T5 – Exploring hydrological and agricultural risks in Europe under tipping scenarios

Short description

Climate tipping points are critical thresholds at which small perturbations can trigger large, potentially abrupt and persistent shifts in the climate system [1]. Growing evidence suggests that some tipping elements may become more likely as global warming approaches ~2 °C above pre-industrial levels [2]. Yet the hydrological consequences of such tipping scenarios remain largely unexplored, despite their importance for climate adaptation and policy support. A particularly policy-relevant tipping element is the Atlantic Meridional Overturning Circulation (AMOC), which has a crucial role in regulating the global climate, and whose weakening or collapse is expected under anthropogenic climate change. AMOC changes could substantially alter European temperature and precipitation patterns, with effects on droughts and water availability, as shown in recent analyses of coupled global climate models (GCMs) simulations [3].

So far, only one study has analyzed how the water balance and drought conditions may change over Europe under weakened AMOC regimes [3]. However, they used only GCM simulations with imposed freshwater perturbations, as done in most AMOC-related climate studies, relying on so-called freshwater “hosing” experiments, i.e., imposing an external flux over the North Atlantic. A recent climate study found a more “spontaneous” AMOC transition, without prescribed perturbations, indicating the possibility of AMOC collapse due to internal variability [4]. Exploring the hydro-climatic changes under such scenarios, both “hosed” and “spontaneous” AMOC transitions, is therefore needed to better understand the implications for hydrological and agricultural risks over Europe. Moreover, existing studies have mainly focused on meteorological drought indicators (based on precipitation and temperature), leaving hydrological drought (river flows, soil moisture) and flood characteristics less explored, especially across multiple AMOC experiment designs.

This thesis will analyze European hydro-climatic changes under AMOC weakening and collapse scenarios, using four paired climate-simulation sets, comparing weakened-AMOC states against stable-AMOC control runs. The scenarios available include simulations from EC-Earth3.3 (pre-industrial, present-day, and future SSP5-8.5 scenario, with freshwater-induced AMOC weakening) and the NASA GISS model (spontaneous AMOC collapse in an extended SSP2-4.5 scenario). The analysis will assess changes in drought characteristics (e.g., intensity, duration, timing), water-balance regimes (e.g., water- or energy-limited), and extreme precipitation (e.g., intensity, frequency), and identify regional hotspots of pronounced hydrological changes. For one or few selected hotspot basins (e.g., Danube, Po, Rhine), the thesis will then evaluate implications for hydrological drought and flood risk using OS LISFLOOD simulations driven by different AMOC scenarios (using CEMS-EFAS setting and historical simulations as baseline). Finally, the insights will be used to discuss impacts and policy implications for water-related sectors, e.g., agriculture, hydropower, navigation, flood protection.

PROPOSED ACTIVITIES

Literature Review

  • Conduct a systematic literature review covering tipping points, AMOC, and water-related risks under climate change scenarios
  • Review the methods to be used, including standard drought indices (e.g., SPI, SPEI, LFI), large-scale hydrological modelling (OS LISFLOOD), potential evapotranspiration (PET) formulas, and water-balance analysis framework (e.g., Budyko diagram)
  • Analyze in detail the most recent studies on hydrological and agricultural risks over Europe, to examine methodologies to be used and understand implications of the work

Data Processing and Analysis

  • Compute several drought indices over Europe, including the Potential Precipitation Deficit (PPD), Standardized Precipitation Index (SPI), Standardized Precipitation-Evapotranspiration Index (SPEI) at different time scales
  • Statistical analysis of the changes in the distributions of hydro-meteorological variables (precipitation, PET) and in the characteristics of extreme events
  • Analysis of spatio-temporal characteristics of droughts and extreme precipitation events, including seasonality statistics

Hydrological Modelling and Drought/Flood Analysis

  • Identification of regional hotspots (river catchments) of water-balance changes
  • Analysis of hydrological simulations from OS LISFLOOD in identified hotspot regions
  • Computation of hydrological drought indices including Low Flow Index (LFI) and flood characteristics (e.g., flood return levels, duration, and seasonality)

References

  1. Lenton et al. (2008). “Tipping elements in the Earth’s climate system”, Proceedings of the National Academy of Sciences, 105 (6) 1786-1793, https://doi.org/10.1073/pnas.0705414105
  2. Armstrong McKay et al. (2022). “Exceeding 1.5 °C global warming could trigger multiple climate tipping points”, Science, 377,eabn7950, https://doi.org/10.1126/science.abn7950
  3. van Westen et al. (2025). “Changing European hydroclimate under a collapsed AMOC in the Community Earth System Model”, Hydrology and Earth System Sciences, 29(22), 6607–6630, https://doi.org/10.5194/hess-29-6607-2025
  4. Romanou et al. (2023), “Stochastic Bifurcation of the North Atlantic Circulation under a Midrange Future Climate Scenario with the NASA-GISS ModelE”, Journal of Climate, 36, 6141–6161, https://doi.org/10.1175/JCLI-D-22-0536.1

Number of students

2, preferably starting in September 2026

Requisites

The student should be comfortable with climate change analysis (impact assessment), statistical analysis (e.g., extreme values), and data handling. Proficient coding skills (Python or R) are mandatory.