During extreme storms ocean waves can be more than 20 metres high. More than just being a product of weather systems, waves are critical for ocean shipping, the stability of beaches, coastal inundation or flooding and determining the design of coastal and offshore structures.
Professor Ian Young and Dr Agustinus Ribal of the University of Melbourne have shown in their new research, published in Science last week, that ocean waves, and the winds that generate them, are increasing in magnitude and have been doing so for the last 30 years.
In order to detect changes in ocean wind and wave climates, a global database of satellite measurements of wind and wave heights needed to be created and analysed. The dataset consists of approximately four billion measurements of wind speed and wave height from 31 satellites that were in orbit between 1985 and 2018.
For the past 33 years, these global satellites have been collecting data on ocean waves, and the winds that drive them. By bouncing energy pulses off wave crests and measuring the time those pulses take to come back, instruments called altimeters aboard satellites can measure wave height; the taller the waves, the faster the signal returns. Other satellite instruments monitor changes in the reflectivity of the ocean surface, which is reduced by wind-generated ripples, to estimate the speed of ocean winds.
In order to make this huge data set useful, all of the satellites needed to be precisely calibrated by comparing the satellite measurements with more than 80 ocean buoys deployed around the world.
The research shows that global average wave conditions are increasing but, more importantly, extreme wave conditions are increasing even more rapidly with the largest increases occurring in the Southern Ocean.
The authors found that extreme winds in the Southern Ocean have increased by approximately 1.5 metres per second, or 8% over the last 30 years. Similarly, extreme waves in this same region have increased by 30 centimetres or 5%. Generally, winds are increasing at a faster rate than the waves.
In addition to the increases in the Southern Ocean, extreme winds have also increased in the equatorial Pacific and Atlantic, and the North Atlantic by approximately 0.6 metres per second over the 30-year period.
Although increases of 5% for waves and 8% for winds may not seem like much, if sustained into the future such changes to our climate will have major ramifications. The potential impacts of climate induced sea level rises are well known. What most people don’t understand is that the actual flooding events are caused by storm surges and breaking waves associated with storms.
The increased sea level just makes these wind and wave events more serious and more frequent. Increases in wave height and other properties such as wave direction will further increase the probability of coastal flooding. Changes like these will also cause enhanced coastal erosion, putting at risk coastal settlements and infrastructure.
The authors note that we still don’t know if the historical increases will be sustained into the future. One of the important uses of the extensive satellite database will be to calibrate and validate the next generation of global climate models which are now including ocean wave predictions. Early results from such models yield similar results to the historical record and particularly point to changes in the Southern Ocean.