Land droughts severely impact water resources, agriculture, and socio-economic sustainability. Recently, ocean-onto-land droughts (OTLDs), characterized by precipitation-minus-evaporation (PME) deficits originating over the ocean and affecting land areas, have been recognized as significant contributors to land droughts. OTLDs are more extensive and intense than land-only droughts. A critical concern is understanding the influence of anthropogenic climate change on OTLDs' spatio-temporal evolution. While studies show anthropogenic emissions intensify droughts, the impact on OTLDs remains unclear. OTLD occurrence is linked to reduced moisture transport from the ocean, driven by atmospheric thermodynamic and dynamic processes. Global warming is projected to affect both thermodynamic (increased moisture-holding capacity) and dynamic (circulation pattern shifts) processes, impacting moisture transport and, consequently, OTLDs. This study investigates past and future changes in OTLD characteristics and explores the underlying physical mechanisms.

Existing literature highlights the severe impacts of land droughts and the increasing evidence of anthropogenic influence on drought characteristics such as frequency, duration, and intensity. Studies have shown that anthropogenic emissions lead to droughts with faster onset and more widespread areas. However, research on OTLDs' past and future changes under climate change is limited, despite their significant impacts on continental systems. Previous research has identified the role of diminished moisture transport (PME deficits) from the ocean in OTLDs, with atmospheric thermodynamic and dynamic processes as key drivers. The effects of global warming on moisture transport are complex and may involve increased moisture transport due to higher atmospheric moisture-holding capacity but also shifts in circulation patterns that might alter the direction of moisture transport. The impact of anthropogenic climate change on OTLDs, however, remains unclear.

This study uses data from the Coupled Model Intercomparison Project Phase 6 (CMIP6) historical simulations (ALL) and Shared Socio-economic Pathway 5–8.5 projections (SSP585), along with the European Center for Medium Range Weather Forecasts Reanalysis (ERA5). A bias correction algorithm (quantile delta mapping) is applied to CMIP6 data to reduce biases compared to ERA5. OTLDs are identified using a three-dimensional clustering approach based on a 12-month accumulation of PME anomalies, calculating a standardized precipitation-evapotranspiration index (SPMEI). Spatiotemporally contiguous drought clusters (STCDCs) originating over the ocean and spreading onto land are identified as OTLDs. OTLD characteristics (frequency, duration, intensity, areal extent, total magnitude) are analyzed in the past and future. The relative anthropogenic index (RAI) quantifies the signal of anthropogenic climate change in OTLD intensification. Changes in OTLD-related moisture transport are investigated, exploring both thermodynamic and dynamic physical drivers. Self-organizing maps (SOMs) assess past and future OTLD risks. The thermodynamic and dynamic contributions to PME changes are quantified.

During 1961–2020, anthropogenic emissions significantly enhanced OTLDs globally. Positive anomalies in OTLD frequency, duration, areal extent, intensity, and total magnitude were found over a large portion of the global land surface, particularly in six hotspots (western North America, southern South America, Europe and northern Africa, southern Africa, eastern Asia, and Australia). The RAI indicates that the anthropogenic signal is detectable in the global intensification of OTLDs. Under SSP585, OTLDs are projected to become significantly more frequent (+39.68%), persistent (+54.25%), widespread (+448.92%), and severe (+612.78%) globally. The anthropogenic signal is strongly detected in OTLD projections. The analysis of moisture transport reveals that reduced landward moisture transport, driven by subtropical anticyclones in the northern hemisphere and complex circulation patterns in the southern hemisphere, contributes to OTLDs. The reduction in moisture transport is mainly caused by atmospheric thermodynamic responses to human-induced warming. Thermodynamic processes are the dominant drivers in enhancing OTLDs. Risk assessment using SOMs shows a substantial increase in high-risk areas for OTLDs under SSP585, with a large expansion of high-risk areas in the six hotspots. Approximately 50.9% of the global population is expected to experience the highest risk of OTLDs in the future.

The findings demonstrate a robust anthropogenic influence on the intensification of OTLDs, both historically and in future projections under high-emission scenarios. The increased frequency, persistence, and severity of OTLDs pose significant threats to water resources, agriculture, and ecosystems. The dominant role of thermodynamic processes, driven by anthropogenic warming, in reducing moisture transport to land highlights the importance of understanding and mitigating greenhouse gas emissions. The substantial increase in high-risk areas for OTLDs underscores the need for adaptation measures to mitigate the negative consequences. The study's results are crucial for informing policies related to climate change mitigation and adaptation.

This study provides compelling evidence of the significant contribution of anthropogenic climate change to the intensification of OTLDs. The projections under high-emission scenarios highlight a substantial increase in the frequency, duration, extent, and severity of these events. Understanding the underlying thermodynamic and dynamic mechanisms is critical for developing effective adaptation and mitigation strategies. Future research should focus on the interactions between OTLDs and other climate modes (like El Niño-Southern Oscillation and Indian Ocean Dipole) and the role of teleconnections in drought propagation.

The study relies on CMIP6 models, which have inherent limitations and uncertainties. The bias correction method may not perfectly capture all model biases. The focus on PME deficits as the primary driver of OTLDs might neglect other potential contributing factors. Further research is needed to explore the interplay between OTLDs and other climate phenomena and teleconnection effects in drought propagation.