Minority Opinion—Mark Boslough, Mitigation Panel Member
The original draft of the table entitled “Expected Fatalities per Year, Worldwide, from a Variety of Causes” (Table 2.2 in Chapter 2 of this final report) included the World Health Organization (WHO)1 estimate of 150,000 deaths per year from climate change. The steering committee made a decision to remove the climate data, giving as reasons (1) caution about having any debate on climate change distract from the issue at hand and (2) irrelevance of climate change numbers to the near-Earth object (NEO) threat.
The first reason is inappropriate. Data should not be removed from a report to avoid the potential for political controversy.
The second reason is incorrect. Climate change is more relevant than the other causes in the table, for several reasons:
The portion of the threat above the global catastrophe threshold—which in the model we quote2 constitutes about one half of the expected annual death rate—is primarily a climate change threat. Estimates of deaths from a large impact are largely based on our model-derived scientific understanding of climate change. The 91 deaths per year assumes a catastrophe threshold significantly lower than the current best estimate (3 kilometer-diameter asteroid). It implicitly assumes a high-sensitivity climate and/or strong dependence of death rate on climate change.
Asteroids and climate change are the only two threats in the original table that can have abrupt and global consequences, and to which everyone on the planet is exposed, regardless of their lifestyle or personal behavior. They are also both to some extent preventable, and in both cases mitigation requires international agreements and cooperation. The climate change death rate is therefore more appropriate to compare to the asteroid death rate than the other threats are. Climate can and has changed abruptly. Evidence from Greenland ice cores and other
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.