paleoclimate data show that these spontaneous changes take place much more frequently than do large impacts and on time scales that can exceed human adaptive capacities.3

  • Asteroids and climate change are the only two threats in the original table that include global catastrophe as a possibility. The best estimate of the global catastrophe threshold diameter for an asteroid is 3 km, but according to Alan Harris,4 all NEOs above this threshold, except for long-period comets, have been discovered. The best estimate of the probability of a global catastrophe this century from an asteroid impact is therefore zero. If Earth and its inhabitants are assumed to be much more sensitive to global change, then a low threshold of 1.5 km (a factor of 8 lower in kinetic yield) can be assumed. Harris estimates around 30 undiscovered asteroids larger than 1.5 km. The probability of impact by one of these before the end of the century is 0.0005 percent. However, recent models5,6 suggest a 2 percent probability of global catastrophe from anthropogenic climate change this century, assuming realistic greenhouse gas emissions scenarios and a threshold temperature change or sensitivity of 8°C. If the threshold sensitivity is 4°C, the probability of global catastrophe exceeds 20 percent. With sensitive assumptions, it is therefore 40,000 times more probable that Earth will be faced with an anthropogenic climate change catastrophe than with an asteroid catastrophe. With best assumptions it is infinitely more probable.

The WHO climate change estimate of 150,000 deaths per year is a lower bound, because of its conservative assumptions that do not include increasing temperatures since 2000. It also does not consider the probability of global catastrophe from human-triggered abrupt climate change comparable to the speed or magnitude of the Bölling/Allerød or Younger Dryas boundaries, which are not impact related.7 The Harris (2009) asteroid estimate of 91 deaths per year is an upper bound, because it assumes a low catastrophe threshold. The inclusion of these figures for intercomparison is the only way to provide policy makers with an objective basis for the prioritization and allocation of resources that is commensurate with the relative threat from various causes.


National Research Council, Abrupt Climate Change: Inevitable Surprises, National Academy Press, Washington D.C., 2002, p. 230.


A. Harris, Space Science Institute, personal communication, 2009.


P. Huybers, Compensation between model feedbacks and curtailment of climate sensitivity, American Geophysical Union 2009 Fall Meeting.


A. Sokolov, Relative contributions of uncertainty in anthropogenic emissions and climate system response to the uncertainty of projected 21st century climate, American Geophysical Union 2009 Fall Meeting.


M. Boslough and A. Harris, Global catastrophes in perspective: Asteroid impacts vs. climate change, American Geophysical Union 2008 Fall Meeting.

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