"From Disaster to Catastrophe -- What's Obama's Endgame In the Gulf?"  Fox News headline, July 15. 

The headline suggests that there is a difference between a disaster and a catastrophe, and that catastrophes are worse. I suppose most people would agree, but it's interesting to delve a little deeper into what we mean by a catastrophe. The term is derived from Greek words having to do with 'growing down' which makes sense. In biological terms, a catastrophe might be defined as a natural or human impact on a population of organisms that exceeds one or more of the physical or chemical limits within which that population can continue to exist. In a word, extinction.

So a catastrophe leading to extinction is probably as bad as it gets. By far the most spectacular catastrophes altering the course of life on the Earth are the result of extraterrestrial  impacts.  We can use such events as natural experiments that reveal how robust life is when challenged by extreme excursions in the environment.

At the most general level, catastrophes involve a positive input of energy or mass to the local or global environment that kills large numbers of organisms in a given population. A couple of recent examples are the heat energy of global warming that is becoming catastrophic for Mexican lizard populations, and volcanic ash burying lichens in eastern Iceland.

How robust is life to a major catastrophic excursion? To answer this question, we need two logarithmic scales, one for energy and one for  mass. We also need a catastrophe unit.  For my purposes here, a  convenient unit is the extraterrestrial impactor that struck the Earth 65 million years ago, bringing to a close the Cretaceous period and ushering in the Tertiary. This impactor is referred to as the K-T object, taken from the Cretaceous-Tertiary boundary layer in which extraterrestrial iridium was discovered thirty years ago by Walter Alvarez and his research associates.

The impactor was most likely a stony asteroid-sized object around 10 km in diameter. It arrived with a velocity somewhere between 12 and 18 km per second, delivering l0^15 kg of mass and a total kinetic energy equal to l0^20 kJ.

This amount of energy is not difficult to comprehend in ordinary terms. It is approximately equal to the energy reaching the Earth as sunlight in one week. The problem arises from the rate and intensity at which the energy and mass were delivered. Instead of a week, the energy arrived in a few seconds, and instead of a global surface area, the energy and mass were delivered to an area of about a hundred square kilometers.  This was a true catastrophe, in which well over half the genera of marine fauna disappeared from the fossil record, not to mention large land-dwelling species like the dinosaurs.

I propose two logarithmic scales based on the K-T event. One will be Kate, from the energy of delivered by the event, and the other is Katm, to designate the mass delivered by the event.

Now we can make some comparisons. The first is the moon-forming event early in the Earth's history, which was equivalent to a gigaKate, approximately a billion times the energy delivered by the K-T impactor. If there had been anything alive at the time, it would have ended up as vaporized atoms.  At the other end of the energy scale, in the micro- and nanoKate ranges, there have been much smaller energy excursions in the historical record.  For instance, in  l908,  an enormous explosion occurred in the sky over the Tunguska river basin in  Siberia. The blast was heard hundreds of miles away, and trees were  leveled in an area of several thousand square kilometers.  This  event most likely was caused by a stony meteoroid or perhaps a comet nucleus that burst at an altitude of six and ten kilometers,  so that  only  an  energy pulse reached ground. The Tunguska event released 10^13 kJ, equivalent to  a microKate. By comparison, a large thermonuclear explosion delivers about a tenth as much energy.

What about mass input? Using Katm unit, the mass injected into the atmosphere by a large catastrophic volcanic eruption such as Krakatoa is approximately a milliKatm  (10^12 kg)  a thousandth the mass of the K-T object.

Now we can answer the question concerning the robustness of life on the Earth. Although a one Kat catastrophe, the K-T event, led to extinctions of many species, some forms of life survived and others even prospered. We humans probably would not be here if the ruling dinosaurs had not been wiped out. My guess is that extraterrestrial impacts between 10 and 100 Kats might send us back to the microbial world of 3.5 billion years ago, and 1000 Kats would be more than catastrophic. Perhaps annihilation is the right word?

Getting back to the oil gusher in the Gulf, where does it rank on the catastrophe scale? The oil spilled by the BP well so far is approximately 10^9 kg, equivalent to a microKatm. I'll let readers decide for themselves whether they agree with Fox News that this qualifies as a catastrophe.

You might be wondering what inspired this exercise in perspective. Well, it started with a paper by Gregory Ryskin that was published several years ago in Geology, which calculated how much methane could be released from the ocean floor. Ryskin speculated that a sufficiently large burp of methane could have contributed to global extinction events in the past. The speculation was picked up by Terence Aym and posted lasted week as a somewhat hyperbolic article in the Helium website: "Doomsday: How BP Gulf disaster may have triggered a 'world-killing' event". And then it went viral. If you have a few free minutes, it's fun to follow the thread and see how the internet responded.