Wednesday, September 2, 2015
Sea levels are rising globally at an increasing rate—three inches in the last 25 years or so--but we’re not seeing that much in Hawai`i.
So what’s up?
One answer to this mystery is that our islands are in a kind of temporary sweet spot. Satellite imagery shows that while most of the globe has seen dramatic rises in sea levels—as much as 3 inches in the past 25 years—Hawai`i has been flat to actually lower.
(Image: The red shows areas of dramatic sea level rise. Blue shows areas where it’s flat or down. Hawai`i appears in the blue zone. But how long will that last? Credit: NASA.)
At a human scale, this makes no sense. If you fill a bathtub, clearly it fills all around the tub.
But global scales are different. There are humps and valleys in the oceans across the scale of thousands of miles. Winds can push water up against a coast, creating a hump. Eddies can change sea levels regionally. Currents and storms and tides and even temperatures can all impact the height of the ocean.
“In a nutshell it's due to changes in winds and ocean circulation that counteract the global sea level signal regionally. That should shift as part of a long-term fluctuation but no projections on when that is likely to occur. This big El Nino may herald the start of a shift, but we have to see how that plays out,” said University of Hawai`i oceanographer Mark Merrifield.
Here is NASA’s recent report on accelerating sea levels and related issues.
“Sea levels are rising rapidly—much more rapidly than they have any time in the last several thousand years,” said NASA’s Joshua Willis, at the Jet Propulsion Laboratory.
And the rate of increase has been increasing as well, he said. It was about 1 millimeter annually in 1900, rose to 2 millimeters annually in the mid 1900s, and is now at 3 millimeters annually. That works out to more than an inch a decade.
I wrote to Willis to ask whether our islands can continue to dodge this bullet.
“In the long run sea level rise will affect Hawaii as well. Because it is in the central Pacific, the impacts of the long-term natural cycles may not be quite as large. Eventually, however, the global rates of rise will be felt in Hawaii also,” he said.
University of Hawai`i coastal geologist Charles “Chip” Fletcher agreed.
“The global oceans cannot keep rising without us experiencing the rise as well - we just may be able to avoid the worst aspects of the variability. On the other hand, models show that the tropics as a region will experience the upper end of global sea level change, so that makes us part of a more dangerous region.
“I have held for several years that Hawaii should plan for one meter of sea level rise by end of century and as far as I can see that is still a valid number,” he said.
© Jan TenBruggencate 2015