Closing the Loop on Sea Level Rise

Sea level rise is potentially the most important consequence of manmade climate change. The global mean sea level has been rising since the great continental ice-age glaciers began to melt, some 20,000 years ago.  years ago. A sufficient network of tide gauges has been in place since 1900, and the generally  accepted rise global average rise since then is around seven inches.

That’s not a lot, but there’s been considerable uncertainty associated with this figure. Tide gauges are often sited on coasts that may be geologically moving up or down, giving rise to an apparent change in sea level that in fact has little or nothing to do with recent climate change.

The main cause for these movements is the geologically recent release of the burden of miles-thick ice that covered much of our hemisphere’s  continents. This mass, which was up to two miles thick in places as far south as what is now Chicago, depressed the surface while the margins around the ice rose. It’s quite analogous to what a couch looks like after you’ve been lying on it—it takes some time to resume its original flatness.

Consequently, much of the part of North America that was under the ice mass is still rising, while the land on its edges, which bulged upward, is falling. The fall is especially large along the U.S. east coast, where this movement roughly doubles the apparent sea level rise caused by expanding the oceanic volume.

The main causes of the rise are net ice loss from nonpolar glaciers, and, to a lesser extent, from Greenland, along with a tiny contribution from Antarctica. There’s also a thermal expansion of the ocean caused by gradually rising surface temperatures. But previous estimates for these figures never seemed to add up to the generally accepted seven inches.

This is noted—and rectified—in a landmark study published in Nature last month by CalTech’s Thomas Frederikse and an international team of 10 co-authors. Auspiciously titled “The causes of sea-level rise since 1900,” it closes the loop and provides improved estimates of the size of the various contributing factors.

For some perspective, the nonpolar glacial contribution has been a bit over three inches, Greenland’s ice loss is good for two inches, and Antarctica has chipped in less than a half-inch. The water storage behind dams takes away one inch, while the thermal expansion adds around 2.4 inches. All told, these sum up very close to the observations—according to Frederiske et al., yielding about 7.1 inches since 1900.

They estimate that thermal expansion in the last quarter-century (1993-2018) has approximately doubled, compared to the entire 1900-2018 period, and the contribution from Greenland has gone up by nearly 50 percent. While this might appear alarming, it’s still puny, with the island-continent chipping in only about 0.6 inches of sea level rise since 1993, while Antarctica contributed a mere quarter-inch.

Meanwhile, the latest pronouncement from the United Nations forecasts a median 21st century rise of nearly 22 inches, assuming no serious reductions in emissions. There’s only been a 2.5-inch rise since 2000, so things are going to have to ramp up pretty much immediately to get to the U.N.’s bad-case forecast by 2100.

The current generation of climate models, with one exception, dramatically overestimates the amount of tropical warming at altitude, since global satellite data became available in 1979. The one model that gets it right, the Russian INM-CM4, has the least warming of all 102 runs in the current family. Its prospective warming for doubled atmospheric carbon dioxide is a mere 2.1⁰C. Its successor recently hit the literature, and it is even cooler, at 1.8⁰.

If we attribute most of the warming that began in 1976 to greenhouse gas emissions, we should see about 1.2⁰C more for the remainder of this century if the new Russian model is right. That’s pretty small beer, not much greater than the observed warming since 1900.

Beside the prospect that future warming may be modest, the news about sea level is that we should have increased confidence that our measures of its small rise are accurate.