO2 rose together. This remarkably detailed archive of climatological evidence clearly allows for CO2 acting as a cause for rising temperatures, while also revealing it can be an effect of them.
(Part of the How to Talk to a Global Warming Skeptic guide) Objection: Correlation is not proof of causation. There is no proof that CO2 is the cause of current warming. Answer: There is no "proof" in science -- that is a property of mathematics. In science, what matters is the balance of evidence, and theories that can explain that evidence. Where possible, scientists make predictions and design experiments to confirm, modify, or contradict their theories, and must modify these theories as new information comes in.
(Part of the How to Talk to a Global Warming Skeptic guide) Objection: H2O accounts for 95% of the greenhouse effect; CO2 is insignificant. Answer: According to the scientific literature and climate experts, CO2 contributes anywhere from 9% to 30% to the overall greenhouse effect. The 95% number does not appear to come from any scientific source, though it gets tossed around a lot.