• Heatwaves are amongst the deadliest natural hazards with thousands of people dying from heat-
related causes each year. However, the full impact of a heatwave is often not known until weeks
or months afterwards, once death certificates are collected, or scientists can analyse excess
deaths. Many places lack good record keeping of heat-related deaths, therefore currently
available global mortality figures are likely an underestimate.
• Early heatwaves and associated drought conditions also threaten the yield for many crops such
as wheat, because it hinders grain filling. This heatwave has come at a critical time for the crop
season in the Western Mediterranean countries.
• While Europe and North Africa have experienced heatwaves increasingly frequently over the
last years, the recent heat in the Western Mediterranean has been so extreme that it is also a rare
event in today’s warmer climate. Our estimate of observed temperatures averaged over 3 days
were estimated to have a return period of approximately 400 years (at least 60 years) in the
current climate, meaning they have approximately a 0.25% chance of happening in any given
• To estimate the influence of human-caused climate change on this extreme heat we combine
climate models with the observations. Observations and models both show a strong increase in
likelihood and intensity but the change is systematically lower in the models than in the
observations. The fact that extreme heat is increasing faster than climate models simulate is a
known problem in summer in Western Europe, in all climate models, and is also found here.
• The combined results, giving an increase in the likelihood of such an event to occur of at least
a factor of 100, is therefore likely too conservative. At the same time, a heatwave with a chance
of occurrence of 0.25% in any given year (return period of 1-in-400 years) would have been at
least 2C cooler in a 1.2°C colder world.
• This discrepancies between the modelled and observed trends and variability also hinders
confidence in projections of the future trends. In a future 0.8°C warmer climate (reaching a
global warming of 2C above pre-industrial levels) such a heatwave would be another 1°C hotter,
but as above, this is probably a very conservative estimate.
• Heat-related fatalities have decreased in cities with urban planning for extreme heat. This has
proved effective in Spain, and notably in Lisbon, Portugal, where the urban heat island effect DOI: https://doi.org/10.25561/103833.
has been reduced through incorporating more green and blue spaces. In addition, early warning
systems for heat, simple self-protective behaviours such as drinking enough water, city heat
action plans, strong social ties, and improved risk perception have been shown to reduce heat-
related health impacts.
Sjoukje Philip, Sarah Kew, Robert Vautard, M Vahlberg, R Singh, F Driouech, R Lguensat, C Barnes, F Otto. Extreme April heat in Spain, Portugal, Morocco & Algeria almost impossible without climate change