Obtaining a reliable trend can require many years of data, which is why the satellite record in Figure 1 has been supplemented with a longer-term reconstruction based on tide gauge measurements. Some changes in relative and absolute sea level can be due to multi-year cycles such as El Niño and La Niña, which affect coastal ocean temperatures, salt content, wind patterns, atmospheric pressure (and thus storm tracks), and currents. Tide gauge measurements such as those in Figure 2 generally cannot distinguish between these two different influences without an accurate measurement of vertical land motion nearby. Relative sea level trends represent a combination of absolute sea level change and any local land movement. 1 (The 10 to 12 inches are an average: differences in both land and ocean height mean the rise will vary regionally along U.S. coastline will rise 10 to 12 inches by 2050-as much as the rise measured from 1920 to 2020. Evidence suggests sea level along the U.S. In contrast, absolute sea level change refers to the height of the ocean surface above the center of the earth, without regard to whether nearby land is rising or falling. Relative sea level change refers to how the height of the ocean rises or falls relative to the land at a particular location. Scientists account for these types of changes by measuring sea level change in two different ways. Changes in ocean currents such as the Gulf Stream can also affect sea levels by pushing more water against some coastlines and pulling it away from others, raising or lowering sea levels accordingly. In other areas, land can sink because of erosion, sediment compaction, natural subsidence (sinking due to geologic changes), groundwater withdrawal, or engineering projects that prevent rivers from naturally depositing sediments along their banks. Land can rise through processes such as sediment accumulation (the process that built the Mississippi River delta) and geological uplift (for example, as glaciers melt and the land below is no longer weighed down by heavy ice). The sea level changes that affect coastal systems involve more than just expanding oceans, however, because the Earth’s continents can also rise and fall relative to the oceans. Higher sea level also makes coastal infrastructure more vulnerable to damage from storms. Rising sea level inundates low-lying wetlands and dry land, erodes shorelines, contributes to coastal flooding, and increases the flow of salt water into estuaries and nearby groundwater aquifers. As water warms, it expands slightly-an effect that is cumulative over the entire depth of the oceans (see the Ocean Heat indicator).Ĭhanging sea levels can affect human activities in coastal areas.Changes in the volume of water and ice on land (namely glaciers and ice sheets) can increase or decrease the volume of water in the ocean (see the Glaciers indicator).Temperature and sea level are linked for two main reasons: 3 Relative sea level also has not risen uniformly because of regional and local changes in land movement and long-term changes in coastal circulation patterns.Īs the temperature of the Earth changes, so does sea level. While absolute sea level has increased steadily overall, particularly in recent decades, regional trends vary, and absolute sea level has decreased in some places.At those sites, even though absolute sea level has risen, land elevation has risen more rapidly. Meanwhile, relative sea level fell at some locations in Alaska and the Pacific Northwest. coastline between 19, particularly the Mid-Atlantic coast and parts of the Gulf coast, where some stations registered increases of more than 8 inches (see Figure 2). ![]() Relative sea level rose along much of the U.S.Since 1993, however, average sea level has risen at a rate of 0.12 to 0.14 inches per year-roughly twice as fast as the long-term trend. 2 When averaged over all of the world’s oceans, absolute sea level has risen at an average rate of 0.06 inches per year from 1880 to 2013 (see Figure 1). After a period of approximately 2,000 years of little change (not shown here), global average sea level rose throughout the 20 th century, and the rate of change has accelerated in recent years.
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