Based on the geology of impact craters and other evidence, earth science suggests that there have been about 60 very large asteroid impacts on the earth over the last 600 million years, or one impact in 10,000,000 years, a probability of (given a more than generous lifetime of 100 years) of 1 in 100,000 that there will be such an event in any given human lifetime. Since such an event would likely kill us all, this means there is a 1 in 100,000 chance of global human extinction happening in our lifetime.
Should this be something on which we should act? Compare this to the chance that our theories about such impacts are wrong. Theories in science undergo major shifts, enough to change such probability calculations significantly, at least once every 500 years. Thus, there is only 1 chance in about 200 that the above risk calculation will stand the test of geologically scaled time.
Once an event is lower in probability than the chance I may be wrong about its possibility, it is not clear whether actions based on that event becoming actual are warranted. We have to seize the days, and the consideration of extremely low probability events should not loosen our grip.
Pragmatism suggests that disease or war are the kinds of disasters that will happen, so decreasing the negative impact of war, disease, and other disasters which occur commonly during our generation on earth are practical norms.
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