Until this year, the US National Weather Service had never once needed to issue a coastal flood advisory for the San Francisco Bay Area in September. Last month alone, it issued two. Streets disappeared under water, beaches were stripped away, and officials up and down the California coast began using words usually reserved for hurricanes rather than tides. Behind this lies El Niño, a periodic warming of the central Pacific Ocean that reshapes weather far beyond its borders.

This year's event has been nicknamed, half-seriously, "Godzilla," and the data support the drama: the US National Oceanic and Atmospheric Administration puts the odds of a "very strong" El Niño above 90%, with a 75% chance it becomes one of the most powerful ever measured. For California, that could mean storms, landslides and flood tides arriving together rather than in sequence. For other regions, from drought-prone parts of southern Africa to flood-exposed South and Southeast Asia, the consequences could be just as severe, disrupting monsoons and harvests alike. The human toll being projected is sobering: scientists estimate that more than 450,000 people worldwide could die from temperature spikes linked to this El Niño by February, with additional deaths expected from the floods, droughts and wildfires it intensifies.

Tens of millions more may face food insecurity as crop yields falter and global food prices climb, compounding pressures already created by a warming planet. California's governor, Gavin Newsom, declared a state of emergency on 21 September, triggering efforts to shore up infrastructure and warn residents ahead of the coming months.