A tale of two cities: What drove 2024’s Valencia and Porto Alegre floods?

Neither Porto Alegre, Brazil, nor Valencia, Spain, are in the news this week. But they have been before, and almost certainly will be again, as bouts of extreme drought, followed by deluge, become more frequent and fierce. The media cameras are gone from 2024’s inundations, but in Valencia 100,000 wrecked cars remain to be disposed of, and in both cities there’s a visceral trauma felt each time rain starts falling. And there’s the daily grind, as residents pick up the pieces in communities where year by year extreme weather makes it harder to live.

The people ask why it’s happening, and scientists try to provide answers. Perhaps surprisingly, researchers are finding similar patterns of causation in these cities on two widely separated continents — patterns serving as a warning to the world.

What residents know in their very bones is that repeated massive flooding sweeps away historical and cultural diversity, homes, livelihoods, financial and family well-being. When last year waves of torrential rain fell on both communities — Porto Alegre in the south of Brazil and Valencia on the Mediterranean coast of eastern Spain — they ended up looking much the same, drowned in mud and misery.

When the water receded, many causes were pointed to involving dynamic Earth systems, though causation and solutions are proving more complex and daunting than expected.

Twin deluges

Porto Alegre was the first hit in 2024. Torrential rains began in April and lasted six weeks, bringing overflowing rivers and massive mudslides. A hydroelectric dam partially collapsed. At least 180 people died and half a million were driven from their homes.

As with all such recent weather disasters, the stories of loss resonate in a warming world, where people have begun asking if their community might be next. Brazil alone has seen a 460% increase in climate-related disasters since the 1990s, according to one landmark study.

Returning to his home in São Sebastião do Caí, a town north of Porto Alegre, businessman Guilherme Cardoso confronted utter devastation: “It seemed like the aftermath of war. Huge amounts of garbage and mud in the streets, cars turned upside down and families begging for help. I’ve experienced other floods, but nothing like this,” he told Brazilian daily Folha de S.Paulo. Parts of Porto Alegre remained underwater for weeks. It was the worst flood in the history of Rio Grande do Sul state, with 1.6 million hectares (2.5 million acres) affected.

Valencia’s floods in October 2024 impacted 450,000 hectares (1.1 million acres), less than the area flooded in Rio Grande do Sul. But size doesn’t define terror: The speed at which the deluge unfolded and toll in human lives was greater than in Brazil. Flood alerts came too late. At least 205 people died — Spain’s worst disaster in decades.

“It was absolutely horrific, I don’t think anything can prepare you for seeing it with your own eyes,” Valencia resident Zoe Wilkes told the BBC. “Every single street had 50 cars piled on top of each other; they were wedged between tree trunks and up in the branches. Pavements had been completely ripped up, houses were missing walls — debris was everywhere. It was completely bizarre and terrifying.” Rescue teams discovered seven bodies in an underground parking garage.

According to the Intergovernmental Panel on Climate Change, extreme weather events causing highly impactful floods and droughts are becoming more likely and severe due to human-driven climate change, which has destabilized the historic hydrological cycle.

In Valencia and Porto Alegre, scientists continue doing careful forensic analyses of the disasters. While the fingerprint of CO₂-induced climate change is all over the catastrophes, other fingerprints are also being detected — some identified decades ago by a little-known climatologist.

Immediate causes: Extreme weather events

Scientists agree that the immediate cause of the horrific flooding in both countries was a confluence of extreme meteorological conditions, which they’ve now described.

In Brazil, several weather systems collided over Rio Grande do Sul, according to Paulo Brack, a professor at the Federal University of Rio Grande do Sul’s Institute of Biosciences. Unusually high humidity arrived on winds from the west, partly due to Pacific Ocean warming during El Niño. This ran into a surge of humid air from the Amazon. These moisture-laden winds then encountered cold fronts coming up from the south.

