Mar 05 2010

Reaching Scientific Consensus – On Dinosaurs

Controversies in science are fun, and the spectacle of such controversies being worked out by competing groups of scientists is a wonderful way to learn about the process of science. But as science progresses, we hope to occasionally resolve controversies and come to a reasonable consensus.

One such controversy that I have been following is the question over what killed the dinosaurs, and much of other life on earth, 65 million years ago. The two leading contenders were an asteroid impact at the Chicxulub crater near Mexico, and volcanic eruptions at the Deccan trap in India. Now a new review of the literature has resulted in a solid consensus supporting the asteroid theory.

Actually, the consensus is somewhat of an anti-climax in that this consensus has been slowly building for years. In recent years the vast majority of scientists already agreed on the impact theory, with just a handful of holdouts rooting for the volcano alternative. So this latest report is no surprise.

In fact, this is often how such things will play out. It would be premature to try to impose consensus on a controversy when the controversy is still raging and the data is not yet definitive. On the other hand – why bother formalizing a consensus that already exists de facto?

In this case, at least, while there was a solid consensus, there was still a robust minority opinion and new data had been coming in fairly quickly. So it seemed like a good time to step back, take a thorough look at all the evidence, and see where the science really is. In this case, a thorough review of the evidence strongly supports the impact theory.

The biggest piece of evidence is that there is a layer of debris spread around the earth. This layer contains iridium, which is more common in asteroids than the earth’s crust. It also contains things like shocked quartz, which is created only during a sudden violent impact. There are also spherules – melted rock that hardens into spheres as it rains back down onto the earth.

Moreover, this layer varies in thickness around the world, pointing like a bull’s eye to Chicxulub. But most importantly, when you look at those layers that are about half-way around the world from Chixculub, you find nicely organized debris layers with a sharp demarcation of dinosaur fossils. Below the line – dinosaurs. Above the line – no dinosaurs. Get too close to the impact and the layers are jumbled by the impact itself. Get too far away and they are too thin to separate out the fossils nicely. But at the perfect distance, the layering becomes clear.

The consensus also dispenses with some of the objections. The timing of the mass extinction does line up with the impact. And the global effects of the massive volcanoes at the Deccan traps were small and short-lived – not enough to explain the mass extinction.

I wonder if this consensus will have any effect on those few remaining holdouts. I also wonder if it will affect future media reporting (probably not). One of my peeves of science news reporting is that the media will report a science news story by a scientist in the small minority as if it represents the new or consensus opinion. So whenever the volcano advocate published a study, the media declared that the impact theory was being rejected – not quite.

The same is happening with the dino-to-birds theory – this is by far the consensus opinion. But a few holdouts grab headlines occasionally and get the media to declare that the birds from dinosaurs evolution story is in doubt. It’s the journalists’ job to put the minority opinion in context, and they rarely do, which is irritating. The public is left confused by each new headline that seems to contradict the last.

And that may be the biggest benefit of such a consensus – if we can get the word out, it may help to make sense of the science to the public.

11 responses so far