Study links major floods in North America and Europe to multi-decade ocean patterns

The number of major floods in natural rivers across Europe and North America has not increased overall during the past 80 years, a recent study has concluded. Instead researchers found that the occurrence of major flooding in North America and Europe often varies with North Atlantic Ocean temperature patterns.

This new study is by far the largest scale analysis of major flood trends for watersheds that are minimally disturbed by human activities. It provides vital information to help understand the most common and widespread of all natural hazards on Earth—a hazard that causes substantial losses of life and property.

“This study is unique in that it examined trends in major floods only—those with 25-year or longer return periods—that typically cause the most damage to infrastructure,” said USGS research hydrologist Glenn Hodgkins, who led an international team of scientists in the study. “We examined historical streamflow data from more than 1200 diverse but minimally altered watersheds across two continents.”

"We are fortunate that agencies in many countries had the foresight to establish reference hydrologic networks that provide the high quality long term streamflow records that make this type of study possible,” said co-author Paul Whitfield of the University of Saskatchewan.

These study findings are consistent with the Intergovernmental Panel on Climate Change conclusion that globally there is no clear and widespread evidence of changes over time in flood magnitude or frequency in observed flood records.

A common assumption is that because warmer air holds more water, climate warming will cause more frequent and intense precipitation and therefore more major flooding. However, flood generating processes are often more complex. Other factors influencing the magnitude of floods include snowmelt runoff and soil moisture prior to rain events.

Significant trends over time in the number of major floods varied according to watershed size, location, climate region, flood threshold, and period of record, indicating that generalizations about flood trends across large areas or a diversity of watershed types are ungrounded.

The number of major floods in both North America and Europe varied with the Atlantic Multidecadal Oscillation (AMO), a measure of North Atlantic sea-surface temperature variability. During warm phases of the AMO, the number of major floods for many large watersheds in North America was reduced while the number increased for medium-sized watersheds in Europe.

“Little research has been completed on the relation between major floods and ocean patterns such as the Atlantic Multidecadal Oscillation and the Pacific Decadal Oscillation. Our study shows that more research is needed to fully understand these connections,” said study co-author Jamie Hannaford of the United Kingdom-based Centre for Ecology & Hydrology.

To be informative about climate-driven flood trends, watersheds that were subject to confounding human influences on flooding, such as urbanization and reservoir regulation, were screened out of this study. Watersheds used in this study varied in size and other characteristics and were chosen to provide the most reliable and pertinent data. The study has produced the most comprehensive international dataset of minimally altered watersheds yet assembled and is notable for the coverage of 13 countries across two continents.