The cold fronts normally travel north without difficulty. But this time, Brack said, they met an obstruction and stalled. “The atmospheric blockage, called a heat dome, was related to deforestation and the lack of vegetation [in central Brazil]. It blocked these rains, preventing them from traveling to other states.”

Marcelo Seluchi, from CEMADEN, the national center for natural disaster monitoring and alerts, continued the story: “With nowhere else to go,” he told Mongabay, “the rains eventually descended chaotically on Rio Grande do Sul, with 420 mm [16.5 inches] of rain falling between 24 April and 4 May.”

In Valencia, the flooding was attributed to a “cut-off low pressure storm system,” as cold autumn winds came down from Northern Europe to collide with a heavily warmed mass of air and moisture sitting over the Mediterranean. The result was a sudden catastrophic storm, with more than a year’s worth of rain — 445.5 mm (17.5 in) — falling in a day.

A second set of fingerprints: Local land-use change

Scientists recognize that local changes in land use can make these climate change-driven extreme meteorological events more intensely damaging.

In Rio Grande do Sul, they point to a loss of about 3.5 million hectares (8.6 million acres) of native vegetation, about 22% of the state’s total coverage, between 1985 and 2022. Much of the forest cleared was replaced by soy farms, with the crop now Brazil’s leading agricultural export.

Eduardo Vélez, a researcher at MapBiomas, which uses satellite imaging to track changes in soil use, told BBC News Brasil that a third of this conversion occurred in the Guaíba River Basin, where Porto Alegre is located. Researchers suggest that if vegetation along the banks of the Guaíba had been preserved, water levels would have been as much as 1.5 meters (4.9 feet) lower, limiting the scale of the Porto Alegre disaster.

Local land-use change also exacerbated Valencia’s floods. Hossein Bonakdari, an associate professor of civil engineering at the University of Ottawa, Canada, noted: “Rapid urban development … has significantly contributed to flood severity by increasing impermeable surfaces, such as roads and buildings, which prevent water from being absorbed into the ground.”

“In rural areas, practices like soil compaction from agricultural expansion and deforestation have reduced the landscape’s ability to naturally retain water, causing rapid runoff that intensifies downstream flooding,” he added.

Blaming carbon emissions and climate change

Most scientists today agree that while these local land-use changes play a role in flooding, increased carbon emissions are the most important causal factor.

“No doubt about it, these explosive downpours were intensified by climate change,” Friederike Otto from World Weather Attribution at the Centre for Environmental Policy, Imperial College London, told Euronews. She added: “With every fraction of a degree of fossil fuel warming, the atmosphere can hold more moisture, leading to heavier bursts of rainfall. These deadly floods are yet another reminder of how dangerous climate change has already become at just 1.3°C [2.3°F] of warming [since preindustrial times].”

Linda Speight, a lecturer at the University of Oxford’s School of Geography and the Environment, agreed, telling Euronews: “Unfortunately, these [violent storms] are no longer rare events. Climate change is changing the structure of our weather systems creating conditions where intense thunderstorms stall over a region leading to record-breaking rainfall — a pattern that we are seeing time and time again.”

But other scientists suggest that while the focus on increased emissions is valid, it gives the false impression that climate change can be combatted solely by reducing emissions, which, they say, conceals another extreme weather intensification fingerprint.

The emphasis on emissions, they say, downplays the possibility that climate change is being driven by more than just rising CO₂ levels. Climate change, these scientists argue, is also driven by large-scale land-use change (especially deforestation), along with changes in other interlinked Earth systems — with each system potentially reinforcing others. The health of the planet will only be restored, these researchers say, if this complex interrelationship is recognized and the multiple causes dealt with in a more holistic manner.

Cutting emissions isn’t enough, these scientists say. In fact, land-use change (and the outsized effect it has on the water cycle) could be having a bigger and more immediate impact on climate — especially helping trigger extreme events like flood and drought.

To properly address the growing crisis, we must repair local ecosystems and reinvent infrastructure (regrowing riverine forests and making cities flood-proof, for example). Likewise, we must also restore national — and even continental — forests, marshlands and other vegetation to stabilize the hydrological cycle. This message often runs counter to the world’s dominant economic development paradigm, a standard rallying cry for politicians.

