Sea Level Change

One of the greatest climate change concerns expressed by low lying coastal and island nations is the threat which would result from rising sea levels, caused by a combination of thermal expansion and the melting of glacial and land ice. Sea level, like near-surface temperatures, has been generally increasing over the ~200 year instrumental record, as would be expected during the warming after a major ice age as well as the recovery from the Little Ice Age just prior to the beginning of the instrumental record.

The surfaces of the global oceans are never truly at rest, but rather constantly in motion, which makes the accurate measurement of sea level challenging. The equipment used to measure sea levels has changed progressively over the period of the instrumental record, as has the extent of the ocean surfaces measured. Until 1993, the beginning of the satellite measurement era, almost all sea level measurements were taken at coastal stations, usually by instruments which were actually in contact with the ocean surface. NOAA currently uses satellites to measure sea level at 10 day intervals, with a claimed uncertainty of 3-4 millimeters, or more than twice the reported long term trend in annual sea level rise and approximately equal to the reported annual rate of sea level rise since 1993.

The satellite measurements are made from satellites in orbits 1336 kilometers above the oceans’ surfaces. The measurements are used to report annual changes in sea level to a precision of 0.1 millimeters, or one part in 13.36 billion. Arguably, since the measurements are based on the time for microwave signals to travel from the satellite to the ocean surface and return, 0.1 millimeter precision might actually represent one part in 26.72 billion, since the microwaves travel 26.72 billion millimeters from the satellite to the ocean surface and back to the satellite.

There is no single, generally accepted explanation for the apparent doubling of the annual rate of sea level rise beginning in 1993, essentially the beginning of the satellite measurement era, nor is this apparent doubling reflected in the measurements in geologically stable coastal areas. One has to wonder whether the explanation is as simple as the failure to recognize that a 1 millimeter rise in sea level results in a 2 millimeter reduction in the round trip distance between the ocean surface and the satellite.