Volcano Watch — Sea level and surf

On Monday and Tuesday of this past week, large waves generated by a storm near New Zealand caused considerable damage on the west coast of Hawai'i, particularly in the town of Kailua. The newspaper accounts attributed the damage to the size of the waves and the fact that they were superposed on a high-high tide of 2.6 feet. These high-high tides are comparable to, or a few inches higher than, the high-high tides recorded in all but a few months of the year.

There is another factor, geologic in nature, that worsened the situation and contributed to the damage. In the nearly 90 years since the sea wall was constructed in Kailua, relative sea level has also changed.

Tide gauge records over the last 50 years show that relative sea level is changing at a rate of 3.6 millimeters per year. Some of the change is due to changes in the volume of the oceans as ice from the polar ice caps melts, but most of it is caused by subsidence of the island. The measured rate of relative sea level change translates into sea level being nearly 13 inches higher when the sea wall was constructed.

Global sea level is rising at rates of about 4.5 inches per 100 years, although the exact rate is the subject of considerable debate. This rise in global sea level has been occurring since the last glacial maxima about 18,000 years ago. At the glacial maxima, global sea level was slightly more than 400 feet below present levels. During much of the intervening time, global sea level rose at rates several times the present rate.

These changes in global sea level are directly related to global climate. When the Earth warms, polar ice caps melt and sea level rises, whereas when the Earth cools, global sea level falls as water is removed from the oceans and stored in the polar ice caps. The warming and cooling cycles of the Earth are caused by cyclic changes in the Earth's orbit around the Sun.

There is also considerable debate about how man's activities, such as the burning of fossil fuels and the resultant slow increase in atmospheric carbon dioxide, might affect global climate. One of the inevitable results of global warming, if it occurs, will be that sea level will rise at accelerated rates. If the polar ice caps continue to melt at present rates, or if their rate of melting accelerates, then many coastal regions will become increasingly vulnerable to high waves in the future.

The second factor in relative sea level changes is the continued subsidence of the island of Hawai'i. The outermost layer of the Earth, the lithosphere, is nearly rigid, but it can bend when a weight, like a volcano, is placed on it. This is exactly what occurs in Hawai'i where the growing weight of the volcanoes flexes the lithosphere downward and results in islandwide subsidence. This subsidence occurs at rates nearly twice that of global sea level rise, or about 2.3 millimeters per year, which translates to about 8.5 inches in the last 90 years.

The older islands in the Hawaiian chain do not continue to sink at these high rates. Tide gauge data from Maui shows a change of 2.2 millimetrers per year, where 1.3 millimeters per year is caused by global sea level change. This means that Maui is subsiding at only about 0.9 millimeters per year, or less than half the rate of Hawai'i. Similarly, the tide gauges at Honolulu and Nawiliwili harbor on Kaua'i show relative sea level changes of about 1.4 millimeters per year, roughly all of which is due to changes in global sea level. From the tide gauge data, we can conclude that O'ahu and Kaua'i are no longer subsiding, or are subsiding only very slowly.

These tiny amounts of subsidence and sea level change slowly add up, and over a period of decades, relative sea level changes significantly. Kailua is now about 13 inches lower than it was when the sea wall was built in the early 1900s, a factor that clearly contributed to the extent of damage from the high surf early this week. With continued subsidence and rise in global sea level, we can expect such damaging periods of high surf more and more frequently in the years and decades to come.