Current and future drivers of humid heatwaves over Europe

Background and Rationale

Europe is the fastest warming continent, with temperatures rising at around twice the global average rate. As an example, southern Europe was subjected to very strong heat stress for prolonged periods of summer 2023 (Fig. 1). There are already clear links between heat extremes and mortality across all of Europe and these impacts will increase under climate change.

Humidity is a key component of human heat stress because it reduces the body’s ability to expel heat through sweating. In the current climate including information about humidity does not explain heat-related mortality better than dry-bulb temperature alone (Armstrong et al. 2019). However, there is concern that elevated humidity during European heatwaves will become much more important in a future, warmer climate.

Fig. 1: Number of days that experienced ‘very strong heat stress’ (Universal Thermal Climate Index between 38 and 46°C) during June, July, August and September 2023 [taken from ESOTC (2023)].
The main atmospheric driver of European heatwaves is a blocking anticyclone. Whilst this is fairly well understood, the role of humidity in heatwaves not. As an example, Fig. 2 shows the daily maximum wet bulb temperature (a combined measure of temperature and humidity) over London during May to September in 2022 and 2023. Whilst the summer 2022 UK heatwaves were record breaking, including the first recorded temperatures above 40°C, humidity appears to be of limited importance. In contrast, the September 2023 heatwave was much more humid.

There is very limited understanding of the synoptic and mesoscale controls on humidity in European heatwaves. It is possible that situations such as ‘atmospheric rivers’, which transport water vapour to Europe over long distances, are a key driver. Coastal areas of the Mediterranean may be at particular risk, due to the potential for advection of humidity from the ocean over hot land, although this process is unquantified. The interplay between the impacts of increasing humidity through the duration of a heatwave and the intense rainfall that often follows heatwave breakdown is also unknown. This PhD project will address these questions for the first time and contribute to new understanding of the health risks of future European heatwaves.

Fig. 2 May to September daily maximum wet bulb temperature in central London in 2022 (left) and 2023 (right) from ERA5. Note the elevated values during the humid September 2023 heatwave.

The project could take a number of directions, such as:

  • Identifying the most relevant heat stress metrics for humid heat in Europe’s range of climates
  • Quantifying the synoptic and large-scale drivers of European heatwaves where humidity plays an important role. Drivers may include the position of the jet stream, character of the blocking anticyclone, presence of atmospheric rivers, forcing from the tropics etc.
  • Quantifying the role of the sea breeze and related mechanisms in elevating coastal humid heat extremes around the Mediterranean basin.
  • At sub-daily timescales, understanding the relationship between humidity in European heat extremes and intense rainfall that often occurs during the breakdown of heatwaves.
  • The ability to skilfully forecast the risk of humid heatwaves in Europe on sub-seasonal timescales, which may be useful for Early Warning Systems.
  • Quantifying projected future changes in European humid heat under a range of climate change scenarios.

Datasets

Relevant datasets include:

  • E-OBS daily gridded land-only observational dataset over Europe
  • Copernicus regional reanalysis for Europe (CERRA) and ERA5 global reanalysis
  • UKCP18 regional (12km) historical and future projections over European model domain
  • CORDEX-EURO domain historical and future projections

Project outputs

This project will, or the first time, quantify the drivers and predictability of humid heat over Europe. There is ample opportunity of novel, high impact research to be conducted in this project, with a potential for results to be published in high impact journals.

Key references

Armstrong B, Sera F, Vicedo-Cabrera AM, et al. (2019) The Role of Humidity in Associations of High Temperature with Mortality: A Multicountry, Multicity Study. Environ Health Perspect. doi: 10.1289/EHP5430.

Birch CE, Jackson LS, Finney DL, Marsham JM, Stratton RA, Tucker S, Senior CA, Keane RJ, Guichard F, Kendon EJ. 2022. Future changes in heatwaves over Africa at the convection-permitting scale. Journal of Climate. 5981-6006 35.18, https://doi.org/10.1175/JCLI-D-21-0790.1.

ESOTC (2023) European State of the Climate 2023, Copernicus Climate Change Service and WMO, https://climate.copernicus.eu/europe-experiences-widespread-flooding-and-severe-heatwaves-2023#c4b9e7d4-cce7-4f5d-b543-2eda8001eebe

Rogers, C. D. W., Ting, M., Li, C., Kornhuber, K., Coffel, E. D., Horton, R. M., et al. (2021). Recent increases in exposure to extreme humid-heat events disproportionately affect populated regions. Geophysical Research Letters, 48, e2021GL094183. https://doi.org/10.1029/2021GL094183.

Simpson, C., O. Brousse, K. Ebi, C. Heaviside (2023), NPJ Clim. Atmos. Sci., https://doi.org/10.1038/s41612-023-00408-0.

Zhang, Y., Boos, W. R., Held, I., Paciorek, C. J., Fueglistaler, S. (2024). Forecasting tropical annual maximum wet‐bulb temperatures months in advance from the current state of ENSO. Geophysical Research Letters, 51, https://doi.org/10.1029/2023GL106990.