Global sea level rise is directly tied into Global Warming. As the world’s temperatures quickly rise, this is having a dramatic effect on the ocean. Sea level rise is being caused by three primary factors. The first factor is melting glaciers, ice sheets, and snow fields. The second factor is seawater will expand when heated. The third factor is the increasing loss of freshwater sources through evaporation that later falls on the ocean as rainwater. Also, as precipitation levels drop on glaciers, ice sheets, and snow fields, these physical areas are not being replenished, thus contributing to their shrinking or retreating.
Earth’s sea levels have greatly fluctuated over its geologic history. With the Last Glacial Maximum about 20,000 years ago, or about 17800 B.C., when a large portion of the planet was covered with immense ice sheets and glaciers, the sea level has risen by more than 125 meters. Initially, sea level rise was rapid as the earth warmed and the ice sheets melted. Then about 8,200 years ago, or about 6000 B.C., sea level rise started to slow. It remains in statis for about 2,500 years, or until 3500 B.C. then restarted as the planet warmed again. The most recent trend of sea levels rise has been estimated to have started at either the end of the 19th Century or the beginning of the 20th Century.
Since 1880, the global mean sea level has been estimated to have risen about 21–24 centimeters overall. In 2021, the global mean sea level was 97 millimeters about the 1993 base level measurement. Using global satellite measurements, it is the highest annual average over the past 30 years. Now the rate of acceleration has more than doubled from 1.4 millimeters per year throughout most of the 20th century to 3.6 millimeters per year during the 2006-2015 observation period.
In many locations along the U.S. coastlines, the rate of local sea level rise is greater than the global average. This phenomenon is due in part to due to land changes including land subsidance, soil erosion, and gas, oil, and groundwater pumping. High-tide flooding has been estimated at between 300 to 900 percent more frequent when compared to the average of half a century ago. It is now estimated that by 2100 the average sea level rise will be projected to be 600 millimeters higher than it was in 2000. However, current models for the contiguous U.S. have projected the average sea levels could be between 2.2 meters and 3.9 meters.
Sea levels are measured using two main methods. The first are tide or tidal gauges. The second is satellite altimeters. Tide or tidal gauges around the world have been measuring daily high and low tides for more well over a century. It uses a variety of manual and automatic sensory equipment. Using this data, scientists have been able to calculate a global average and adjust it for seasonal variances. Satellite measurement has been employed since the early 1990s. A radar pulse is beamed down from the satellite to the ocean’s surface. The return speed of the pulse is measured along with the intensity of the return. The higher the sea level is, the faster and more intense the return will be.