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The problem is not how high the temperature may go, but how fast it is changing

(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, …

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‘It’s the sun, stupid’–Very bright, yes, but not getting brighter

(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. Turns out it's more complicated than one might think to detect and measure changes in the amount or type of sunshine reaching the earth. Detectors on the ground are susceptible to …

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‘Historically, CO2 never caused temperature change’–Not so

(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. During the glacial/interglacial cycles, temperatures …

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‘Geological history does not support CO2′s importance’–Just not true

(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. What we do have is an unfortunate lack of comprehensive and well-resolved data. There is always the chance that new data will turn up shortcomings in the models and unforeseen new aspects to climate theory. Scientists …

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‘CO2 doesn’t lead, it lags’–Turns out CO2 rise is both a cause and an effect of warming

(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 …

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There is no proof in science, but there are mountains of evidence

(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. In the case of anthropogenic global warming, there is a theory (first conceived over …

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Water vapor is indeed a powerful greenhouse gas, but there is plenty of room for CO2 to play a role

(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. Please see this paper (PDF), the textbook referenced here, and this article at RealClimate. There is a very important distinction to be made, as you will read if you follow the link to Real Climate, between …

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‘Climate scientists dodge the subject of water vapor’–No, they really don’t

(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. …

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‘We are just recovering from the LIA’–Why should we expect this to happen?

(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? There is no evidence of such a baseline. The climate is influenced by many factors, which change or remain stable in their own ways. The current understanding of …

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‘The CO2 rise is natural’–No skeptical argument has been more definitively disproven

(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 …

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