Posted by Daniel Hall on June 20, 2008
First off, however, I’ll acknowledge that the headline tag — “The odds that a potentially devastating space rock will hit Earth this century may be as high as one in 10.” — is overblown.* This has been the subject of the most vigorous objection (see Brad DeLong) so in this sense the critics are right. But I’d argue they’re missing the forest for the trees: the specifics of Easterbrook’s argument may be oversold, but the question he poses in the second half of the headline tag — “So why isn’t NASA trying harder to prevent catastrophe?” — is exactly the right one to ask.
The critics miss two key things: First, the ‘expected value’ calculation for an asteroid impact could show a very large cost indeed, even if the vastly most likely outcome is no cost at all (i.e., no asteroid). And second, we must consider both benefits AND costs… if preventing asteroid impacts is very cheap, it may well be worth doing.
Regarding the first point, let’s do a simple back-of-the-envelope calculation. We’ll assume the value of a statistical life (VSL) is a (very conservative) $1 million. An asteroid impact that killed 100,000 people would thus cost $100 billion. Easterbrook’s tagline seems to imply that in any year there is about a 1-in-a-1000 chance of such an event, which would imply we should be willing to spend $100 million per year for “asteroid defense”. This would be true even though there was a 90% chance we’d reach 100 years in the future and that money would have been a total waste every single year. Meanwhile, an expert in Easterbrook’s article thinks the whole asteroid diversion thing could be done for $400 million — total.
Now you may disagree with the numbers in my example (e.g., you think such an event is far less likely) but the specific example is not the real point. The important intuition is that one large catastrophe can dominate your expected value calculation.
Of course the actual calculation you would want to do is a full expected value calculation — the sum of all possible futures (their probabilities times their costs). This means counting up all the very likely worlds with no asteroids, the very unlikely worlds with asteroids that wouldn’t be so bad, and the one-in-50-million type asteroids that could cause mass extinctions. But just dismissing the problem out of hand because it hasn’t wiped us out yet strikes me as irresponsible.
The second mistake some critics have made is comparing asteroid impacts to other types of disasters without considering whether we have leverage over these disasters and what this leverage costs. This guy, who mercilessly shreds the article, is quite guilty of this:
Easterbrook is panicked by the thought that maybe-once-a-century event like Tunguska could occur over the negligible percentage of Earth’s surface which is covered by dense cities. He writes:
The blast had hundreds of times the force of the Hiroshima bomb and devastated an area of several hundred square miles. Had the explosion occurred above London or Paris, the city would no longer exist.
A comparable destructive energy can be expected from a good-sized hurricane or typhoon, which have a tendency to strike coastal areas, a popular location for cities. … Or, if you don’t like that analogy, you can also get a much larger, and similarly rapid, yield from an 8.0+ earthquake, of which we get about one a year. Localized disasters of this sort happen hundreds or thousands or times more frequently than similarly-powerful asteroid/comet impacts, and yet somehow we don’t characterize them as the deadliest threats ever.
… There have been extinctions of large numbers of species in Earth’s history – we have an idea of when they happen, and how big they are – and some of them may have been precipitated by some kind of catastrophic extraterrestrial impact. How does this threat compare with the expected results of human-caused climate change?
The widely-accepted science on global warming, much like the highly speculative situations Easterbrook is fantasizing about, would have similar effects on the planet – mass extinction, starvation, disease, and massive physical destruction. According to a 2004 study in Nature, mid-range estimates for global warming could cause the extinction of 15-37% of all plant and animal species. The last extinction event which even approaches this magnitude was 33.5 million years ago, which may or may not have had something to do with some kind of extra-terrestrial impact. Sixty-five million years ago we have a mass extinction likely caused by the impact of one (or many fragments of an) asteroid, wiping out perhaps 30% of all species. Before that, we have to go back 200 million years. So, a survey of the last 200 million years tells us that at most we have extinctions from all causes on the order expected from global warming every 60-70 million years.
I have a couple of responses to this:
1. Yes, earthquakes and hurricanes are more likely than asteroid impacts but we don’t have a good way of stopping or diverting them. (Not that we shouldn’t be researching this.) This means we must either pay the (very large) opportunity costs of not inhabiting hurricane- or earthquake-prone areas, or pay the costs of building more resilient infrastructure and rebuilding destroyed infrastructure. (Anyone remember what Hurricane Andrew cost? Maybe we shouldn’t have rebuilt Florida but the fact that we did gives you at least a hint about the opportunity cost.) Paying for asteroid defense, by comparison, looks cheap.
2. Yes, climate change is far, far more likely to be a problem than asteroids but you have to compare the proper counterfactuals. For asteroids we are talking a world with a very tiny risk of asteroid disaster versus a basically identical world ($400 million is nothing) with zero risk of asteroid disaster. For climate change, unfortunately, we are talking about a world perhaps warmer by 6-8 degrees Centigrade versus a world that is perhaps only warmer by 3-6 degrees but where we have literally spent trillions of dollars to do this. There is no free lunch. This doesn’t mean I don’t think some level of climate protection could well be the best lunch we ever buy, but let’s be honest about what we’re spending and what we’re getting.
My ultimate point is that the article has value because it highlights our screwy priorities when it comes to spending money on space. Why does almost all our $17 billion NASA budget go to getting humans in orbit and bases on the moon and Mars? We need a more Earth-centric NASA. It could be doing far more good developing enhanced Earth monitoring systems — satellite data is going to be invaluable to understanding climate change in the next century — and yes, protecting us from space debris. Anyway, I think Easterbrook’s article is thought-provoking and you should read it.
*Although I’d suspect Easterbrook would fall back on the definition of “potentially”.