Spring flooding along many of Europe's rivers is starting weeks earlier now than it did 50 years ago, and some winter flooding is starting later as the climate changes, a new study shows.
The changes in timing are geographically nuanced, but the trends and their connections to rising global temperatures are clear and are likely to continue into the next several decades, a team of international scientists wrote Thursday in the journal Science.
The researchers, from across Europe, analyzed data from more than 4,200 river gages between 1960 and 2010, covering all major European river basins.
In Scandinavia and the Baltics—where snowmelt is the main driver of high water—spring floods are coming a week earlier than they did 50 years ago. Along the North Atlantic coast from Portugal to England, winter flooding in many places occurs 15 to 36 days earlier, driven by changes in the timing of winter rain storms and changes in soil moisture.
"The trends we found are in line with projections from climate models. We can see climate change very clearly," said co-author Berit Arheimer of the Swedish Meteorological and Hydrological Institute.
The study focused on the timing of floods, which is considered one of the most sensitive indicators of climate change because it's less affected by land use changes, like deforestation, urban development or agricultural uses—all of which can affect the magnitude of floods.
The trend to earlier spring floods in Scandinavia is already requiring hydropower facilities to reconsider when to store and release water, and seasonally drying up wetlands that have evolved with certain seasonal flooding patterns, Arheimer said. The shift in earlier flooding will also require water reservoir managers to recalibrate their release strategies to ensure enough water for irrigation, power systems and communities throughout the year, the authors said.
Officials who are responsible for protecting communities from flood dangers also need to consider the trends as they plan defensive measures like urban stormwater runoff systems, she said. Globally, river flooding affects more people than any other natural hazard, with an estimated global annual average loss of $104 billion. Most climate change research projects those damages to increase in regions that will experience intensified rainfall in a world warmed by greenhouse gases.
In snow-dominated Scandinavia, for example, river flows follow distinct seasonal patterns linked directly with temperatures. As Scandinavia has warmed, there's less snow to begin with and it's melting considerably earlier, Arheimer said.
"For the 50-year study period, 80 percent the stations (in Scandinavia) showed a shift toward earlier floods, with a very clear rise in temperature across the domain," she said.
The study documented similar changes in northwestern European rivers flowing to the Atlantic Coast, but for different reasons, said Jamie Hannaford, a researcher at the UK Center for Ecology and Hydrology. In this region, from Portugal through Spain, France and England, heavy rainfall is usually the main driver of floods, and the primary flood season is winter.
It's a much more complex picture here than in Scandinavia, Hannaford said.
Some of the changes in rainfall and flooding in the region are linked with large-scale changes in the way weather systems move across the mid-latitudes of the Northern Hemisphere, and those changes are probably linked with the shrinking polar ice cap, according to recent research. Hannaford said the earlier flooding is also clearly linked with earlier saturation of soils across the region, as early winter storms soak the area.
"Our study suggests flooding is sensitive to climate in northwestern Europe, and that we should continue to monitor for timing ... because the changes could have serious implications for flood risk management," he said. Half the stations in the Atlantic Coast region between Portugal and England showed a shift toward earlier floods by at least 15 days in the 50-year study period, and 25 percent showed a shift of more than 36 days.
The study also documented later flooding around the North Sea in southwestern Norway, the Netherlands, Denmark and Scotland, where 50 percent of the stations show a shift toward later floods by more than eight days in the past 50 years. Along parts of the Mediterranean coast, including the northeastern Adriatic and northeastern Spain, the shift toward later floods is smaller, at about five days. In both regions, the researchers attributed the change to a later onset of winter storms.
"Later winter floods around the North Sea would lead to softer ground for spring farming operations, higher soil compaction, enhanced erosion, and direct crop damage, thereby reducing agricultural productivity," Hannaford concluded.
Risks to Dams Show Need for More Data
"This paper highlights the importance of understanding uncertainties," said Loughborough University geographer and hydrologist Louise Slater, who was not involved in the study but wrote a related article in Science describing some of the implications of the changes. As an example, she cited February 2017 flood damage to the Oroville Dam in northern California, where 188,000 people had to be evacuated on short notice.
It's important that climate change impacts are considered in the governance of dams, she said. Safety inspections and other operational measures should be based on the best and latest scientific information.
"Some of these recent cases show us we should be looking at our understanding of floods in a more probabilistic way," she said.
The data from the new study can help give resource managers some advance notice on how river flows and floods may shift. That affects how dam operators adjust flows to balance the competing needs for hydropower and downstream flood control, for example.
In the UK, Slater said a new training facility for flood rescue is using climate change projections to make sure that first responders are prepared for conditions they may encounter in the future. River flow and flood data can also be used for "smart monitoring" to inform regional early warning systems that help keep people safe. she said.
"This is by far the most comprehensive flood dataset for European river flooding. It allowed us to see continental patterns. Yes, climate change has impacted flood timing in different ways in different parts of Europe," said lead author Günter Blöschl, a hydrology researcher with Vienna Technical University. The results will help verify existing climate change projections and make future projections more accurate, he added.