Drought and deluge: ‘The terrible twins’

One scientist who spent most of his life warning that ignoring this hydrological truth would threaten the very survival of humanity was Spanish scientist Millán Millán Muñoz. Before he died in 2024, at age 83, he feared that few had listened, lamenting: “I failed all of us.”

When Millán trained as a scientist in the 1960s, there was broad consensus that vegetation, soil and water played crucial roles in regulating global climate, moderating weather.

Indeed, scientists believed this for centuries. Greek natural philosopher Theophrastus more than 2,000 years ago demonstrated a keen understanding that when forests were clear-cut, weather changed: “The greater part of the district was dried up and put into cultivation,” he wrote. “[T]he clearing of the woodlands has opened up the land, exposing it to the sun and bringing about a warmer climate.”

In 1800, explorer Alexander von Humboldt wrote about devastation due to deforestation in Venezuela: “When forests are destroyed, as they are everywhere in America by the European planters, with an impudent precipitation, the springs are entirely dried up, or become less abundant. The beds of the rivers remaining dry during a part of the year, are converted into torrents whenever great rains fall on the heights.”

The message was clear: Change the land, change the weather. Wangari Maathai, winner of the Nobel Peace Prize in 2004, echoed millions of Kenyan women when she warned: “If you destroy the forest, then the river will stop flowing, the rains will become irregular, the crops will fail and you will die of hunger and starvation.” From the 1970s on, she was part of a huge Kenyan women’s movement that nurtured and protected millions of trees.

Millán’s conviction that land-use change was a key factor impacting Earth’s hydrological cycle and climate was strengthened in 1991 when he and nine other scientists were asked by the European Commission to determine why the weather in Valencia and the entire Mediterranean region was rapidly changing, with a decline in summer rains, causing drought and desertification, punctuated by an increase in sudden, ferocious super storms.

Using extensive climate data, Millán became certain that the droughts and storms were linked, calling them “the terrible twins.” He and his team discovered that the rain clouds that came in from the Mediterranean historically no longer contained enough moisture to make rain. The clouds had a water content of just 14 grams per cubic meter of air (about 0.01 ounce per cubic foot), while they needed 21 g/m3 (0.02 oz/ft3) to precipitate.

The research team also determined the reason for the moisture loss: In the past, as clouds reached the Mediterranean coasts, they crossed large, vegetated marshlands, picking up added grams of moisture as well as cloud condensation nuclei. Those moisture-laden clouds would then float above the great rainmaker, Spain’s oak forests. Those forests did more than send up the needed grams of moisture and cloud condensation nuclei. They also cooled the air, a vital step in making rain. When the westward-flowing clouds then reached a mountain range such as Spain’s Sierra Nevada, they rose even higher, cooled more, and as they headed back east toward the Mediterranean Sea, released rain.

Then came the Mediterranean region’s intensive development over the 20^th century. Coastal marshes were paved over to make way for roads, homes, hotels, water parks, mega oil and gas installations, and more. Oak forests were cut for timber and agriculture. The winds blowing ashore, instead of cooling, picked up heat from concrete and compacted soils. And for every degree Celsius of warming those clouds experienced, they were able to contain 7% more water vapor without making it rain. When those clouds hit the inland mountains, they rose and returned but still didn’t release their precipitation. Day after day the clouds piled one atop the other, helping make the Mediterranean Sea one of the hottest in the world. These cloud formations could reach 4 kilometers (2.5 miles) in height.

Then, when cold autumn winds drove down from Northern Europe, this immense moisture-laden cloud formation sitting over the Mediterranean Sea and eastern Spain could be triggered to produce a super storm, dumping a deluge on a landscape shorn of its forests and wetlands.

