Climate change caused the catastrophic severity of Cyclone Daniel over Libya in 2023

Cyclone Daniel was the deadliest Mediterranean storm on record and struck Greece and Libya in September 2023. In our study, we aim to disentangle the factors contributing to the severity of the event, with a focus on the influence of anthropogenic climate change. To this end we utilized a process-based, conditional attribution approach and simulated storylines of the event with a convection permitting regional climate model under actual and counterfactual conditions. Specifically, we tested how cyclone Daniel would have unfolded (1) in a 1970s world with 1°C less climate change; (2) without the prevalent Mediterranean sea surface temperature anomaly of +1.3 °C; and (3) with decreased soil moisture in the Balkans assuming no rainfall anomalies had occurred in the months prior to the event. Climate response uncertainties have been approximately accounted for by imposing climate change signals from different GCMs. 

Our simulations show that 1°C of climate change only moderately influenced the cyclone's extreme precipitation during its early phase in Greece. In contrast, during its tropical-like Libyan phase, this level of climate change has amplified the severity of the event by a staggering 30 to 60%. Increased energy availability and convection led to the formation of a rare and destructive Medicane with a warm and rapidly deepening core. Artificially lowering only the sea surface temperatures reduced the meteorological hazards in both phases and underpins the importance of the Mediterranean as an energy and moisture source. Reducing soil moisture over the Balkans alone, although an important source for evapotranspiration during the early phase, did not substantially affect the intensity of the cyclone.

Our results demonstrate that current climate change can already be a game changer for individual extreme events and highlight the power of storylines to analyze the potentially destructive influence of climate change on rare extreme weather events.