Dangerous waves

These waves, which are generated from a major disturbance to the water surface, are known as tsunami. (Most scientists don't like the popular name "tidal wave" because tsunami have nothing to do with the tides. However, tsunami sometimes surge ashore like a huge, fast-moving tide rather than breaking like a classic surfing wave.) 

Tsunami can travel at around 400 mph in deep water. When they reach shallow water they slow down, and that's when the real danger begins. The front of the wave slows first and the effect is like a pile-up on a freeway, with the rear of the wave catching up to the front. The wave increases in height from this bunching effect. The final height of the wave depends on several factors, but the shape of the sea floor has the greatest impact. Estuaries, harbours, cliffs, reefs, and the topography of the continental shelf all play a role. 

For a typical shoreline, the final tsunami height is usually about three times its height in deep water, but in some locations the ratio (known as "run-up factor") reaches 40. In other words, a 1-foot wave in deepwater can amplify to a 40-foot wave at a shoreline that is exceptionally vulnerable to tsunami, as are some parts of Hawaii.

Splashdown

Based on NASA estimates, about once every 2,000 years an asteroid with a diameter of about 100 yards can be expected to hit one of Earth's oceans. Larger asteroids collide with the Earth much less frequently -- a 500-yard rock from space might hit an ocean once every 80,000 years and a 1,000-yard (1 k) asteroid perhaps once every 200,000 years. 

Atomic bombs and ocean impacts

The largest aboveground H-bomb test by the United States was like a firecracker compared to an asteroid impact. That "Bravo" explosion at Bikini Atoll in 1954 was equivalent to fifteen megatons (million tons) of TNT but was only about one-thousandth of the energy of a 500-yard asteroid moving at 50,000 mph.

The Bikini Atoll H-bomb tests enabled scientists to develop computer models of the destructive effects (on shipping) of explosions at the water surface. In the early 1990s these models were applied to asteroid impacts. Initial results suggested that even relatively small impacts could pose a grave tsunami threat over large areas of ocean.

More recent modelling indicates that the tsunami generated by an asteroid impact tend to dissipate, or die out, rapidly (the computer program, developed by Sandia National Laboratories, accurately predicted the consequences of the plummet of Comet Shoemaker-Levy 9 into Jupiter in 1994).

According to this work, a 500-yard-diameter asteroid is predicted to generate a water crater nearly 3 miles in diameter. At a distance of 10 miles from "ground zero" the resulting deepwater tsunami will be about 200 yards high, but by the time the wave has travelled 100 miles it will be reduced to a height of about 14 yards. After 1,000 miles it will have dropped to less than 1 yard in height. Due to the amplification in shallow water, however, this size tsunami could still become a 120-foot wave at a vulnerable shore. 

Extra hazard to coastal areas

People in these vulnerable locations need not lose sleep, however, because the odds of a major asteroid-generated tsunami in any one year are about one in 200,000. On the other hand, as astronomer Duncan Steel has pointed out, asteroid impacts don't run to a timetable like busses.

The estimate of impact tsunami risk is based on the limited search for Near Earth Asteroids carried out so far and assumes that impacts are randomly distributed in time. There is some evidence that impacts may come in clusters (some busses seem to do the same). If this is the case, then it is well worth finding out if we are approaching the next barrage so that coastal areas can be better prepared. 

Climate disruption

The giant plumes from the Jupiter impact of Comet Shoemaker-Levy 9 clearly showed how a comet or asteroid tunnels through the atmosphere and creates a temporary chimney. This draws the impact debris into the upper atmosphere. Scientists are only beginning to understand this effect in the case of an impact into Earth's oceans. 

An ocean impact by a 500-yard-diameter asteroid will vaporise about 20 cubic miles of water. At first sight this appears to be insignificant since it is less than one tenth of the total amount of water that evaporates from the world's oceans every day (assuming 1 inch of rain over 10 percent of the Earth's surface each day).

Scientists caution, however, that an ocean impact would send the water vapour high into the atmosphere, compared with the lower atmosphere, or troposphere, in the case of evaporation. The upper stratosphere is normally extremely dry and the effects of a sudden injection of a large quantity of water vapour are simply unknown. Other effects of concern are greenhouse warming (water vapour is a strong greenhouse gas) and ozone depletion. Unlike evaporation, an ocean impact would send salt (sodium chloride) into the air. The chlorine in the salt may affect upper atmosphere ozone levels in the same way as chlorofluorocarbons. 

The same impact on land would pulverise an equivalent amount of rock (20 cubic miles -- about 1,000 times the volume of the asteroid) and send much of it into the upper atmosphere, where it would circulate around the globe and disrupt agriculture for many months.

A lesson from violent volcanoes

In the case of Tambora, it has been estimated that 10,000 people died directly from the explosion and 80,000 more died in the region from indirect effects, such as starvation. In addition, the ash is thought to have caused the "year without a summer" in 1816, when there were widespread crop failures across North America. The final death toll was probably in the hundreds of thousands. A similar event today might kill millions. 

Because of the chimney effect, an asteroid impact is much more efficient at sending dust into the upper atmosphere than a volcanic explosion, and the climatic disruption is probably much greater with an asteroid impact. Even so, the events of 1815 serve as a clear warning of the global danger from land impacts by asteroids.

With much less dust released into the atmosphere, an ocean impact will have very different, and perhaps less damaging, effects than a land impact. If an asteroid struck thick ice formations, such as Antarctica or the extensive ice sheets of the last Ice Age, the result would likely be similar to a water impact.

It's possible that our species has been saved from extinction several times because a large asteroid hit the ocean or ice rather than the land. Every million years or so it can be expected that a mile-wide asteroid will hit the Earth. A land impact would probably cause severe climatic disruption and regional extinctions. If the global effects of an ocean/ice impact are less severe than one on land, then the impact by a mile-wide asteroid into the ocean might not be as hazardous to life. 

Evidence of ocean impacts

Despite this presumed destruction to coastal areas, there is no evidence of global climate change or regional extinctions around this time, when our early ancestors, Australopithecus, were roaming Africa. We don't know whether they would have been wiped out if the Eltanin asteroid had struck land in South America or Africa, instead of splashing into the ocean. To solve that puzzle, to understand which type of impact most threatens our existence, we need a much better understanding of the consequences of asteroid impacts.