See climate change in action along Sonoma coastline

Glaciers melting, oceans warming, climate change. What does it mean for Sonoma County?

It’s possible to see climate change in action by heading for the coast, where you can walk on evidence of past climate events and see where it’s happening today. Some of the best viewing spots are easy to find.

The level of the ocean is a good yardstick for climate change because sea levels change with the warming and cooling of the Earth. The Pacific Ocean along California’s coastline has risen and fallen many times in the distant past, sometimes by as much 300 feet, during periods of climate cooling and warming.

When Earth cools, ice caps at the poles and glaciers grow to enormous size, locking up vast amounts of water as ice, which lowers the height of the oceans around the world. When the planet warms again, the ice melts and fills the seas like adding water to a bathtub, and warming also swells the oceans. These events raise or lower the height of the sea at the coast over time. The evidence is still there to see.

Ten minutes south of Jenner, near Goat Rock State Beach, Highway 1 passes a long, flat and nearly level stretch of coastal plain dotted with towering rocks. That flat plain, called a marine terrace, was ocean bottom and underwater 80,000 years ago, covered by surf and waves. The giant rock towers, called sea stacks, once stood out in the surf, just as others do today.

The Kortum Trail now crosses the terrace and passes close by the land-locked sea stacks, and there’s a great view of the entire terrace from the side of the road on the hillside leading to Goat Rock Beach.

Goat Rock itself has a wide, flat top because it was once part of the marine terrace, but the relentless pounding of surf has cut and nearly severed the connection with the mainland, turning it into a virtual island.

That erosion, which wears down rock and cuts cliff faces into the tall coastal bluffs, is a continuous process all along the California coast. But it’s also accelerated by rising sea levels.

Since ice began to melt at the end of the last Ice Age, roughly 20,000 years ago, our Sonoma coastline has shifted eastward and inland by about 15 miles. Before the ice sheets began melting, sea levels were so low it was possible to walk all the way to the Farallon Islands, which today is a 25-mile sail from the Golden Gate.

Today, sea levels are still rising, as the Earth and seas warm, and glacial and polar ice continues to melt. In fact, experts now believe the pace of sea level rise is increasing faster than at any time in the past 2,000 years.

The result at the beach can be dramatic. One effect can be seen by driving a bit further south along Highway 1 near Gleason Beach. The coastal homes standing high on the bluff there are in the process of disappearing because the land they’re standing on is crumbling into the ocean surf below. In 1998, 30 homes sat on this scenic stretch; only a handful remain, and all that’s left of some are dangling concrete pads and exposed pipes and piers.

In fact, the eroding edge of the cliff has now reached the highway blacktop, and next year CalTrans will be forced to move this section of highway 400 feet inland, out of harm’s way.

As a result of climate change, sea levels along California are expected to rise by another 12 inches in the next 20 years, 2 feet by 2050 and as much as 5 feet by the end of this century. Depending on differences in how parts of the coastline resist erosion, that could mean a substantial change in where the actual shore will be in coming years, according to Gary Griggs, director of the Institute of Marine Sciences and Long Marine Laboratory at UC Santa Cruz. His research cites studies of past erosion that suggest surf will break up to 500 feet further inland by 2100 than it does today, with just a moderate 5 feet of sea level rise over that time.

Further inland, the history of past climate change is written in additional flat marine terraces, rising one above the other like stair steps, connected by sloped, steeper faces. These are much older, dating to 100,000, 120,000 and 150,000 years ago, and can be seen and hiked in Salt Point State Park, an hour north of Jenner.

All of the marine terraces along California’s coast have been preserved because they are being slowly pushed up by another force of nature, the collision of coastal plates that cause our earthquakes. Climate change is not something entirely new, as the evidence suggests. But this is the first time humans have lived in such numbers near the water’s edge, and it can be instructive to see some of the scale and effect of such events in the past as a way of preparing for what’s coming.

Views and walks on marine terraces are accessible along the Kortum Trail in Sonoma Coast State Park near Goat Rock, at Fort Ross State Park and at Salt Point State Park, where trails climb several ancient terraces.

Stephen Nett is a Bodega Bay-based Certified California Naturalist, writer and speaker. Contact him at snett@californiaparks.com.