Future heatwaves in Europe will be most severe in the south

Heatwaves in Europe are likely to become more frequent and devastating, according to recent research. Climate change will lead to extended periods of high day and night-time temperatures, coupled with high humidity, and will particularly affect the Mediterranean coast and southern European river basins, where there are many densely populated urban areas.

The summer heatwave of 2003 was thought to cause more than 70,000 early deaths across Europe. The elderly, the very young and those with heart and respiratory conditions are especially susceptible to long periods of hot and humid conditions.

As part of the European project ENSEMBLES1, the researchers compared projections from six climate models to assess the likelihood of heatwaves occurring across the entire European continent for the period 1950 to 2100. Health impacts were estimated from changes to three key heatwave characteristics: the length of the heatwave, the minimum night-time temperature and the relative humidity.

Hot nights prevent recovery from high daytime temperatures and have further impacts on health, through the effects of sleep deprivation. High humidity impairs sweating, which helps people keep cool.

Despite uncertainties associated with modelling climate change, all the climate models produced a consistent pattern of regional changes in heatwave characteristics up to 2100. For example, the regions where the increase in number of dangerously hot days will be most pronounced are found in southern European river basins, such as the Po and lower Danube river valleys, and along the Mediterranean coast. These impacts are strongly related to the extremely high temperatures found in low-altitude areas and the closeness to adjacent seas causing relatively high humidity levels.

Specific future projections include:

• The average number of heatwaves will increase from one every three to five summers in 1961-1990 to two or three every summer in 2071-2100.
• In the Mediterranean region, the number of heatwave days each summer will increase from about 2 days, as found in the period 1961-1990, to about 6-24 days in 2021-2050 and to 27-67 days in 2071-2100.
• The frequency of combined hot days (above 35°C) and tropical nights (above 20°C) will increase across all regions by 2021-2050, with more northern areas becoming affected too. Southern European river basins and the Mediterranean coast will be most affected.
• Although southern regions will experience reduced relative humidity, this does not offset the combined effects on health of higher temperatures and remaining humidity from proximity to the sea, especially along Mediterranean coasts and southern river valleys.

The urban heat island effect (city areas being hotter than rural areas) was not included in the projections, which may mean that the health risk to people living in southern densely populated cities (e.g. Athens, Bucharest, Marseille, Milan and Rome) could be underestimated.

ENSEMBLES was supported by the European Commission under the Sixth Framework Programme. See: ensembles-eu.metoffice.com

Source: Fischer, E. M. and Schär C. (2010) Consistent geographical patterns of changes in high-impact European heatwaves. Nature Geoscience. DOI: 10.1038/NGEO866.

Contact: erich.fischer@env.ethz.ch

Science for Environment Policy, issue 208: A service from the European Commission