A year’s worth of rain could fall in a day, and with no vegetative sponge to soak it up, waters rose and raged. These deluges fell on pavement and hard-packed Spanish soils that had suffered years of intensive industrial agriculture. Healthy soils would have absorbed much moisture. Instead, the Spanish soils turned to mud, making the floodwaters more destructive.

A vicious hydrological cycle was created and intensified, with more deforestation pushing more regional warming and the “terrible twins” of drought and deluge. That one-two punch stressed and degraded the remaining forests, further impacting the water cycle, Millán said. Add global greenhouse gas emissions to the mix, and the puzzle obscuring the causes of the escalating floods in Valencia and Porto Alegre becomes clear.

Millán emphasized that all elements in the natural world interact and depend on each other. “Water begets water, soil is the womb, vegetation is the midwife,” he said. Water, soil and vegetation — these were the pillars of life. Within this biogeological worldview, profound changes in land use and freshwater systems could be seen in conjunction with surging emissions and global climate change to fill out the fingerprint of extreme weather events.

In fact, over the short term, land use and water change were much more dramatic in their effects than increased emissions, he argued. A case in point at the heart of Millán’s theory: the 2024 Valencia flood.

Recognition, then resistance

Millán’s study was initially well received. He was invited by the U.N. Intergovernmental Panel on Climate Change (IPCC) to contribute to its Third Assessment Report in 2002. But that was also a time when climate scientists — who built models based on projections of increased global carbon emissions and rising temperatures — were becoming dominant.

Millán found that the modelers were not interested in his analysis of interacting factors and “questioned every result we presented.” He found himself involved in endless losing arguments and eventually left the IPCC.

Politicians, it seemed, also preferred the modelers’ straightforward analysis to Millán’s complex accounting. With countries the world over attempting to improve living standards via rapidly expanded industrial agriculture, mining and big infrastructure projects, people were often infuriated when told deforestation and development were seriously damaging the climate and destabilizing the hydrological cycle, perhaps irrevocably.

The modelers’ view, that the main climate change culprit was an increase in global carbon emissions, became dominant. For a time, the focus shifted almost exclusively to emissions control. Even today, when forest cover is mentioned in reporting, as it often is at Mongabay and in other media, an emphasis is placed on the benefits of forest carbon storage or biodiversity; and far more rarely on the harm deforestation does to the hydrological cycle.

‘An exquisitely fine-tuned system … driven by life’

In 2009, Johan Rockström’s international team of scientists proposed the planetary boundary framework — then a hypothesis, now a respected theory — that postulates nine Earth systems critical to maintaining the planet’s “safe operating space for humanity,” sustaining life as we know it. Of those systems, scientists now say human activities have dangerously destabilized six, including land system change (including deforestation), freshwater change (disruption of the hydrological cycle), climate change, and loss of biosphere integrity. This framework encompasses and greatly expands on many of Millán’s ideas.

Today, Millán’s supporters have swelled in number, as they argue for the integration of an Earth systems approach to climate solutions. In a 2024 Mongabay commentary, for example, author Judith D. Schwartz argues that solar power projects, installed to reduce carbon emissions, should not be sited on natural forest land. Doing so, she says, ignores that “Earth has developed an exquisitely fine-tuned system for regulating temperature and moisture that is driven by the life that dwells here, the flora, fauna and fungi, and the interactions among them.”

She goes on: “This does not mean that CO₂ concentrations are irrelevant. Rather, carbon in the atmosphere can be seen as a lever, part of an overall climate-regulation system.” Solar power and other renewables can play a part in tackling the crisis, she adds, but a systems approach balancing all inputs is needed. That’s why solar installations should be located “in abandoned industrial sites, above parking lots and on warehouse roofs.”

Klaas van Egmond, faculty professor of geosciences at Utrecht University in the Netherlands, told Mongabay that Millán was “absolutely right to point out early the need to look integrally at the total process.” Van Egmond adds that he’s long worried science has become too hyper-focused on single causes with singular solutions.

