Climate change: time for the sceptics to put up or shut up
Say I were to ask you to prove that the dinosaurs were wiped out when an asteroid collided with the Earth 66m years ago, in what is now snappily called the Cretaceous-Paleogene extinction event.
If you were as weirdly obsessed by these catastrophes as I am, you would maybe start by citing the worldwide layer of sediment known as K-Pg boundary, which was first discovered near Gubbio, in Italy, and is thought to be the fallout from a massive explosion. You would mention the soot that is associated with this layer, the site of a huge impact in the Yucatán region of Mexico 66m years ago and, finally, you'd ask what else could have caused the dinosaurs to die out more or less overnight. A sceptic might respond that this is all supposition, evidence tenuously linked to fit a very recent theory: none of it constitutes proof and no one can ever know why the dinosaurs vanished to allow the rise of mammals and the eventual evolution of man.
So you would quote more evidence, such as the presence in the K-Pg layer of iridium, an element rare on Earth but not in asteroids, as well as the altered state of quartz, which can only be made under extremely high pressure, such as is caused by a huge impact of a 10km asteroid. You would mention the long darkness when only ferns grew and the fact that the seas were emptied of all but the most tenacious species.
Ah, but this is still all very hypothetical, the sceptic would say, at which point you might give up and tell him, yes, a spacecraft might have visited Earth and exterminated 75% of the world's species, but you're going with the best available evidence. The sceptic would walk away, satisfied that he had achieved a draw, not from the merit of his argument, but simply because he had not let you convince him.
http://www.theguardian.com/commentisfree/2014/feb/16/climate-change-deniers-put-up-or-shut-up
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