Polar bear on ice floe. Melting iceberg and global warming. Climate change
Sea level rise — mostly due to glacial melt largely caused by anthropogenic climate change — has been a hot button topic for the past half century. But historically defining the basic parameters of global sea level has never been a trite exercise.
Yet in a revelatory new book, Sea Level: A History, author Wilko Graf von Hardenberg, a senior research scholar at Germany’s Max Planck Institute for the History of Science in Berlin, deftly chronicles just how difficult it was to come up with a standard method of measuring mean sea level.
In the process, Hardenberg gives us a richly detailed yet very accessible history of how over five centuries, the concept of measuring mean sea level was a painstaking process that advanced in fits and starts.
Mean sea level is quite simply the average height of the ocean’s surface over time. But the boundaries between land and sea are often skewed.
The concept of sea level as an average is only one step in the long-term pursuit of a point of reference in a space that is never at rest, Hardenberg writes. Neat distinctions between land and sea are relatively recent products of the modern age, he notes. Coasts are actually ecotones, spaces in which different ecosystems meet and interact, porous regions that are part land, part water, he continues.
Early Data
The earliest available data series on sea level relative to land began to be produced consistently almost five centuries ago in Amsterdam, Hardenberg writes in Sea Level. Elsewhere, tide gauges—the tools necessary to collect sea-level measurements—were installed in an unorderly, piecemeal fashion, he notes. Such haphazard and unbalanced development and the consequent unevenness of available series has produced biases in our historical understanding of sea-level rise, Hardenberg writes.
Long before the advent of advanced geodetic buoy systems, altimeters and satellites, our ancestors took a credible stab at defining a global mean sea level. They weren’t altogether successful, but they tried. And one must remember that even with today’s global satellite technology, measuring changes in sea level from on high remains a somewhat iffy proposition.
Sea Level: A History
That’s arguably one reason why recent dire predictions about climate change have thankfully not always come true.
In a 2011 article for Climate Central, I point out that the state of Georgia’s 100-mile coastline has waxed and waned for thousands of years, surviving by shifting miles in the process. Sea level rise may be even more pronounced in areas such as the Georgia coastline, where the curve of the land already amplifies high tides of 8.2 ft, I write.
In fact, according to NOAA, global mean sea level has risen about 8–9 inches since 1880. The rising water level is mostly due to a combination of meltwater from glaciers and ice sheets and thermal expansion of seawater as it warms, NOAA reports.
Getting a handle on mean sea level.
At the 1867 meeting of the American Association for the Advancement of Science, the Ohio-based geologist and surveyor Charles Whittlesey presented a paper on the impact of glaciation on the level of the oceans, Hardenberg writes. Whittlesey concluded that the melting of existing ice caps would cause a catastrophic rise, the author notes.
Satellites have allowed scientists to overcome the limits of a sparse network of tide gauges and gravimeters, along the coasts and on midocean islands, finally supplying continuous—and nearly global—sea-level data, Hardenberg writes.
Yet satellites’ precision is not absolute, Hardenberg writes. In fact, valid, long-term series of observations reliably collected on the ground are essential for calibrating the instruments deployed on satellites, he notes.
Dire Predictions
Models predict a rise of at least 15 centimeters by 2050 and possibly more than a meter by 2100, Hardenberg writes.
As Sea Level points out, Miami has been dubbed the “most vulnerable” major coastal city in the world. And with its deltaic geography, the country of Bangladesh is virtually defenseless against tidal floods and sea-level rise, Hardenberg notes.
Trial and Error
Sea Level offers a fascinating account of how science is often a piecemeal undertaking of trial and error. But the book also offers a sobering reminder of the inexactitude of today’s global sea level measurements.
As for sea level rise caused by anthropogenic climate change?
Even though there’s no question that anthropogenic climate change has played a major role in the current run-up of the global mean sea level, some of the most histrionic predictions about pending inundations have mercifully remained just that. Predictions. Not reality.
That’s not to say that we should not take the subject seriously. But sea level rise linked to anthropogenic climate change is both insidious and nuanced. Those who are not well versed in the history of our planet’s long-term geology are often not aware that Earth’s climate and its oceans have waxed and waned over millions and billions of years.
The Bottom Line?
There are no easy solutions; the climate change genie is out of the bottle, and we still have much to learn. But for those interested in learning how sea level fluctuations have historically been measured, the book Sea Level is well worth a read.
