Analysis of Observed and Projected Climate Change and Trends in Extreme Rainfall and Temperature in the Tana and North Gojjam Sub-basins, Upper Blue Nile Basin, Northwestern Ethiopia

This study provides new insights into climate extremes across elevation gradients in Ethiopia's agriculturally critical Tana and North Gojjam sub-basins. Using bias-corrected CMIP6 models, we analyze observed trends (1990–2014) and future projections (2015–2099) to address critical gaps in regional climate risk assessment. Our approach significantly improves upon previous basin-wide studies by revealing distinct elevation-dependent climate responses that demand localized adaptation strategies.

The research reveals three major findings with important implications. First, we identify plain elevation contrasts in climate extremes: lowland areas near Lake Tana face projected temperature rises up to 6.6 °C by 2100 (SSP5-8.5), exceeding highland warming by 1.5–2.6 °C. Conversely, highland stations show 10–34 % increases in heavy rainfall (R99p), contrasting with drying trends in southern lowlands. Second, our bias-correction framework reduces uncertainties in CMIP6 rainfall projections by 25–40 %, demonstrating particular skill in capturing orographic precipitation patterns through multi-model ensemble improvement. Third, we document an emerging Lake Tana warming hotspot with minimum temperatures rising up to 7.1 °C by 2100 - a previously unreported microclimate anomaly threatening lakeside agriculture and aquatic ecosystems.

These results advance climate risk assessment methods for tropical highlands while providing actionable data for policymakers. The identification of compounding risks in northern sub-basins (increasing hot days and intense rainfall) highlights the need for integrated flood-heat stress management. Our findings emphasize that effective adaptation in the Upper Blue Nile Basin requires strategies tailored to specific agro-ecological zones, particularly for protecting vulnerable highland food systems and lowland water resources.