As ocean waves rise and fall, they exert force on the seafloor below, creating seismic waves. These seismic waves are so strong and widespread that they appear as a constant sound on seismographs, the same instruments used to monitor and study earthquakes.
This wave signal has become more intense in recent years, reflecting increasingly stormy seas and higher ocean waves.
In a new study published in the journal Nature Communications, my colleagues and I tracked this increase worldwide over the past four decades. These global data, along with other ocean, satellite and regional seismic studies, show a decades-long increase in wave energy coinciding with increasing storms attributed to rising global temperatures.
What does seismology have to do with ocean waves?
Global seismographic networks are known for monitoring and studying earthquakes and allowing scientists to create images of the planet’s deep interior.
These highly sensitive instruments continuously record a wide range of natural and human-caused seismic events, including volcanic eruptions, nuclear and other explosions, meteor impacts, landslides and glacial earthquakes. They also capture persistent seismic signals from wind, water and human activities. For example, seismographic networks observed a global reduction in human-induced seismic noise as quarantine measures were implemented worldwide during the coronavirus pandemic.
However, the most common of the seismic background signals globally is the incessant hum created by storm-driven ocean waves, called global microseismism.
Two types of seismic signals
Ocean waves produce microseismic signals in two different ways.
The most energetic of the two, known as secondary microseismism, pulsates for a duration of approximately eight to 14 seconds. As wave clusters move in various directions across the oceans, they interfere with each other and create pressure changes on the sea floor. However, interfering waves are not always present, so in this sense they are an imperfect proxy for overall ocean wave activity.
The second way ocean waves produce global seismic signals is called the primary microseismic process. These signals result from traveling ocean waves directly pushing and pulling on the seafloor. This occurs in areas where the water depth is less than about 1,000 feet (about 300 meters), since water movement in waves decreases rapidly with depth. The primary microseismic signal can be seen in the seismic data as a steady hum with a period between 14 and 20 seconds.
What does the shaking planet tell us?
In our study, we estimated and analyzed historical primary microseism intensity at 52 seismograph sites around the world with a long history of continuous records dating back to the late 1980s.
We found that 41 (79%) of these stations showed extremely significant and gradual increases in energy over decades.
The results show that global average ocean wave energy has increased by an average of 0.27% per year since the late 20th century. However, since 2000 the global average increase in this rate has increased by 0.35% per year.
We found the largest microsis energy in very stormy Southern Ocean regions near the Antarctic peninsula. But these results show that North Atlantic waves have intensified most rapidly in recent years compared to historical levels. This is consistent with recent research suggesting that North Atlantic storm intensity and coastal hazards are increasing. Storm Ciarán, which hit Europe with powerful waves and hurricane-force winds in November 2023, was one record-breaking example.
Decades-long microsis records also show seasonal oscillations of strong winter storms between the Northern and Southern hemispheres. It captures the wave-dampening effects of growing and shrinking Antarctic sea ice, as well as the multi-year highs and lows associated with El Niño and La Niña cycles and their long-term effects on ocean waves and storms.
These and other recent seismic studies complement results from climate and ocean research showing that storms and waves are intensifying as the climate warms.
coastal warning
Oceans have absorbed nearly 90% of the excess heat due to increased greenhouse gas emissions from human activities in recent years. This excess energy can translate into more damaging waves and stronger storms.
Our results offer another warning for coastal communities where increasing ocean wave heights could batter shorelines, damage infrastructure, and erode land. The effects of increased wave energy are exacerbated by climate change and ongoing sea level rise due to subsidence. They highlight the importance of mitigating climate change and building resilience in coastal infrastructure and environmental protection strategies.
This article is republished from The Conversation, an independent, nonprofit news organization providing facts and authoritative analysis to help you understand our complex world. The Conversation has a wide range of fascinating free newsletters.
Written by: Richard Aster Colorado State University.
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Richard Aster receives funding from the US National Science Foundation.