365 Days of Climate Awareness 213 – Dynamic Equilibrium

“Stable” is a dangerous word in science, because it can mean physical stasis. In a world where matter and energy are in constant motion, calling something “stable” can be a grave mistake. In the case of sedimentary geology, the physical form of a beach or a river or stream bed changes in response to changing conditions. While at any given moment a beach or a river bed isn’t tottering in instability, it is in a constant process of being molded by the forces around it: the flow of water, the crash of waves, the wind overhead. The concept of a geological form which doesn’t disappear, but is in constant evolution due to changing conditions is called “dynamic equilibrium”.  

There is no end state to a beach. Its current form is the product of the influences on it to that point. Tidal regime, climate (with its changes), sediment supply, mineralogy of the sand—all of these factors affect what form a beach takes when you see it. You might go see a beach today. The next time you go, there will be some differences. Wait long enough, and there might be some big differences.  (And “he who sees the most beaches wins,” my advisor Jon Boothroyd used to tell me.)

The beach which has just been hollowed out by a massive coastal storm is not an aberration. It is the form circumstances have created at that time. Two years later after steady onshore swash and longshore transport have brought sand in and created a beach which resembles the one before the storm…that might be form we consider typical, but it is no more natural than the depleted version. The hydrodynamics of waves, currents and tides combine with the natural supply of sediment to build the form we see, and that form is always undergoing change.

Barrier island erosion, pre- and post-Katrina (August 2005).

Barrier islands are built by the combination of tides, large waves, and large sediment supply. Being essentially huge dunes which rise above the ocean surface, they are not consolidated, and are subject to motion. And they do move, in some cases eroding rapidly due to the regular action of waves and tides. Sediment which is removed from one part of an island might be deposited on another part, or brought to another island altogether. A storm could punch one or several holes through a barrier island, or destroy it altogether. But across centuries, though the specific forms of barrier islands change greatly, they continue to exist along stretches of coast. That is dynamic equilibrium.

Barrier island depostion, 2005-07.

One of the more egregious bad-faith arguments by climate change deniers in recent years was by a senator who opined [paraphrasing], “If the sea’s rising, why haven’t we lost the beaches?” Total sea level rise since pre-industrial times now is a few tens of centimeters. The very simple answer lies in the beaches we see now, after sea levels have changed—both rising and falling, depending on location—by hundreds of feet over the past 10,000 years. The beach moves with the sea.

Are there locations where an ocean highstand has left a former beach perched on the upland? Yes! On the coast of Maine, there are hills where you can see, under the leafdrift, old rounded cobbles when from when the ocean lapped at the hill’s flanks before postglacial rebound lifted it far above the ocean. Where the ocean is encroaching on land—called a transgressive coast—the beach is continually rebuilt along with the sea level rise. In some few cases, the flooding might be so rapid as to leave the beach submerged, but that is very rare.

Tomorrow: return to Kiribati.

Be brave, and be well.