How High or Low Can the Oceans Go?

Hanalei Bay

We think of sea level changes in inches over periods of decades, but over longer time spans, the evidence is for far more dramatic rises and falls.

How dramatic? Hundreds of feet different. Whole Hawaiian valley floors today were once underwater when oceans were higher. And some shallow inter-island channels—notably in Maui County — were land bridges.

It’s all about time.

Once you stop thinking about years and centuries, and start thinking in terms of tens and hundreds of thousands of years, the sea level turns out to be dynamic, with vast amounts of our planet’s water pulsing between being stored in the ocean and being stored in huge ice sheets on land.

Just a few years ago, researchers were warning that climate change would result in sea levels rising six or seven inches over a century, and that’s been the rate of increase generally over the past century.

The best estimates today are that it is rising faster than it was last century. Maybe a few feet instead of a few inches over the next century.

That means children born today will see dramatically different coastlines by the time they’re nearing retirement age.

But they’ll see nothing like what’s been happening on a geologic time scale.

All we’ve heard about in recent decades is sea level rise, but the world isn’t just about rising sea levels. The last ice age was 100,000 years ago, and as little as 18,000 years ago ocean levels were 400 feet lower than now.

Today, you’d need sophisticated diving equipment or submarines to get down to where the coastline was back then.

“Hanalei was not a bay then. It was a big river valley” that went out beyond the outside of the current bay, said Kaua`i geologist Chuck Blay.

Blay is the author of “Kaua‘i’s Geologic History: A simplified overview.” The book is available online, but also at places like Koke‘e Museum, Koke‘e Lodge, Kaua‘i Museum, Kilauea Light House, NTBG Visitor Center in Po‘ipū, the Kauaʻi Store in Kapaʻa, Kilauea Lighthouse, Talk Story Bookstore in Hanapepe and the St. Regis hotel bookshop in Princeville.

When water levels were 400 feet lower, the islands of Maui County were connected, and the combined island of Maui Nui — including Maui, Molokai, Lanaʻi and Kahoʻolawe — created an island 25 percent bigger than the Big Island is today, Blay said.

What else was different?

When sea levels were that low, the beach on Kaua`i would have been two to five miles seaward of where beaches are now.

But just 3,000 to 4,000 years ago, seas were six feet higher than they are today. Hanalei Bay extended farther inland than it does now, and some of the inner taro field lands were likely salty marshes.

These high sea levels are natural to us, but are actually quite unusual, Blay said. The ocean has only ever been this high rarely — just 15 percent of the time over the last 435,000 years, he said.

• Jan TenBruggencate is a Kaua‘i based writer and communications consultant.