The Slow Emergency

What the index measures

This is not rainfall. It is a water balance: precipitation set against how much the atmosphere pulls back out through evaporation, standardised so that zero is each region's own long-run normal. Negative means drier than normal, and the eight regions can be read on one scale despite entirely different climates.

• SPEI: the Standardised Precipitation-Evapotranspiration Index, from CSIC's SPEIbase. It captures the supply of water and the thirst of a warming atmosphere together.
• Below −1: drought conditions. Below −2 is roughly a one-in-forty dry month; the deepest reading in this record is −3.1.
• The same yardstick: because the index is standardised, a −2 in the Amazon and a −2 in the Sahel are equally unusual for each place.

Why drought is counted in months

Every reading here integrates the previous twelve months of water balance, because drought is not a day, it is a persistence. A single dry month is weather. A year of them empties reservoirs, fails a second harvest and cracks the ground beneath buildings.

That is why this series has no landfall and no epicentre. A drought is detected when the index stays below threshold for months on end, and it is the duration, not the depth alone, that turns a dry spell into an emergency. The longest event in this record ran fifteen months.

Where the deficit becomes loss

Drought reaches the balance sheet slowly and through many doors at once. It is the compound peril: the same missing rain hits crops, water supply, power and the built environment together, over the same long season.

• Crops and water: failed yields and depleted reservoirs drive the direct agricultural and municipal loss, and the price shocks that follow.
• Power and rivers: low water cuts hydropower and halts river freight, turning a farming problem into an industrial one.
• Subsidence and famine: clay soils shrink and crack foundations in insured markets; where resilience is thin, the same deficit becomes famine.