Are the heat waves getting more intense?

DOMINIC ROYE Physical Geography Researcher, University of Santiago de Compostela ALEJANDRO DIAZ POSO Predoctoral researcher in Physical Geography, University of Santiago de Compostela

For the time being, and not yet finished, the large number of absolute extreme temperature values ​​that have been exceeded during the heat wave in southwestern Europe undoubtedly indicate that it is historic, both in terms of intensity and of spatial extension and duration.

It should not be a surprise to see more and more extreme events if we remember the phrase "of the Iberian Peninsula, forget about it", pronounced by the German physicist Hans Joachim Schellnhuber in the face of the unbearable heat that we will experience in the extreme south of the old continent due to climate change. This is just the beginning.

The data recently provided by the Intergovernmental Panel on Climate Change (IPCC) leaves no room for doubt: in 2030-2052 we will find ourselves 1.5 ℃ above pre-industrial levels. This will translate into more intense, frequent and prolonged heat waves throughout Europe, especially in the Iberian Peninsula, a critical area within the Mediterranean region.

Since the beginning of the century, the old continent has suffered extreme heat waves on an increasingly common basis. The 2003 heat wave left mortality figures high, with more than 70,000 deaths. This episode is not an isolated case in terms of mortality. In 2010, 54,000 people died in the heat wave that hit western Russia. In addition to the 2015 event, this trend of extreme events is illustrated by the recent set of consecutive exceptionally hot and dry summers in 2018, 2019, and 2020.

Definition of heat wave

There is no uniform criterion to define heat waves, since different criteria will be followed depending on the perspective from which it is analyzed. However, all definitions imply at least one form of temperature (be it maximum, minimum or average) and require a threshold to be exceeded for a given period of time, usually at least three days.

These events can be characterized according to four dimensions: frequency, duration, intensity and spatial extension.

Although numerous indices describe heat waves, the dimension of their spatial extension and, above all, their intensity, have been less studied, especially in the Iberian Peninsula.

From a biometeorological perspective, the Excess Heat Factor (EHF) index directly incorporates the aspect of intensity and the acclimatization process of the human body, by including the average daily temperature for a period of three days compared to the previous 30 days.

More intense, extensive and lasting

Recently, we have verified that for the historical reference period 1971-2000, the intensity, duration and spatial extension of heat waves show significantly increasing trends.

The maximum intensity has increased at a higher rate than the average intensity, with increases of between 2 ℃² and 6 ℃² per decade, while the number of heat wave days has increased by 3.8 days/decade. The average extension of heat waves has also increased for this period by 1.71% per decade, the increase in maximum extension being even more notable, reaching 4.3% per decade.

What can we expect in the future?

Forecasts for the coming decades do not invite optimism. All the trends observed in the aforementioned historical period will worsen in the near future (until 2050) for the entire Iberian Peninsula, with an increase in the average annual number of heat wave days of 104%, reaching 150% on the coast Mediterranean and Pyrenees.

These percentages imply an increase for the peninsula as a whole of 6.4 days/decade in the most moderate scenario of the IPCC (RCP4.5), increasing to 7.6 days/decade in the most pessimistic scenario (RCP8.5). , doubling the trend observed in the last 3 decades of the last century.

Similarly, the maximum intensity increases by more than 50% in most of the peninsula for the moderate scenario but reaches almost 100% in much of the territory in the pessimistic scenario.

In addition, heat waves will not only be more intense and frequent, but will cover an increasingly larger area of ​​the Iberian Peninsula. The value of the maximum extension shows a more notable increase than that of the average extension, ranging between 6% and 8% per decade. This supposes a scenario with more population affected, greater risk of forest fires and greater energy demand.

The preliminary results that we have for the second half of the century show continuity in the behavior observed in the first half of the century, but with higher values ​​in the trends and greater differences between the scenarios analyzed.

Regional variability of the Iberian Peninsula

Heat waves in the Iberian Peninsula tend to show great regional variability in terms of intensity and duration.

A clear example is the last great heat wave that occurred in Spain, back in August 2018. Somewhat similar to the heat wave that is hitting the peninsula hard these days, this last great wave was the result of an intense dorsal (anticyclonic situation) in height, located in the west of the peninsula. For this reason, the highest intensities were recorded in the west and northwest of the peninsula, as well as in mountainous areas, as a result of their presence at height.

According to the data relating to the historical period (1971-2000), in the areas in which a greater intensity (also called severity) is recorded, the duration of the heat waves is shorter, since these areas are more exposed to the rapid transport of air masses from the Atlantic Ocean to the west of the peninsula.

An example of this great regional variability is that the duration of heat waves on the Mediterranean coast and the Balearic archipelago doubles the duration of the phenomenon in the west of the peninsula. In the most intense days of the 2018 event (August 2-3), the heat wave covered more than 99% of the territory, affecting a total of 50 million people. More than 14 million suffered conditions of extreme severity.

Adaptation and mitigation measures

In the coming days we will see how the west and northwest of the peninsula will no longer be under heat wave conditions, while in the east of the peninsula, and more specifically on the Mediterranean coast, these conditions will still persist.

In recent decades, heat waves have become an extreme atmospheric phenomenon par excellence, due to their implications not only for the health of the population but also for agriculture, livestock and transport.

It is urgent to take adaptation and mitigation measures: develop warning, action and communication plans, carry out studies on changes in the population's sensitivity to heat and implement improvements in governance practice and urban design.

This article has also been written by María de las Nieves Lorenzo González, Professor of the Area of ​​Physics of the Earth, University of Vigo; and has been published in 'The Conversation'.

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