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‘Natural emissions dwarf human emissions’–But emissions are only one side of the equation

(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 …

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‘Climate is always changing’–That doesn’t mean it isn’t different today

(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. For example, we see in ice core records from Antarctica and Greenland that the world cycled in and out of glacial periods over 120Kyr cycles. That climate …

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‘The null hypothesis says warming is natural’–An inappropriate test, and one that would fail anyway

(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. But putting aside inappropriate …

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‘Volcanoes emit more CO2 than humans’–Not even close …

(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. …

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‘Mars and Pluto are warming too’–No they aren’t — and what if they were?

(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, …

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‘Global warming is part of a natural cycle’–This idea is one short step above appealing to magic

(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 …

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‘Vineland was full of grapes’–Or was it an early advertising campaign?

(Part of the How to Talk to a Global Warming Skeptic guide) Objection: Newfoundland was so warm in the Medieval Warm Period that when the Vikings landed they called it Vineland and brought boatloads of grapes back to Europe. Answer: Once again: you can't draw conclusions about global climate from an anecdote about a single region, or even a few regions. You need detailed analysis of proxy climate indicators from around the world. These proxy reconstructions have shown that the Medieval Warm Period (around the time the Vikings are said to have discovered North America) was not as pronounced or …

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‘The hockey stick is broken’–Well, no … but who’s playing hockey anyway?

(Part of the How to Talk to a Global Warming Skeptic guide) Objection: The Hockey Stick graph -- the foundation of global warming theory -- has been shown to be scientifically invalid, perhaps even a fraud. Answer: The first order of business here is to correct the mischaracterization of this single paleoclimate study as the "foundation" of global warming theory. What's going on today is understood via study of today's data and today's best scientific theories. Reconstructions of past temperatures are about, well, the past. Study of the past can be informative for scientists, but it is not explanatory of …

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Yes, the last ice age started thawing over 20,000 years ago, but that stopped a long time ago

(Part of the How to Talk to a Global Warming Skeptic guide) Objection: Global warming has been going on for the last 20,000 years. Answer: It is true that 20,000 years ago the temperature was some 8 to 10° C colder than it is today. But to draw a line from that point to today and say, "look, 20K years of global warming!" is dubious and arbitrary at best. If you have look at this graph of temperature, starting at a point when we were finishing the climb out of deep glaciation, you can clearly see that rapid warming ceased …

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‘Greenland used to be green’–Don’t judge a book by its cover, much less a land by its name

(Part of the How to Talk to a Global Warming Skeptic guide) Objection: When the Vikings settled it, Greenland was a lovely, hospitable island, not the frozen wasteland it is today. It was not until the Little Ice Age that it got so cold they abandoned it. Answer: First, Greenland is part of a single region. It can not be necessarily taken to represent a global climate shift. See the post on the Medieval Warm Period for a global perspective on this time period. Briefly, the available proxy evidence indicates that global warmth during this period was not particularly pronounced, …

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