Absolute sea level in the Gulf of Alaska has been falling, contradicting a global trend, according to a new study that focused mainly on the "good news-bad news" situation in Chesapeake Bay on the United States' East Coast. But Alaska scientists have found a more complicated picture.
The study, reported last week by researchers at Virginia Institute of Marine Science, found that the "absolute" level of the ocean in Chesapeake Bay area was rising only about 1.8 millimeters per year (the good news). But after they calculated how fast the land in that region was subsiding, it turned out that the "relative" level of the sea in the bay was rising much, much faster than a couple of millimeters per year (the bad news).
Climate and marine scientists have warned that absolute global sea level is now rising 3.1 millimeters annually -- about one-tenth of an inch per year -- due to the thermal expansion of warming ocean water combined with glacial and ice sheet runoff. But this satellite-generated figure doesn't say much about boating conditions in Southern Alaska, home to huge tidal ranges and dramatic ongoing shifts in the height and shape of the landscape.
A release about the Chesapeake study made one cryptic reference to the Gulf of Alaska, identifying it as one of the few places on the Earth where "absolute" sea level is falling. It stated the Gulf's "absolute" level is dropping due to currents, glacier melt-water and chilly sea temperatures -- colder water is denser and takes up less space.
But as any Alaskan captain with a coffee-stained, duct-taped tide book will tell you, "absolute sea level" means absolutely nothing when you're trying to navigate across mud flats to the fishing hole.
It's the relative sea level that counts -- the depth of the water over those hull-gouging pinnacles and rock piles.
So what can we say about sea level trends along Alaska's coast? It's the geology, stupid.
"The land surface in Southern Alaska is moving faster than global sea level is presently changing," says Jeff Freymueller, a scientist with the Geophysical Institute and Department of Geology and Geophysics at the University of Alaska Fairbanks. "But rising sea level is likely to 'catch up' eventually."
In a presentation posted on the web earlier this month by the Alaska Center for Climate Assessment and Policy, Freymueller reported that land along Cook Inlet and the Kenai Peninsula is rising two to four times faster than the ocean, and that means "relative" sea level is falling. Blame this geographic uplift on the colossal collision between the North American and Pacific tectonic plates, plus some rebound from the melting of massive glaciers at the end of the ice age.
For instance, land at the Homer airport has been rising about 8.6 millimeters per year since 1997, according to Freymueller. In Seldovia, land is rising a bit faster -- almost 11 millimeters per year since 2001.
Parts of Prince William Sound, on the other hand, have been seeing a rapid rise in relative sea level, Freymueller said. The land in that area is now subsiding due to tectonic pressures building ever since the 1964 earthquake, the second largest ever recorded on the planet.
In Alaska, geology trumps climate when it comes to sea level.
"Transporting water to the ocean from melting glaciers and ice sheets changes Earth's gravity field and causes uplift of the surface due to removal of the ice load," Freymueller explained. "Both of these effects cause regional variations in relative sea level, which can be larger in magnitude than the global average rise. In addition, vertical tectonic motions along large regions of the Alaska coast are more rapid than sea level change."
You can view Freymueller's presentation via a video, podcast or PDF file at the ACCAP website.
Doug O'Harra is an Anchorage writer.