Intrumental Records of Sea Level Rise

Instrumental records for sea level date back about 140 years, when systematic measurements by tide gauges became available (NRC, 1990b). During the past few decades, tide gauge records have been augmented by satellite measurements that give precise sea level maps across the entire globe. Together, these modern records indicate that sea level has been rising since the mid-19th century, and that the rate of increase has been accelerating in recent years (Figure 7.2). For example, in the late 19th century, when tide gauge readings begin to provide accurate global sea level estimates, the rate of sea level rise was about 0.02 inches (0.6 millimeters) per year

Years (x103) before present

FIGURE 7.1 Illustration of relative sea level rise since the last ice age: 26,000 years ago, sea level is estimated to have been about 400 feet (120 meters) lower than it is today. This curve was assembled using analyses of coral reefs all over the world. The abbreviation MWP refers to various meltwater pulses, which caused sea level to rise relatively rapidly. MWP-1AO, ~19,000 years ago; MWP-1A, ~14,600 to 13,500 years ago; MWP-1B, ~11,500 to 11,000 years ago; MWP-1C, ~8,200 to 7,600 years ago. SOURCE: Gornitz (2009).

(Church and White, 2006); in the last half of the 20th century, this increased to approximately 0.07 inches (1.8 millimeters) per year (Miller and Douglas, 2004); and over the past 15 years, the rate of sea level rise has been in excess of 0.12 inches (3 millimeters) per year (Katsman et al., 2008; Vermeer and Rahmstorf, 2009). Ice core records show that atmospheric CO2 concentrations have been rising since about 1830 (see Chapater 6), so sea level and CO2 increases are generally coincident. Clear indications of interan-nual and decadal variability can also be seen in Figure 7.2. Distinguishing the effects of natural climate variability from human-caused warming is one of the challenges of understanding the details of past sea level rise and anticipating its future course.

1880 1900 1920 1940 1960 1980 2000

year

FIGURE 7.2 Annual, global mean sea level as determined by records of tide gauges (red curve with error bars, from Church and White [2006]; blue curve, from Holgate and Woodworth [2004]) and satellite altim-etry (black curve, from Leuliette et al. [2004]). For the last half of the 20th century, the rate of sea level rise can be estimated as being about 0.07 in/yr (1.8 mm/yr), with the most recent decade exhibiting a rate of sea level rise over 0.12 in/yr (3 mm/yr). The red and blue curves show deviations in sea level relative to the 1961 to 1990 period; the black curve shows deviations from the average of the red curve relative to the 1993 to 2001 period. SOURCE: Bindoff et al. (2007).

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