284 IMPACT STRUCTURES transmission would drop to 10–20% of normal (a cloudy day) for a few decades It may be, therefore, that the devastating effects of the K–T impact are due to the character of the target rocks at Chicxulub Whatever the case, the temporal association of an extremely large impact crater with a worldwide ejecta layer and a global mass-extinction event is well established The cause–effect relationship is less well established and is the subject of current research The frequency of K–T-sized events on Earth is on the order of one every 100 Ỉ 50 My Smaller, but still significant, impact events occur on shorter timescales Dust loadings from the formation of impact craters as small as 20 km could produce light reductions and temperature disruptions on relatively short time-scales Such impacts occur on Earth with a frequency of a few every million years or so and are not likely to have an effect upon the biosphere The most fragile component of the present environment, however, is human civilization There is little doubt that if it lasts long enough it will suffer severely or may even be destroyed by an impact event The greatest threat (the product of probability and the expected death and destruction) is likely from ocean-wide impactinduced tsunamis, which occur on time-scales of tens of thousands of years For example, model calculations indicate that the impact of even a relatively small body (approximately 200 m in diameter) would result in a wave in the open ocean that would still be 10 m high 1000 km from the point of impact Potential impact events do, in fact, occur on human timescales For example, the Tunguska event on 30 June 1908 was due to the atmospheric explosion of a relatively small,