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The Honolulu Advertiser
Posted on: Monday, November 13, 2006

Big shifts in sea level over time

By Jan TenBruggencate
Advertiser Columnist

In the discussion of coastal erosion and sea level change, it's interesting to look at Hawai'i evidence of how sea levels have shifted over tens of thousands of years.

For example, during the last interglacial period roughly 125,000 years ago Hawaiian coastlines looked dramatically different from today's. Global temperatures were similar to what they are now. Ocean levels were high, scientists suggest, both because glaciers were melting and because the ocean water was warmer, and warm water takes up more space than cooler water.

Sea levels were not inches but feet higher than they are today. The evidence is that across the state, you can find former reefs that now stand on dry land.

"I think there is evidence that the shoreline was quite a bit farther inland than it is today," said Kaua'i geologist Chuck Blay.

He said there is a coral reef in Hanalei Valley that is 6 feet higher than the present sea level. There is also evidence at Maha'ulepu of a time when sea levels were higher, and thus the beach was farther inland than today. Similar research has been done on O'ahu, where geologists have mapped inland limestone reefs with identifiable kinds of corals corals that grow today on our still-wet reefs.

A U.S. Geological Survey paper, available online at http://esp.cr.usgs.gov/info/lite/hawaii/hawaii.html, cites corals inland at Waimanalo, Diamond Head, Kahuku and elsewhere. Some of the Waimanalo corals have been dated using radioactive element techniques and found to date to the time of the last interglacial period.

Blay said it's important to understand that O'ahu itself has also been rising, and lifting the old reef evidence with it so where the old reefs stand doesn't accurately reflect where the water was. The USGS report suggests sea levels at that time were actually just 3 to 6 feet higher than now.

None of this necessarily means that global climate conditions will drive sea levels up several feet just that they once did.

And sea levels can go the other way, too. Divers have found numerous "drowned" reefs in Island waters, which form deepwater terraces on the underwater slopes of the Islands. Some are several hundred feet deep.

During the last "glacial maximum" about 20,000 years ago, a lot of the planet's water was locked up in ice, and sea levels were more than 350 feet lower than today. The shallow channels of Maui County were dry then, forming the super-island Maui Nui.

If you have a question or concern about the Hawaiian environment, drop a note to Jan TenBruggencate at P.O. Box 524, Lihu'e, HI 96766 or jant@honoluluadvertiser.com. Or call him at (808) 245-3074.