Changing wave conditions: the other coastal threat

A guest blog by Christie Hegermiller

February 2016: Large storm waves generated in the middle of the Pacific Ocean reach the California coast with incredible energy. During El Niño winters, storm waves result in massive beach and cliff erosion. In Santa Cruz, CA, parts of a well-used cliff path eroded into the ocean during a storm (Photo credit: Christie Hegermiller, USGS).

February 2016: Large storm waves generated in the middle of the Pacific Ocean reach the California coast with incredible energy. During El Niño winters, storm waves result in massive beach and cliff erosion. In Santa Cruz, CA, parts of a well-used cliff path eroded into the ocean during a storm (Photo credit: Christie Hegermiller, USGS).

A winter walk along the cliffs in Santa Cruz this year reveals the dynamic nature of the ocean. Sections of well-used paths slump down the bluffs, kelp seaweed and sand lie across the roads, coastal properties are evacuated, and powerful waves pose great danger for those witnessing the spectacle. On the other side of the country, the Northeast faced an unusual start to the winter, with record warm temperatures on Christmas Eve. These two distant, abnormal experiences have the same root cause: a strong El Niño that is impacting not only the coastal communities of California, but also regions thousands of miles away. El Niño is an atmospheric and oceanic phenomenon that begins as a weakening of normal winds in the tropical Pacific and a resulting pulse of warm water traveling eastward across the tropics toward the coast of South America. This bulge of warm water then travels both north and south. As a result, the California coast experiences temporarily elevated sea levels, unusually warm water, increased rainfall, and large storm waves approaching from west-southwesterly directions. These changes can have serious impacts, ranging from decreased algal growth in coastal waters, damage to kelp forests, and, notably, increased beach and cliff erosion due to waves.

Oceanic conditions also change as a result of global climate change. California’s coastline is particularly vulnerable to these changes. With 21 million people living in coastal counties as of 2010, California contains roughly 30% of the United States’ coastal population. Projecting the effects of climate change on California’s coastline is analogous to assembling a puzzle, in which each piece interacts with the next in a complex manner. Though much scientific research and public effort has focused on sea level rise projection and adaptation, it is the combination of sea level rise and changes in waves that threatens coastal systems. As the atmosphere responds to global climate change, winds and storm tracks shift and alter their strength, affecting the height, period, and direction of waves. If we look to El Niño as an example, it is clear that changes in normal and extreme waves can have dramatic negative effects on California’s cliffs and beaches. As a result, California’s coastal homes, ports, power plants, wastewater treatment facilities, agriculture, and highways are threatened. Coastal managers and engineers increasingly require rigorous predictions of future wave conditions in order to develop strategies to protect coastal communities. To provide these data, I am collaborating with U.S. Geological Survey scientists to develop robust methods for making such predictions.

Extreme waves during this 2015-2016 El Niño result in cliff erosion at Ocean Beach, CA. A major road sits atop this cliff (Photo credit: Alex Snyder, Pat Limber, Christie Hegermiller, USGS).

Extreme waves during this 2015-2016 El Niño result in cliff erosion at Ocean Beach, CA. A major road sits atop this cliff (Photo credit: Alex Snyder, Pat Limber, Christie Hegermiller, USGS).

Global climate models are used to predict changes in important aspects of climate, such as changes in temperature, precipitation, and ice extent. These models are extremely complex and represent the critical, interrelated components of the climate system, including obvious parts such as the ocean, atmosphere, land, and ice sheets, and less obvious parts such as aerosols, land-use changes, and the carbon cycle. Though global climate models are irreplaceable for predicting worldwide changes, they are unable to provide useful predictions at regional and local scales, such as the California coast or San Francisco. Thus, I work to “downscale” winds and other atmospheric fields from global climate models to predict how waves will change along our coastline. Our work – the first attempt to project future waves for the California coast – suggests we may see unexpected changes in the height, period, and direction of waves. By providing this information, my work helps local communities identify vulnerable coastlines and infrastructure to prepare for future changes. As we continue to add pieces to the puzzle of climate change’s local effects, we move towards a more complete picture of what the future might look like. 

Christie Hegermiller is a Ph.D. Candidate in the Ocean Sciences Department at UCSC.