Scientists are striving to unravel the complex interplay of factors contributing to these extreme weather events, particularly the lethal rains in Europe. Two key theories emerge regarding the human-induced climate change connections: first, that warmer air can hold more moisture, leading to increased rainfall; second, that changes in the jet stream—a high-altitude river of air that drives global weather systems—may also be influencing severe weather occurrences.
The immediate cause of the flooding has been linked to a cut-off low-pressure storm system, which has been generated by an unusually wavy and stagnant jet stream. This system stalled over the region, unleashing relentless rain. Meteorologists have noted that such systems, known by their Spanish acronym DANA, frequently occur in Spain.
In contrast, a persistent high-pressure system in the U.S. has created a “dome” of dry air, effectively pushing away storms and resulting in widespread drought conditions. Jeff Masters, a meteorologist and co-founder of Weather Underground, explained, “If we are experiencing dryness, someone else is getting all the rain.”
Jennifer Francis, a climate scientist at the Woods Hole Climate Research Center, highlighted the interconnected nature of these weather patterns. She noted that the same wavy jet stream contributing to drought in the U.S. is also responsible for the severe flooding in eastern Spain. This theory attributes the changes in the jet stream to climate change, particularly as the Arctic warms at a rate faster than the rest of the planet. Although this idea is gaining traction, the scientific community has yet to reach a consensus on its implications.
Maria Jose Sanz, scientific director at BC3 Basque Centre for Climate Change, emphasized that climate change is causing more pronounced fluctuations in the jet stream, leading to increased occurrences of DANAs, which are often more frequent during the winter months.
Erich Fischer, a climate scientist at ETH Zurich, acknowledged the impact of stalled low-pressure systems on this autumn’s flooding in Europe. He pointed out that the region has witnessed a series of flooding events in recent months, with notable incidents occurring in France, Italy, and the Balkans. Fischer noted that the systems have remained fixed, allowing for hours of continuous rain in the same valleys.
The underlying physics of these storms may also suggest that, regardless of jet stream changes, the basic principles are making such storms wetter. The Clausius-Clapeyron relation, a fundamental equation in physics, indicates that for each degree Celsius of warming, air can hold about 7% more moisture. Frederick Otto, a climate scientist at Imperial College, mentioned that global temperatures have risen by approximately 1.3 degrees Celsius due to greenhouse gases, which could result in a 9% to 10% increase in heavy rainfall events.
Masters pointed out that the increased moisture in the atmosphere can lead to a feedback loop. When moisture condenses, it releases heat energy, which energizes storms, enhances updrafts, and allows storms to draw in even more moisture, potentially increasing rainfall by 20%.
Historical comparisons reveal alarming trends; Fischer recalled a similar storm in 1957 that resulted in about 250 millimeters (10 inches) of rain. However, recent events have seen almost double that amount falling in a matter of hours, largely due to increased moisture retention in the atmosphere, exacerbated by warmer conditions.
Adding to this recipe for disaster is the unusually warm Mediterranean Sea, which recorded an average surface temperature of 28.47 degrees Celsius in mid-August. Carola Koenig, from the Flood Risk and Resilience Center at Brunel University London, noted that warmer sea temperatures can significantly enhance moisture retention in the atmosphere, resulting in heavier rainfall as autumn progresses.
While methods for attributing the impacts of climate change may vary, the consensus remains clear: the burning of fossil fuels is driving climate change, which in turn leads to increased death and destruction due to extreme weather events. As Valencia faces the aftermath of this catastrophic flooding, the implications of climate change on our weather patterns have never been more apparent.
