We have some influence over how long we can delay human extinction. Cosmology dictates the upper limit but leaves a large field of play. At its lower limit, humanity could be extinguished as soon as this century by succumbing to near-term extinction risks: nuclear detonations, asteroid or comet impacts, or volcanic eruptions could generate enough atmospheric debris to terminate food production; a nearby supernova or gamma ray burst could sterilize Earth with deadly radiation; greenhouse gas emissions could trigger a positive feedback loop, causing a radical change in climate; a genetically engineered microbe could be unleashed, causing a global plague; or a high energy physics experiment could go awry, creating a “true vacuum” or strangelets that destroy the planet (Bostrom, 2002; Bostrom & Cirkovic, 2007; Leslie, 1996; Posner, 2004; Rees, 2003).
Farther out in time are risks from technologies that remain theoretical but might be developed in the next century or centuries. For instance, self-replicating nanotechnologies could destroy the ecosystem; and cognitive enhancements or recursively self-improving computers could exceed normal human ingenuity to create uniquely powerful weapons (Bostrom, 2002; Bostrom & Cirkovic, 2007; Ikle, 2006; Joy, 2000; Leslie, 1996; Posner, 2004; Rees, 2003).
Farthest out in time are astronomical risks. In one billion years, the sun will begin its red giant stage, increasing terrestrial temperatures above 1,000 degrees, boiling off our atmosphere, and eventually forming a planetary nebula, making Earth inhospitable to life (Sackmann, Boothroyd, & Kraemer, 1993; Ward & Brownlee, 2002). If we colonize other solar systems, we could survive longer than our sun, perhaps another 100 trillion years, when all stars begin burning out (Adams & Laughlin, 1997). We might survive even longer if we exploit nonstellar energy sources. But it is hard to imagine how humanity will survive beyond the decay of nuclear matter expected in 1032 to 1041 years (Adams & Laughlin, 1997). Physics seems to support Kafka’s remark that “[t]here is infinite hope, but not for us.”