“This is the root cause of all our current problems,” he said. “As science has progressed — and particularly with the advent of computers — there has been this overvaluation and overconfidence that we can ‘control the world by scientific-technological means.’ But the last few decades have shown that this will not be the case.”

Brazil’s ‘dreadful twins’

In Brazil, some scientists are sympathetic with Millán’s holistic views underlining the existential urgency of landscape-wide forest conservation — ideas unwelcome among policymakers keen to promote economic growth.

One of these scientists, now retired, is Antonio Donato Nobre, who worked at Brazil’s National Institute for Space Research (INPE). He has argued there is “a profound connection between deforestation in the Amazon and the intensification of extreme climatic events in Brazil, such as the catastrophic floods in Rio Grande do Sul and the prolonged droughts in the Pantanal and elsewhere.”

Like Millán, he sees droughts and floods as “dreadful twins,” bred in this case by Amazon deforestation and rising global temperatures.

Nobre stresses the role of the rainforest not only in guaranteeing Brazil’s traditionally benign climate, but in regulating Earth’s climate. The tropical forest, he says, acts as a powerful natural “air conditioner” through the process of evapotranspiration, and is associated with a biotic pump that drives moist winds far inland. Trees, particularly in dense forest ecosystems, not only cool air at the Earth’s surface but also release moisture that rises high into the atmosphere, forming heat-reflecting clouds and triggering rainfall.

Such thinking is in line with the biotic pump theory first proposed in 2007 by Russian physicists Victor Gorshkov and Anastassia Makarieva. The biotic pump “is a mechanism in which natural forests create and control ocean-to-land winds, bringing moisture to all terrestrial life,” Gorshkov and Makarieva told Mongabay in 2012. When proposed, their theory turned traditional textbook thinking about climate on its head by proposing that it isn’t atmospheric circulation that drives the hydrological cycle; rather it’s the world’s forests and the hydrological cycle that drive the mass circulation of air.

Whether the biotic pump theory is proven out or not, what is clear to scientists today is that forests play a much more complex and comprehensive role in climate regulation than simply being sequesters of CO₂.

For centuries, a dynamic and healthy Amazon Rainforest ensured a stable and productive climate system, especially in regions dependent on what scientists dubbed the Amazon’s “flying rivers” — massive streams of water vapor formed over the Atlantic Ocean by dominant trade winds which then blow across the Amazon, picking up more moisture from the forest until they are finally redirected southeast by the Andes. That southeast turn brought regular rains to the farmlands of central and southern Brazil, Paraguay, Uruguay and northern Argentina.

These nations benefited from this reliable precipitation pattern over what Nobre calls the “lucky quadrangle,” an agricultural area still responsible for 70% of South America’s GDP. He contrasts this bounty with desert landscapes found at similar latitudes on the other side of the Andes and on other continents like Australia or in Namibia, which lack an Amazon forest upwind.

Now, Nobre fears the ongoing destruction of the Amazon is destabilizing this once robust system, leading to the formation of a “hot air bubble” similar to the build-up of moisture-sodden clouds described by Millán over Valencia. This bubble, Nobre warns, will trap heat, block rain systems and exacerbate droughts and floods, while pushing “flying rivers” into uncharacteristic pathways that intensify extreme weather events.

In recent years, studies have found that the Amazon rains that fed southern Brazil and nations to the south for generations are declining due to intensive loss of rainforest, along with rising global temperatures.

Catastrophe tomorrow?

The 2024 floods in Porto Alegre and Valencia have receded. In Rio Grande do Sul, concerns are now growing over indications of a new threat: drought, the other terrible twin.

In Valencia, the recovery effort continues; not least among the daunting demands is the need to find a final resting place for those aforementioned 100,000 wrecked cars. People are dreading the arrival of the hot summer, with temperatures reaching 40°C (104°F), and regular summer rains now a fading memory. People are also dreading the next flood.

2024 was the first year in history in which average global temperatures exceeded preindustrial levels by 1.5°C (2.7°F) — the upper relatively safe limit set by the Paris climate agreement. Soaring temperatures, persistent heat domes, super storms, droughts and fires battered the globe. Modelers have so far been unable to fully account for all of 2024’s heat.

