A pin in the map of global climate disaster:

Mexico City, a sprawling metropolis of nearly 22 million people and one of the world’s biggest cities, is facing a severe water crisis as a tangle of problems — including geography, chaotic urban development and leaky infrastructure — are compounded by the impacts of climate change.

Years of abnormally low rainfall, longer dry periods and high temperatures have added stress to a water system already straining to cope with increased demand. Authorities have been forced to introduce significant restrictions on the water pumped from reservoirs.

“Several neighborhoods have suffered from a lack of water for weeks, and there are still four months left for the rains to start,” said Christian Domínguez Sarmiento, an atmospheric scientist at the National Autonomous University of Mexico (UNAM).

Politicians are downplaying any sense of crisis, but some experts say the situation has now reached such critical levels that Mexico City could be barreling towards “day zero” in a matter of months — where the taps run dry for huge swaths of the city.

The Aztecs who built Tenochtitlan in a lake bed worked with the water, but the Spanish who invaded it stamped the water out. Bad move.

Their decision paved the way for many of Mexico City’s modern problems. Wetlands and rivers have been replaced with concrete and asphalt. In the rainy season, it floods. In the dry season, it’s parched.

Around 60% of Mexico City’s water comes from its underground aquifer, but this has been so over-extracted that the city is sinking at a frightening rate — around 20 inches a year, according to recent research. And the aquifer is not being replenished anywhere near fast enough. The rainwater rolls off the city’s hard, impermeable surfaces, rather than sinking into the ground.

The rest of the city’s water is pumped vast distances uphill from sources outside the city, in an incredibly inefficient process, during which around 40% of the water is lost through leaks.

Tick tick tick.