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Ebb Tide

“The physics of sand is not rocket science; it may be even harder.”
— Albert Einstein, warning his son to steer clear of coastal geology.

It is a cool, crisp September day on Plum Island, Massachusetts. Evidence of change is everywhere.

The early morning sun glints off a gelatinous mass of squid eggs stranded in a sandy tidal pool. Last night millions of the fleshy mollusks congregated offshore to mate and lay their eggs. Males darted in and out of the school trying to herd individual females away from the pack. They had to grapple, struggle, and occasionally bite their opponents to secure an unattached female.

But this is a new ritual on this beach. Last year, 2012, was the first time that large numbers of squid started to swim north of Cape Cod to spawn. Global warming has finally warmed the oceans enough to cause such biological change.

There are other changes in this land. The sand dunes are shifting. In the spring the prevailing winds turned to the southwest, and now each incoming tide is repairing the beach from last winter’s gales. It is a subtle process. Each wave stirs up a few sediments, suspends them in the water column, then redeposits them in a delicate filigree of sand grains a few inches down the beach. But the ultimate result is dramatic: Waves can return as much as 60 feet of sand from offshore sandbars to the beach in a single tidal cycle.

The sediments are also being transported to this spot from a reservoir of Paleolithic sand left by Ice Age glaciers. Unprotected by wide summer beaches, these drumlins had been exposed to the full fury of last winter’s storms. Now the sand is being carried in longshore currents that travel parallel to the beach in the runnels that lie between the sandbars and the shore. It is this constant destruction and renewal that keep barrier beach islands healthy. As long as such a beach can move, pulsate, and grow, it can continue to reform and gradually migrate landward.

Changes also are occurring on a geological time scale. I am standing in the center of the island. In 1879, a beachcomber discovered the remains of a woolly mammoth protruding out of a 50-foot-high sand dune just east of here. The prevailing winds had been blowing the entire sand dune steadily landward, leaving the massive skull, leg bone, and backbone exposed. In this way, both winds and waves constantly cause barrier beaches and sand dunes to migrate by essentially rolling over themselves.

Ten thousand years ago, this woolly mammoth had been walking through an upland area covered with a half-mile of ice, snow, and emergent arctic vegetation. The sea level was 250 feet lower, and the coast was three miles further east.

But change was in the air. The atmosphere was warming, the glaciers were retreating, and the island itself was 3½ miles from its present location. It has been slowly migrating inland ever since, but in only the last 20 years has the rate of erosion increased from about two feet a year to 13 feet a year — enough to catch the attention of humans. Last winter, six houses pitch-poled down this dune, and 30 more were declared uninhabitable.

All the lost houses had been immediately downstream of man-made groins built to stop this beach from eroding. But a barrier beach is a living being; it needs to be able to grow. As long as a beach can move, it can repair itself after every storm. But once you build an immovable structure on a beach, you stop that flow of sand and doom the beach to the kind of destruction we witnessed last year.

Nature is telling us that there are no technological solutions to living on a barrier beach. You cannot force a beach to stop moving. We have to start thinking of leaving these fragile areas, so they can continue to protect the mainland from the full effects of sea-level rise.