What is thermal stress and how will it affect the 30 most populated Spanish cities in the coming years

It is common to worry about the heat when it exceeds 38 °, but the thermometer is not the only indicator to measure how it affects: that temperature in a dry place, with breeze and clouds becomes bearable, but if there is a lot of humidity, direct sun and no air runs becomes a health risk. Humidity, radiation or wind are the factors that, in addition to temperature, determine how weather affects the body, which is called thermal stress. Measuring this variable is not easy, to the point that there are several indicators to do so; one of them is the UTCI (Universal Thermal Climate Index), which tries to assess how those atmospheric elements interact with us. Lobelia Earth, an expert company in generating data from geophysical characteristics, has made projections on how this index will affect the 30 most populated Spanish cities in the coming years.

The forecast is that in 2030 there will be almost one degree UTCI more than 20 years earlier in Spain and up to 5.4 degrees UTCI more in 2090. It does not affect all localities equally: where it will grow the most will be in Zaragoza, Valladolid, Pamplona and Madrid, cities with a continental climate, where it is expected to increase progressively to almost 8 degrees UTCI in 2090. In these cities the maximum thermal stress will be more than 43 degrees in July, which will make life on the street very difficult in the central hours of the day, especially if their urban centers, with more vegetation and shadows, are not adapted. These figures are consistent with the climate predictions of the IPCC – the UN's panel of experts on climate change – which estimates that temperatures will increase exponentially if there is no reduction in emissions.

In the graph below you can see the variation in degrees UTCI of 30 cities with respect to 2010. In the table below, the UTCI value for each one is detailed, calculated as an average of the maximums in the month of July (you can consult here the methodology used for the calculation).

UTCI thermal stress is measured in degrees and, although logically related to temperature, it does not have to coincide with what the thermometer marks. Lobelia Earth's calculations, based on weather prediction models validated by the scientific community, show how much will change during the month of July – the warmest of the year – in the 30 main Spanish cities from 2010 to 2090 in the worst possible scenario, that is, if measures are not applied to limit the emissions of greenhouse gases that drive global warming.

Two southern cities on the banks of the Guadalquivir, Seville and Córdoba, which already suffer very strong heat waves, will withstand the greatest thermal stress also at the end of the century, although in this case the change compared to today is minor. In both cities, ideas such as awnings in the streets of the center or areas with water to cool off and other solutions are already applied that should also reach the rest of the cities in the coming years. On the other hand, Las Palmas de Gran Canaria and Santa Cruz de Tenerife will be the Spanish capitals where the UTCI increases the least, softened by the sea and a more equatorial climate.

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The heat wave is concentrated in Andalusia and the Canary Islands on its last day in the Peninsula

How the body reacts to heat stress

Heat stress is increasingly used, rather than temperature alone, to understand how heat can affect it, but it is rarely used for forecasting. These data, worked by Lobelia Earth in the framework of a project funded by the Copernicus Climate Change Service (C3S) of the European Union, use historical values and predictions based on IPCC climate models. As summarized by Suso Peña, climate scientist at Lobelia Earth, its importance lies in the fact that UTCI "approximates our body's response to heat stress."

That is why the projected future temperature and thermal stress may not coincide: "In our data it is seen that in coastal cities, such as Barcelona, Valencia, Murcia, the UTCI temperature will rise more than the air temperature: this may be, for example, because there will be more humidity and our body is thermoregulated worse", says Peña. In inland cities such as Madrid, Zaragoza or Córdoba the opposite happens: the increase in thermal stress that will be felt can be slightly lower than what the temperature increases.

How does the body respond to this stress, under exceptional heat and conditions? Cristina Linares, researcher in Climate Change, Health and Urban Environment at the Carlos III Health Institute, explains it this way: "Heat stress affects the hypothalamus, which regulates our sweating, the only natural mechanism we have to cool down. When the hypothalamus is affected by overheating, or if there is a lot of humidity, we are very vulnerable to thermal stress, which can translate into heat stroke, with nausea, vomiting, dizziness ...".

This researcher, who has not participated in the study of Lobelia Earth and prefers not to comment on its specific data, points out that it would be "devastating" a perspective in which thermal stress increased in all large Spanish cities. He recalls, in fact, that climate records are being broken earlier than predicted by the IPCC: "In international projections, modelling is being exceeded much earlier than predicted. This is associated with the fact that greenhouse gas emissions are not falling, so the only certainty is that the temperature will continue to increase at a higher rate. In addition, the Mediterranean countries warm up 20% faster than the others. These projections are alarming from a scientific point of view."

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