Extinction debt. It means the future extinction of a species brought on by events already in the past — a lost habitat, a shifted climate, some earlier wound to the environment. The "debt" part is the unsettling bit: the dying can take its time, unfolding slowly, long after the thing that doomed the species has come and gone. Take nuclear war. Atomic bombs can wipe out whole cities and regions in an instant — the people who live there, and the ecosystem that feeds them, gone together. And crushing as the immediate blast is, it's usually the long game that's worst. The radioactive fallout poisons the environment and seeds genetic mutations and illnesses that linger for generations. With the war between Ukraine and Russia, and Putin forever rattling the nuclear sabre, it's a dread Europe hasn't felt since the Cold War. When the war broke out in earnest, millions of iodine tablets vanished from Norwegian pharmacies almost overnight. The apocalypse — doomsday — is one of humanity's oldest stories, and it goes by many names: Ragnarök in Norse myth, the year 2012 in the Mayan calendar, al-Malhama al-Kubra in Islam. But the most famous of all are surely the Four Horsemen of the Book of Revelation. And they map neatly onto extinction debt, each rider standing for a different flavour of chaos that could bring human civilisation crashing down. The first rider is War — the destruction that lays waste to habitats and ecosystems. The second is Famine, which brings hunger and drives down fertility in plants and animals alike. The third is Plague, spreading disease and wearing down our defences — Covid, for one. And last but not least comes Death, the fourth rider, who can ride in on anything from pollution to global warming. All four are already well on their way — and plenty of scientists reckon the gallop is finished, and the debt is looming ominously close. In the end, extinction debt is a reminder of how tightly all living things are bound together. When one species blinks out, the ripples run right through the ecosystem — and through our own human society too.
