I don't think it's my imagination, but people today seem less worried about the possibility of all-out nuclear war than they were twenty years ago. Certainly that's true if we look back fifty years, when Americans were building fallout shelters and nuclear drills formed a standard part of the school curriculum.

A valid reason for lessened worry is the collapse and fragmentation of the Soviet Union. Wars may start (we can make a stronger statement: wars will start) but they are unlikely to include every state of the former Soviet Union acting in unison. Another reason we worry less about nuclear war may be that advances in genetics have made another kind of warfare - biological - seem increasingly possible and increasingly dangerous. The worst reason for complacency is the most probable one: we can only worry about something for a while. When disaster fails to happen for a long enough period, we quite illogically tend to assume that it will never happen. This is the type of philosophy that encourages some people not to write a will.

Today I want to take a different tack, and ask, What if? There are still plenty of "nuclear devices" (a nice, sanitized term for uranium and hydrogen bombs) in the world. Suppose we had a nuclear war in which all were used. Let us take the absolute worst but highly unlikely situation, in which every bomb works, and they all explode within a one-day period. For the sake of this analysis, it makes little difference whether the bomb makes it to its intended target. I want to be simple-minded, and examine the energy that would be dumped onto an unprepared world.

There will of course be radioactive fallout, a huge problem, but one that we should have some time to protect against. But will we have any time at all? Could an all-out nuclear war, under the worst circumstances, kill off everyone on Earth in the first spasm of energy release?

To answer that question, we need two things. First, we need to know how much energy a total war would release. Second, we need a way of comparing this with other possible disasters. I am going to assume that there are 25,000 hydrogen bombs stored away in various places around the world. This may be wrong, but it has been used in other detailed studies. The way that we measure the energy-release of these bombs is a historical one, dating back to the end of the Second World War and the atomic bombs dropped on Hiroshima and Nagasaki. We evaluate the power of a bomb in terms of conventional explosive, TNT. Thus a "one megaton" hydrogen bomb (medium-sized, but nowhere near the biggest) has the explosive power of a million tons of TNT. I am going to assume that a one-megaton bomb is typical, so our all-out nuclear war will release the energy of 25,000 megatons, or 25 billion tons, of TNT.

This is not a standard scientific method of measuring energy, but it's easy enough to make the conversion to other forms. So I am going to measure everything using a billion tons of TNT as my working unit. Let's see how nuclear war compares with other natural disasters.

First, what about volcanic eruptions? These come in two kinds: spectacular, when a volcano literally blows its top; and more apparently gentle, involving great lava flows but no gigantic explosion. The most famous eruption of the past couple of hundred years probably was Krakatoa, an island just west of Java. A movie was made about this eruption, with the title "Krakatoa, East of Java." This tells us something about Hollywood's regard for accuracy, but let's not digress. Krakatoa, whose explosion was heard at least 3,000 miles away, was puny by nuclear war standards. It released an estimated 240 million TNT tons of energy, less than one percent of our assumed all-out war.

However, Krakatoa, although famous, was nowhere near the most energetic explosion of which we have knowledge. Tambora, erupting in 1815, released an estimated 20,000 million TNT tons, very close to our worst possible war. The world went on as usual, though 1816 was known as "the year without a summer" because crops failed to ripen, perhaps because of a layer of dust in the stratosphere that reflected solar energy.

What else might dump this much energy into the Earth in one day?

Let us take something very familiar: the tides. Estimating total tidal energy, and converting that to megatons of TNT, we find that every day the tides deliver energy of about 6,000 megatons, or one quarter of a nuclear war. However, the tides, far from being disastrous, are benign and essential to life on Earth.

What else? Earthquakes do tremendous damage, but they do it locally. They are not candidates as world-destroyers.

How about meteorites? They are a contender. We can expect to be hit by a meteorite half a kilometer across and packing the energy of a full-scale nuclear war about once every two million years. It could, of course, be next year, but most of us will take our chances.

What about the Sun? It is, even more than the tides, essential to all life on Earth. A good part of the Sun's radiant energy is reflected before it hits land or water, but what gets through in 24 hours is the equivalent of 15,000 billion tons of TNT. That's 600 nuclear wars, each day and every day. A slight increase in solar output, by one sixth of one percent, would hit Earth with as much extra energy as the worst nuclear war that we can imagine.

Could it happen? The biggest solar flares since we began to measure them have produced only infinitesimal variations in solar energy output. So far as we can tell from the fossil record, in the past the Sun has been a highly stable star for billions of years.

And in the future? Almost every ancient civilization worshipped the Sun, praying for its continued steady light. Maybe we should be doing the same.

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