The end of life on Earth as we know it

This past Monday my friend and colleague Paul Guinnessy and I attended a press viewing of Interstellar two days ahead of the movie’s US release. Paul’s review appears today elsewhere on Physics Today‘s website. Here, I’d like to discuss the plot element that motivates the movie: the impending doom of humanity brought about by blights that destroy the world’s food crops one by one.

As end-of-the-world scenarios go, agricultural collapse is not entirely implausible. In the 1840s the microorganism Phytophthora infestans devastated Europe’s potato harvests. More than one million people died of starvation; 90% of the victims were in Ireland. In 1943 the fungus Helminthosporium oryzae precipitated a famine in the Indian state of Bengal that killed 3 million people. And the amount of the world’s rice harvest lost annually to rice blast fungus, which arrived in the US eight years ago, could feed 60 million people.

Climate change compounds the threat to agriculture. In a paper published last year in Nature Climate Change, Sarah Gurr of Exeter University in the UK and her colleagues have estimated that since 1960 the geographical range of crop pests has been expanding toward Earth’s poles at a rate of 2.7 ± 0.8 km/y. She concluded: “The observed positive latitudinal trends in many taxa support the hypothesis of global warming-driven pest movement.”

Plausibility aside, there’s a narrative flaw in Interstellar‘s eschatological scenario. The movie’s screenwriters ask us to accept that in their fictionalized version of the near future, humankind will possess the technological prowess to send human astronauts to a wormhole near Saturn yet be unable to defeat a few fungi. Real-world biologists have already identified a gene in a variety of wild potato that protects against most blights. The gene can be incorporated into the genome of cultivated varieties using current technology.

Interstellar‘s audience is also asked to believe that the killer fungi will somehow consume all the oxygen in Earth’s atmosphere, suffocating humanity in the process. But the pests need oxygen-emitting plants to thrive, even as they kill them.

Interestingly—and alarmingly—humanity is indeed destined to suffocate in the future if we don’t leave our home planet, albeit not as imminently as is the case in Interstellar. Earth’s atmospheric nitrogen and carbon dioxide come from outgassing volcanoes, which are ultimately powered by the heat released by radioactive isotopes in Earth’s mantle. As those isotopes decay, their ability to supply heat diminishes. Eventually, volcanic and tectonic processes will cease and no new CO₂ will enter the atmosphere. Plants will grab what CO₂ they can from the atmosphere and die. And when they die, they will stop producing oxygen.

That scenario will take billions of years to play out. On a similar time scale, the Sun will turn into a red giant and engulf Earth and the other planets in vast a cloud of plasma. Humans might not have to leave Earth as soon as they do in Interstellar, but leave we must.