History leaves its mark. It leaves traces not only in the form of monuments or through bequeathed culture, but also in natural environments which bear witness to climatic periods which are rich in lessons for today. The DANA (Isolated High Level Depression) that occurred in Valencia or the episodes of torrential rain that occur throughout Spain can be studied by looking at the past. A survey in Guadix (Granada) and the historic series of river floods in Huelva allow us to draw up an Andalusian map that explains the DANAS of the future. With a conclusion that repeats itself: storms are becoming more and more intense and their effects more severe due to human action.
Andrés Díez-Herrero, technician at the Geological and Mining Institute of Spain (IGME), carried out research between 2018 and 2021 in the archaeological excavations of the Roman Theater of Guadix, in Granada, which allows us to speak of the existence of an “archive climate” which reveals episodes of torrential rains since the 1st century with the climatic conditions which affected eastern Andalusia and, by extension, part of the Levant at that time. A climatic archive which, in the words of the specialist, has left not only physical traces, but also scientific conclusions which force us to rethink urban planning.
The research, published in the scientific journal Global and Planetary Change, allows us to establish parallels and differences between the DANAS of the past and those of today. Using radiocarbon dating (C-14) and using an optically stimulated luminescence (OSL) technique, the team led by Díez-Herrero studied the sediments of the archaeological remains in different layers, being able to determine severe climatic episodes, particularly in the 1st century. and in the Middle Ages.
Global warming as a common factor
At these two times, compared to today, there was a common denominator: global warming. According to the researcher, “the vegetation found then is that usual in the semi-arid Mediterranean zone, common both in eastern Andalusia and in a large part of the Levant.” We can therefore conclude that the climate was similar in Roman times, in the Middle Ages and today there were periods of warming, but it was not as noticeable as today today, scientists explain. This is not the only thing in which all of these periods are similar, depending on what is dated, but human action has also contributed to worsening the effects of torrential rains.
This was verified by observing the remains found in the different layers analyzed between the years 2018 and 2021. According to scientific research, the DANAS “drew more sediment than water”, which suggests, according to Díez-Herrero, researcher at IGME. , which humans of these three eras built in areas prone to flooding during times of high risk of torrential rain. What is striking is that the location of the Roman Theater of Guadix is not one of the areas classified as prone to flooding. It is therefore advisable, according to scientists, to review the maps.
“You have to look at maps to know where you are, especially key buildings like hospitals or fire stations,” emphasizes Andrés Díez-Herrero. Currently, the return period usually used to build is that of the climatic episodes of the last 500 years, but different research suggests updating this period and adapting it to the reality in which we live. “These return periods are measured taking into account floods with clean water from rivers, but not with dirty water from streams, ravines or areas likely to be flooded in the event of torrential rains.”
Climate change and the “acceleration” of episodes
If research on the Roman remains of Guadix suggests that human adaptation to the environment had a strong impact on the region’s climate, the torrential rains that occur in western Andalusia present other historical particularities . Juan Antonio Morales, a geologist at the University of Huelva, has been studying the river cycles of the Guadiana, Tinto and Odiel rivers for years, which has allowed him to establish a solid foundation on the behavior of rain and associated flooding.
These episodes, throughout the western arc, are obviously marked by the Atlantic Ocean. “The climatic periods of El Niño and La Niña define the torrential rains that fall in this region,” Morales explains. Remember that we cannot in any case speak of a “cold drop” in the Atlantic areas because the ocean, unlike what happens in the Levant with the Mediterranean Sea, “is already cold”. The problem is that, also because of human action, “climate change is increasing the temperature of the oceans”.
“We observed that, although the Atlantic does not generally produce cold drops, El Niño brings warm water from the Mediterranean, shifting Atlantic currents and generating episodes of heavy rain in the Gulf of Cádiz.” A trend that has accelerated in recent decades. “We are seeing that storms are becoming more and more violent and they are also occurring at times when they did not occur before. They almost never happened after March and now we have them in May or June.
The story, collected in geological dating, also explains the differences between rivers like the Tinto and the Odiel with a more important river like the Guadiana. In western Andalusia, the Tinto and the Odiel mark the so-called “fluviotidal” floods, which are those that coincide with the overflow of rivers in marshy areas, as happens in Huelva. Guadiana is an example that human action can mitigate the effects of climate change. This river is “fully controlled”, thanks to more than 40 reservoirs along its entire length. “Today it would be almost impossible for it to overflow, unless all the reservoirs were left open. Something that could have happened in the past,” Morales believes.
Regardless and with the data in hand, the geologist recalls that what happens in regions like Huelva or Cádiz cannot always be compared to a DANA. In fact, this is not the case for some recent torrential rains. “Atlantic fronts are common in this region of Spain and, over time, they become more and more severe.” This happens because the ocean temperature increases, but in a stable manner, while the wind fluctuates more quickly, meaning that when both factors come together, the rainfall is heavier and continuous over time.
Rethink the present
Morales and the Díez-Herrero team agree, by observing historical climate models and the traces of the past collected, on the need to “rethink” current urban planning and the way in which the population is informed about flood zones and torrential rains. “Schools need more geological knowledge,” Morales says, while IGME researchers believe the historical periods analyzed are notable enough to “examine flood maps and revise urban plans.”
“The design of urban and rural infrastructure in vulnerable areas results in torrential rains with a large load of solid waste which can clog drains and worsen flooding. » Something that invites us to adapt them to any torrential flows that arise. In fact, in the theater of Guadix, the Romans modified the structure after one of the floods, foreseeing the consequences of future episodes. DANAS episodes which, as happened in Valencia, tend to be more frequent and more serious due to human action which causes global warming and therefore climate change.
