Multilayer soil moisture depletion intensifies drought impacts on global ecosystems.

When soil moisture across all layers of the entire profile is depleted, ecosystems face the highest risk because they lose all vertical buffering capacity against drought. However, these vertically compound soil moisture droughts, in which every soil layer suffers simultaneous water deficits, have been largely ignored because previous monitoring and risk assessments relied on entire-profile mean soil moisture. Here we identify vertically (0-100 cm) compound droughts worldwide during warm seasons of 1981-2020 using multilayer soil moisture products. Their annual duration and ratio have increased across more than half of global lands over the past four decades, with pronounced hotspots in southwestern North America, South America, Africa, mid-latitude Eurasia, southeastern Asia and Australia. Vertically compound droughts cause substantial declines in ecosystem productivity, particularly in forests and croplands, threatening carbon sink stability and food security. By contrast, diagnosing drought solely from entire-profile averages causes an overestimation of vertically compound drought duration by about 128.3% but an underestimation of associated ecosystem losses by about 27.8%. These projections and systematic biases highlight the urgency of developing multilayer monitoring, mitigation and adaptation strategies for vertically compound droughts.