Some suggest they look for it in civilization’s relentless consumption and growing population, triggering complex interactions between transgressed Earth operating systems: including not only rising emissions, but also falling forests, degraded biomes, declining biodiversity, overheated seas, and a super-energized and unstable hydrological cycle.

The planet can live with the “new normal.” The question is: Can humans? Can life as we know it cope?

If governments and corporations offer no major course corrections to address disastrous land and water system change and carbon emissions, then scientists warn the world could continue lurching from extreme floods to extreme droughts, with each disaster leaving the landscape and people more vulnerable, poorer, exhausted, a little less resilient, a little more easily drawn into social conflict, maybe raging at easy scapegoats held up by authoritarians.

A better future

There are ways forward, though the scope for recovery lessens each year the crisis runs unchecked.

When hunting for hope, Millán looked back to the acequias he and his father used to leap over on regular treks toward the Sierra Nevada. The acequias were built by the Moors between the eighth and 10^th centuries, one of many Arab legacies left in Spain. “Acequias were natural and artificial storage networks created in dry lands so that the little bit of water runoff there was could be caught for drinking or other purposes,” Millán explained. He saw them as one of many traditional environmental practices the modern world could learn from. Such practices are universal, with amunas in Chile and eris in India serving similar purposes, to name but two more.

Millán has helped inspire an international movement based around regenerating land and protecting forests. “While Millán’s predictions are now becoming reality in the extreme weather we’re seeing in the Mediterranean, it’s not too late to stop the feedback loop of nature degradation and climate disaster,” Willem Ferwerda, founder of one such initiative called Commonland, told Mongabay. “Integrated land restoration can restore the ecological function of degraded landscapes through making agriculture more regenerative, reviving forests and wetlands to rebuild natural water cycles and stabilize regional climates. However, solving the problem will take time. This is a long-term commitment.”

The organization is working in Spain to heal devastated landscapes because, as Ferwerda explains, “You can’t fix the climate crisis without restoring the land. It’s like trying to rebuild a house on a crumbling foundation. That’s why Commonland is dedicated to supporting local people and organizations to work together.” He goes on: “Through restoring the land, we can not only bring back the vital, life-giving summer rains in the Mediterranean basin but also restore a sense of hope and connection for communities living there.”

Nobre also finds hope in nature’s regenerative power. He embraces the “miraculous technology” of seeds, which encapsulate millions of years of evolutionary intelligence, allowing ecosystems to self-repair, but only if given half a chance.

What needs to be changed above all is the way we view the natural world, particularly water. Writer and poet Rob Lewis was regularly in touch with Millán before the latter died in January 2024 in Valencia, the city that nine months later faced a catastrophic flood. Lewis summarized Millán’s thoughts on water:

Humans are 60% water, birds [roughly] 75%, fish 70 to 84%. A typical cat weighs in at 67%, while [the actively growing parts of trees are] 80 to 90%. How much water a landscape can hold is therefore proportional to how much life is in the landscape and soil. The more life in a landscape, the more water it can “milk” from ocean flows. It’s a self-amplifying circle: water, through life, begetting more water, begetting yet more life, gathering yet more water, and around it goes, the result being increased climate cooling and moderation.

But the reverse is true: Cut down forests and drain the marshes, and you dry the land until it becomes lifeless and loses its ability to moderate climate. It is the tragic story of civilization and of our time.

Banner image: More than 200 people were killed in Valencia by the 2024 flood. The army joined rescue efforts in the worst flash floods ever to hit Spain, leaving victims in the ravaged region begging for aid. Image Courtesy of inkl.

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Rockström, J., Steffen, W., Noone, K., Persson, Å., Chapin III, F. S., Lambin, E. F., … Foley, J. A. (2009). A safe operating space for humanity. Nature, 461(7263), 472-475. doi:10.1038/461472a

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