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Like “all” of the other phenomena climate scientists measure and attribute relative to the effects of climate change (temperature, carbon dioxide, ocean currents, ozone) sea level rise has a short history of understanding. It is also both alarming and uncertain.
We don’t ever actually measure the elevations of the oceans (the oceans are zero; see below), we measure the elevations of the land surface. The problem that had to be overcome, and is still, and will always be, elusive, is that of a “stable” datum, or baseline, or zero starting point.
For more detail on the science and history of elevation surveys, please read this from NOAA.
Prior to our understanding of the constraints, elevations were certainly surveyed, but only relatively. In other words, the builders knew that point A was higher than point B and by how much, but there wasn’t a standard datum to connect one surveyed area to another. It wasn’t until the mid to late 1800s that instrumentation technology had advanced to enable elevation measurements across the continent and along railroads and rivers tied to tide gauges. Eventually, the “point A to point B” surveys and continental surveys were all tied together and connected to a standard reference point for mean sea level (MSL) as the datum. This network has to be periodically adjusted to account for changes in the land surface due to post-glacial uplift, earthquakes, and subsidence. Recent modernization has included the integration of satellite GPS technology with the network for quick and “accurate” elevation control.
As mentioned above, the land surface is not stable and changes in elevation with time due to several factors. Sea level is certainly not a “stable” datum either, given the variability caused by tidal and weather influences, and the fact that, by definition, it is only present due to its juxtaposition with the land. The reason sea level is used as a datum is because, between the two (land and sea), sea level is believed to be the “most” stable due to the properties of water, the main one being that water seeks it’s own level and forms a flat surface when not acted upon by other forces.
However it has always been difficult to discern, upon change, which one (land or sea) is moving. Sea level may appear to be rising when actually the land surface is sinking, and vice versa. In fact, everything is moving. There is no part of the earth, solar system, Galaxy, or universe that is “stable.” That is why even the integration of satellite GPS into the methodologies for surveying elevations only makes the elevations more “relatively ” accurate; satellites are not perfectly stationary.
Sea level may very well be rising, but the whole surface of the earth moves up and down and around, being influenced by tectonic forces, tidal forces, and buoyancy effects. Add to that the relative nature of elevation surveys, and there is a lot to be understood before we can definitively state that sea level is rising, much more that it’s due to climate change.
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