We received a large number of e-mails in response to my report [VIDEO LINK] and blog post about the National Research Council's latest report on global warming. Many of the e-mails were thoughtful and detailed, and we are always grateful for that sort of feedback, whether you agree with what we reported or not. It is also clear that while this is a technical topic, it touches all our lives, and many people hold strong opinions.
There was one notion voiced in several of the e-mails which was not what I reported and not what the report said. Several people asked why we should be concerned about global warming if the Earth was even warmer 400 years ago. I have not heard any scientist say that. If you look at the pdf of the report's summary and pay particular attention to figure S-1 you can see why the panel concluded that is it far hotter on average now than it was 400 years ago –- and probably hotter than in the past 1,000 years.
Still, this question cuts to the key issue about global warming: How much of any trend that is observed can be accounted for by natural variability in the Earth's temperature? And make no mistake, there is natural variability. 18,000 years ago the Earth was so cold that the arctic ice cap extended over what is now Boston and Seattle with ice 1/2 mile thick. Since then, the Earth has warmed considerably and clearly without human intervention. The big question is how much of the huge spike in temperatures in the past few decades could be natural variation and how much of it is human-produced greenhouse gases. My reporting tells me that a consensus of science says that most of the heat comes from human activity.