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‘The Medieval Warm Period was just as warm as today’–Repeating this point does not make it true

(Part of the How to Talk to a Global Warming Skeptic guide) Objection: It was just as warm in the Medieval Warm Period (MWP) as it is today. In fact, Greenland was green and they were growing grapes in England! Answer: There is no good evidence that the MWP was a globally warm period comparable to today. Regionally, there may have been places that exhibited notable warmth -- Europe, for example -- but all global proxy reconstructions agree it is warmer now, and the temperature is rising faster now, than at any time in the last one or even two …

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‘It was warmer during the Holocene Climatic Optimum’–This period was not global and not like today

(Part of the How to Talk to a Global Warming Skeptic guide) Objection: It was warmer during the Holocene Climatic Optimum than it is today -- without any human influence.   Answer: Though some temperatures during that period were in the same range as today, they were confined to the northern hemisphere and the summer months. What's more, the cause is understood (orbital forcing similar to what controlled the Ice Ages), just as today's cause is understood (CO2 emissions), and these causes are very different. NOAA has a page on this that contains the following quote: In summary, the mid-Holocene, …

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‘Hansen has been wrong before’–Maybe, but not about the climate!

(Part of the How to Talk to a Global Warming Skeptic guide) Objection: In 1988, Hansen predicted dire warming over the next decade -- and he was off by 300%. Why in the world should we listen to the same doom and gloom from him today? Answer: While in some instances it is ignorant repetition of misinformation, at its source this story is a plain lie. In 1988, James Hansen testified before the U.S. Senate on the danger of anthropogenic global warming. During that testimony he presented a graph -- part of a paper published soon after. This graph had …

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‘They predicted global cooling in the 70s’–But that didn’t even remotely resemble today’s consensus

(Part of the How to Talk to a Global Warming Skeptic guide) Objection: The alarmists were predicting the onset of an ice age in the '70s. Now it's too much warming! Why should we believe them this time? Answer: It is true that there were some predictions of an "imminent ice age" in the 1970s, but a cursory comparison of those warnings and today's reveals a huge difference. Today, you have a widespread scientific consensus, supported by national academies and all the major scientific institutions, solidly behind the warning that the temperature is rising, anthropogenic CO2 is the primary cause, …

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Understanding what is happening right under our noses does not require paleoclimate perfection

(Part of the How to Talk to a Global Warming Skeptic guide) Objection: Climate science can't even fully explain why the climate did what it did in the past. How can they claim to know what is going on today? Answer: There are two requirements for understanding what happened at a particular point of climate change in geological history. One is an internally consistent theory based on physical principles; the other is sufficient data to determine the physical properties involved. It is extremely hard, in some cases impossible, to gather sufficient data about every aspect of the climate system for …

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‘Chaotic systems are not predictable’–Sure, but who says climate is chaotic?

(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 …

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‘We can’t even predict the weather next week’–But weather is not climate

(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. Climate is defined as weather averaged over a period of time -- generally around 30 years. This averaging smooths out the random and unpredictable behaviour of weather. Think of it as the difference between trying to predict the height of the fifth wave from now versus predicting …

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‘Aerosols should mean more warming in the south’–More North. Hemisphere warming is well-understood

(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. Answer: Aerosol cooling does indeed affect the Northern Hemisphere, …

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‘Climate models are unproven’–Actually, GCM’s have many confirmed successes under their belts

(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. The climate is an extremely complex system. Our observations of it are by no means complete -- even with regard to what's going on today. This is a shortcoming we …

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‘Models don’t account for clouds’–Clouds are complex and uncertain, but unlikely to stop warming

(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. It is true, however, that clouds are one of the largest sources of uncertainty in the GCMs. They are complicated to model because they have …

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