Future Simulations Project a Significant Decrease in Habitability Space of Safe and Just Earth System Boundaries

Human-induced environmental changes are rapidly reshaping the Earth system, with significant implications for human habitability. While existing safe and just Earth System Boundaries (ESBs) have delineated critical planetary thresholds, the future evolution of human habitable conditions remains unclear, especially given the transgression of all eight global ESBs and the underexplored "just" dimension of health and well-being. Here we propose the habitable composite volume (HCV), defined on a three–dimensional environmental phase space P, to quantify the collapsing boundaries of human habitability under the Great Acceleration. During the historical period (1981–2014), global HCV declined by approximately 27%, from 0.66 to 0.48. Under the projections of Shared Socioeconomic Pathways, the high-emission scenario poses the greatest risk, with HCV declining by up to 78% from 2015 to 2100 and collapsing areas encompassing 91.6% of global land, drastically reducing viable living space. Of greatest concern is that, high-risk regions—where collapse coincides with dense populations—expand nearly tenfold (1.7% to 16-18%) under moderate-to-high emissions, disproportionately affecting vulnerable developing regions first before extending to every continent. These findings highlight the escalating risks to human habitability and underscore the urgency of both mitigation and adaptation strategies to address this global crisis.