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“Alarmism” is a term that gets bandied about a lot. It is often said that one should not call out “fire” in a crowded building. But it really depends, one might say, on whether the “calling out” is done in such a way as to simultaneously prevent a stampede and prevent anyone getting burned.

This riddle was very much on my mind as I sat down to write my thoughts on Mark Lynas’s book, Six Degrees: Our Future on a Hotter Planet (London: Fourth Estate, 2007). I don’t read much popular science literature, and I doubt I would have read this book if I hadn’t made the mistake of referring to it (in a negative manner) in the comments section of a RealClimate post. I don’t think my error was very grave. What I actually said was that if what I had heard about the book from the press materials were true, then the book was probably alarmist and not worth reading. But I don’t blame the author for asking me to read the book and see for myself. He said that the press (in this case Sunday Times (London)) had misconstrued what he says in the book, and he assured me that is was all based on very careful review of the scientific literature. I was thus both curious and obligated to read the book.

Mark Lynas will no doubt be pleased that I very much like the book. To be sure, it is alarming, but the question of whether it is alarmist is a more difficult one, and I don’t think the answer lies in debating the book. Rather, it lies in looking closely at the underlying science the book builds on. I don’t intend to do that here, but I do think that all climate scientists (particularly those that talk to the public) ought to read this book, and ask themselves a question. I’ll get to that question at the end, after saying a bit more about the book.

Six Degrees, as the title suggests, is comprised of six main chapters (plus an introduction and a conclusion). Each of the main chapters examine what the earth might look like as we raise the planet’s temperature by 1^o, 2^o, etc. degrees Celsius, based on what the scientific literature has to say about it. Laying out the book this way makes for a good logical progression of ideas, and a fair bit of suspense. Very few people, Lynas says, have got “the slightest idea what two, four or six degrees of average warming actually means in reality, and I’m sure he is right.

In Chapter 1, at 1^o, we have predictions of, for example, an annually ice free Arctic ocean. Yes, quite plausible and supported by the literature, and perhaps occurring a little sooner than expected. At 2^o, we have, “so whilst southern China can expect more flooding as the two-degree line is approached, the oceanic time lag means that it may take much longer for the rain-bearing summer monsoon to reach the drought-stricken north.” Yes, certainly plausible based on the studies Lynas cites. At 4^o, we have “with global sea levels half a meter or more above current levels, [the Egyptian city of] Alexandria’s long lifespan will be drawing to a close. Even in today’s climate, a substantial part of the city lies below sea level, and by the latter part of this century a terminal inundation will have begun. … a rise in sea levels of 50 cm would displace 1.5 million people and cause $35 billion of damage.” Alarmist? Hardly. A 50 cm rise in sea level, is well within the conservative IPCC projections, even for temperature rises less than four degrees.

At 5^o and 6^o, the book really does start to sound alarmist, with the analogy to Dante’s Inferno – used to good literary effect throughout the book – coming very much to the fore. At five degrees, we have “an entirely new planet is coming into being – one largely unrecognizable from the Earth we know today. At six degrees, “… the pump is primed … not for flourishing palm trees in Alaska, but for the worst of all earthly outcomes: mass extinction.”

Aha, say the skeptics! It is alarmist after all. But is it? Lynas’s reference to the “entirely different planet” actually refers to the fact that at five degrees, the “remaining ice sheets are eventually eliminated from both poles.” That’s entirely true. And unlike in Gore’s discussion of sea level in Inconvenient Truth Lynas does emphasize the long timescales (thousands of years) in this case. Furthermore, there is published research that raises the likelihood of the significant loss of ice sheets at lower temperatures, and Lynas could have claimed certainty of a disappearing Greenland ice sheet in an earlier chapter. That he doesn’t do that is characteristic of the book: it doesn’t tend to go beyond the published literature. This is what Lynas claims at the outset — “all of the material in the book comes from the peer-reviewed scientific literature” – and I think he does an admirable job.

And that brings us back to the question I promised to raise at the beginning, which is this:

If a reading of the published scientific literature paints such a frightening picture of the future as Six Degrees suggests – even while it honestly represents that literature – then are we being too provocative in the way we write our scientific papers? Or are we being too cautious in the way we talk about the implications of the results?