Wednesday, September 9, 2015

Do Scientists Ever Change Their Minds? A Flashback to Nuclear Winter

Of course we do!

We scientists get accused of thinking that we know all the answers, but among the scientists I know, this is not the case. It is particularly evident among global-warming denialists: they think that the 99 percent of scientists who accept Global Warming science are closed minded and totally unwilling to consider the facts as presented by the oil companies and the think tanks that they sponsor.

But I have a story to tell that only my older readers will remember, a story that shows how scientists can, in fact, change their minds. It is the story of Nuclear Winter.

The concept of Nuclear Winter entered scientific and public consciousness with something of a splash. In a 1983 issue of Science, two days before Christmas, a pair of articles appeared. The first one claimed that a nuclear exchange between the United States and the USSR would put so much smoke and dust into the air that sunlight would be blocked and the surface of the Earth would freeze. In fact, it would freeze so deeply that it would never thaw. This is because ice reflects sunlight into outer space. A planet covered with ice could not warm up because it would reflect all the sunlight that it would need in order to warm up. The authors did not consider the “dirty snow effect,” in which the dust and smoke would settle on some of the ice, absorb sunlight, and melt the ice, to be important. This one paper, written by Turco, Toon, Ackerman, Pollack, and Sagan (yes, Carl Sagan)—referred to by cognoscenti as the TTAPS paper—painted a stark picture of the fate of the Earth that had not been considered previously. The second paper detailed all the extreme consequences that such a deep freeze would have upon all life on Earth. Merry Christmas, everybody!

By 1984 everyone was talking about nuclear winter. I did not think about it very much, until I had sort of a political conversion experience on Easter Day 1984 and became an anti-nuclear activist. And then I told everyone I could—in particular, my laboratory and discussion sections that I taught as a graduate teaching assistant—about it.

This was about the same time that Global Warming science started to get broadly noticed. While research into Global Warming was already a couple of decades old, it was not talked about very much before about 1984. Global Warming and Nuclear Winter were, therefore, both fledgling theories at that time. In case you wonder how they could both be true, it is simple: Global Warming if you don’t have nuclear war, Nuclear Winter if you do.

That’s when the two theories started to diverge. As more scientists investigated Nuclear Winter, they began to get skeptical. The numbers were a little off. Maybe that dirty-snow effect was a little more important than previously thought. Maybe the smoke and dust would settle fast enough that the Earth would not have a chance to cool off very much. The principle of Nuclear Winter was sound, but the new calculations began to show that the cooling after nuclear war would not be as profound as previously estimated. I remember that by 1986 some scientists were talking about Nuclear Autumn.  And not long after that, Nuclear Winter was just not discussed very much by scientists.

Meanwhile, the evidence for Global Warming became clearer and clearer. Not the least of the evidence was that, since the 1980s, nearly every year has been one of the hottest on record. The arctic ice really has been melting and getting thinner. The Northwest Passage really did open up. Glaciers really have retreated. Global Warming science has advanced as Nuclear Winter science has retreated.

Scientists were willing to give up the theory of Nuclear Winter, even those like me who held it with an almost gospel intensity. Scientists are also willing to give up the theory of Global Warming, should the evidence ever warrant it. I say this because scientists have in fact changed their views about a major global issue that has political consequences. But the evidence supporting Global Warming science is so massive, and of so many different kinds (temperatures, ice, tree buds, bird migrations, methane bubbles in Siberia), that this is extremely unlikely to happen. I guess it could. I guess we might even find out that the geocentrists were correct after all. And if it does turn out that Global Warming scientists, or Copernicus, were wrong, scientists will (eventually) admit it.

Oh, I forgot one little thing. The Great God Donald Trump has proclaimed that Global Warming theory was invented by the Chinese as a way of ruining our economy. Well, Trump has spoken. That takes care of that. Evidence? God doesn’t need evidence.

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