Global Catastrophes in Perspective: Asteroid Impacts vs Climate Change

When allocating resources to address threats, decision makers are best served by having objective assessments of the relative magnitude of the threats in question. Asteroids greater than about 1 km in diameter are assumed by the planetary impact community to exceed a "global catastrophe threshold". Impacts from smaller objects are expected to cause local or regional destruction, and would be the proximate cause of most associated fatalities. Impacts above the threshold would be expected to alter the climate, killing billions of people and causing a collapse of civilization. In this apocalyptic scenario, only a small fraction of the casualties would be attributable to direct effects of the impact: the blast wave, thermal radiation, debris, ground motion, or tsunami. The vast majority of deaths would come later and be due to indirect causes: starvation, disease, or violence as a consequence of societal disruption related to the impact-induced global climate change. The concept of a catastrophe threshold comes from "nuclear winter" studies, which form the basis for quantitative estimates of the consequences of a large impact. The probability estimates come from astronomical observations and statistical analysis. Much of the impact threat, at its core, is a climate-change threat. Prior to the Spaceguard Survey of Near-Earth Objects (NEOs), the chance of dying from an asteroid impact was estimated to be 1 in 25,000 (Chapman & Morrison, 1994). Most of the large asteroids have now been discovered, and none is on an impact trajectory. Moreover, new data show that mid-sized asteroids (tens to hundreds of meters across) are less abundant than previously thought, by a factor of three. We now estimate that the lifetime odds of being killed by the impact of one of the remaining undiscovered NEOs are about one in 720,000 for individuals with a life expectancy of 80 years (Harris, 2008). One objective way to compare the relative magnitude of the impact threat to that of anthropogenic climate change is to estimate the long-term worldwide fatality rate. For asteroids, the average is about a hundred deaths per year--about half of which are climate-change related. By contrast, the World Health Organization (WHO) has estimated that 150,000 deaths per year are currently attributable to anthropogenic climate change. Both estimates are similarly impacted by uncertainty in our understanding of climate change and statistical attribution of indirect causes. The WHO estimate is a lower bound, because it does not account for the unknown probability of a human-triggered abrupt climate change comparable to the speed or magnitude of the Bolling/Allerod or Younger Dryas boundaries, which are not impact related. Nevertheless, by any objective measure the impact threat is minuscule (by a factor of at least a thousand) compared to the threat from anthropogenic climate change. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-94AL85000.