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Regional Interests

The Moon Has Always ‘Wobbled.’ What’s New is Climate Change.

So, about that moon wobble …

Last week, NASA published a new study in the journal Nature Climate Change that found rising sea levels plus higher tides caused by an oscillation in the moon’s orbit will dramatically increase the number of sunny day floods experienced in U.S. coastal communities.

The Bay Area is one of the most populous and flood-threatened shorelines of the entire West Coast. Places like San Francisco’s Embarcadero and the exit off Northbound Highway 101 toward Mill Valley already regularly flood during King Tides, along with communities like East Palo Alto in the South Bay.

NASA researchers say it could get a lot worse — and sooner than expected.

“The assessment we made for the tide gauge in San Francisco is that we’d see about a five time increase in the number of high-tide flood days,” said Ben Hamlington, who co-authored the study.

With that in mind, he says the region’s current sea level rise adaptation plans may not go far enough, and might be moving too slowly.

A regular oscillation in the lunar orbit — what researchers have dubbed a “moon wobble” — will temporarily suppress flooding from rising sea levels this decade, only to worsen across the Bay Area in the mid-2030s as the moon sways back.

Carbon pollution is trapping heat and warming the planet, even as companies and governments continue to emit planet warming gases with little abatement. The wobble will do nothing to limit the melting of ice sheets in Greenland and Antarctica, which is accelerating and being driven by these rising temperatures.

The fact that the moon wobbles is not a new discovery. Scientists have known about the lunar sways, which take 18.6 years to complete, for centuries. The first half of the pattern is marked in the U.S. by lower tides and the second — which the moon is currently in — by high tides.

“There’s nothing new or dangerous about the wobble; it was first reported in 1728,” NASA’s release said. “What’s new is how one of the wobble’s effects on the moon’s gravitational pull — the main cause of Earth’s tides — will combine with rising sea levels resulting from the planet’s warming.”

Scientists worry that as the tides amplify in the next decade, cities around the Bay will see more flooding. The San Francisco Bay has risen by about eight inches since 1990, the Mercury News reported.  But by the midcentury that could increase by up to 1.9 feet, before accelerating even more towards the end of the century.

Phil Thompson, lead author of the study, said cities will likely experience accumulating impacts from what academics call nuisance flooding — water from high tides spilling on streets, into parking lots, parks, and neighborhoods, regularly disrupting life and business. “If it floods 10 or 15 times a month, a business can’t keep operating with its parking lot under water,” he said.

The real doozy, Hamlington says, is that a certain level of sea rise is already baked in, and will surge as the temperatures rise and oceans warm. San Francisco could see 31 additional flooding days per year on average during the 2030s.

The team examined the impacts under low-medium and medium climate warming emissions scenarios. They forecast that flooding will be experienced at different moments across the Bay Area.

Hamlington says natural drivers of flooding like El Niño could also make flooding worse, and the impact of sea level rise will be clearly evident in the next decade.

“A lot of those flooding days that you see now are going to be that much more severe,” he said. “We are going to see some abrupt transitions. I think it’s important for coastal planners to understand that, in a lot of cases, you need to start preparing now. Some of the adaptation strategies that are being discussed need to be put in place sooner rather than later.”

Copyright 2021 KQED