As some people have pointed out, news reports in this country often give areas in two main forms: football fields and New Jersey. And along the same lines, many energetic reactions (hurricanes, volcanoes, comets smashing into Jupiter are given in terms of atomic bomb yields. This is often given in terms of the bomb that destroyed Hiroshima, which is, of course, by today's standards, a rather small atomic bomb. But it is a good measurement anyway.
What if instead of measuring the impact of a mass extinction event in terms of atom bombs, we measured something seemingly smaller and closer to home? This was the question that led me to ask: how much firewood would you need to burn, to equal the explosion of a smaller fission bomb? For entertainments sake, ask yourself this question, write down your answer. You might want to ask some other people and see what they answer. How much normal, innocent firewood (you probably have some lying around) would you need to burn to equal an event able to vaporize people and knock down buildings? It might seem, at first, to be an astronomical amount.
But let us look at the chemistry, physics and arithmetic of it. Much of this math will be rough, but we are assuming some perfectly spherical cows. The explosion that destroyed Hiroshima was measured at 15,000 tons of TNT. TNT has a lot of explosive energy, but that energy is not on a scale different than the combustion of any other organic substance. TNT actually has, per gram, the same energy potential as any fat or oil. (It just explodes instead of combusts). Wood, of course, is not made of fat, it is mostly made of carbohydrate, which is in effect half oxidized already, meaning it has roughly half the energy of a fat. (Of course, wood can also include water and ash, but we are leaving those out of our calculations.) So to equal the explosive power of 15,000 tons of TNT, we would need about 30,000 tons of wood.
How much is 30,000 tons of wood? In terms of volume, it could be a wide range, since wood varies widely in density. However, for the easy of computation, lets assume that wood is exactly the density of water. That means that 30,000 tons of wood would equal 30,000 cubic meters. To get the solution to that, you just take the cube root of 30,000, which turns out to be around 30 meters.
This means that the pile of firewood needed to equal an atomic bomb yield that could destroy a major city would need to be around 100 feet on each side. This is much less massive than most people would guess.
Some readers might be wondering, at this point, if I cheated in my figures. And in certain ways, I did: for one thing, the energy of combustion is not just in the wood, but in the air that it reacts with (how much air it would take to totally oxidize this pile is left as an exercise to the reader). And of course, you can't stack wood as a totally solid pile. Even if you had solid blocks of wood, the nature of combustion means that they need surface area exposed. In addition, some firewood might have much less than half the caloric energy of an oil, and some of it might be much less dense than water. And of course, all of it won't burn, even in a hot fire.
Of course, the square-cube relation can overcome many of these objections. If we move that pile from 30 to 40 meters on a side, we have more than doubled its volume, and that is enough to make up for the shortcomings of our spherical cows.
The real reason that this answer might seem counter-intuitive is just that the firewood, while it will release a lot of energy, will release it rather slowly. Even in a very fast fire, it would take minutes or hours instead of microseconds for the energy to be released, and it would mostly be released in the form of thermal disorder and infrared radiation, instead of in the form of a kinetic shock wave and very high intensity radiation. The natural world isn't less energetic than the technological world, just slower.
Further variations upon this theme can be done at the leisure of the reader: figure out firewood for everything from the Tsar Bomba to the smallest tactical nuclear weapon, the Davy Crockett. What size nuclear weapon would you get if you lit the General Sherman redwood tree on fire. (Don't actually try to find out empirically). What fraction of the world's nuclear article could be found in the biological energy of a cubic kilometer of ocean. All of these are, if nothing else, a fun way to play around with math.