Expert: Courtney Seligman - 9/26/2008

Question
Dear Madam

What would happen if an asteroid hit where the ice caps are melting? Thank you.

Answer
I presume you mean an asteroid large enough that the impact and subsequent explosion would melt and vaporize most of the asteroid, and a much larger part of the surrounding countryside. Such a collision isn't very likely, since even moderately large impacts are a once in several tens or hundreds of millions of years event, and having such an impact take place at a specific location would be a once in many billions of years event, but --

If it happened in the Arctic Ocean, blast effects would extinguish all life within hundreds or thousands of miles, and tsunamis would scour seacoasts and tens of miles of near-seacoast regions throughout the Arctic Ocean and many subarctic regions, but sea level would be essentially unchanged, because sea ice, even when melted, cannot change the height of the oceans (while floating, ice displaces the same volume as it occupies, after melting).

If it happened in Antarctica, blast effects would be the same. Tsunami effects would be reduced, but still be significant, because large amounts of previously land-bound ice would melt and run downhill to the sea. Dumping the meltwater (and a much larger amount of pulverized Earth mixed in with the meltwater) into the Antarctic Ocean would create tsunamis which would scour the southern coasts of Africa, Australia, and/or South America. And, as I suspect you are primarily wondering, sea level would rise all over the Earth. The change in sea level would depend on how big the impact was, and the resulting amount of muddy water dumped into the ocean. Relatively small impacts (with asteroids a few miles in diameter), which would be more likely to occur, would only melt part of the Antarctic ice, and raise sea levels a few feet. Much larger impacts (with asteroids tens of miles in diameter), capable of melting all Antarctic ice, could raise sea level by more than a hundred feet. However, the rise in sea level would be relatively unimportant to the denizens of the Earth, because such an impact would almost certainly destroy all life on Earth. I'm sure you've heard of 'nuclear winter' effects, in which volcanic gases and dust blown into the atmosphere poisons most life-forms, and by reducing the sunlight reaching the surface, kills off most of the others. Huge impacts would create extreme nuclear winters, and plunge the Earth into a deep freeze unlike any it has experienced in its previous history. Fortunately, such impacts have not occurred anywhere in our part of the Solar System in nearly four billion years, and are unlikely to reoccur in time periods which are short, compared to trillions of years.

So, in summary, 'small' impacts, which might occur once every hundred million years or so, could produce devastating effects over a portion of the Earth, and in rare circumstances, raise sea level a few feet. 'Large' impacts, which should not occur in any conceivable period of time, would devastate the entire Earth.

Courtney Seligman
Professor of Astronomy
Long Beach City College

(P.S. It is a minor matter, but as noted on my website, at http://cseligman.com/about.htm I am a Mr., not a Ms.)