Global Sea Level Rise: A Decade of Multi-Satellite Altimeter Observations versus 100 Years of In-situ Observations

Based on observations from TOPEX/Poseidon, Jason-1 and four other satellite altimeter missions, it now appears that global sea level increased over the past eleven years at a rate of about 3 mm/yr, nearly 1 mm/yr greater than the generally accepted 20 th century rate based on tide gauge measurements and 2 mm/yr greater than the lower bound of the most recent assessment by the Intergovernmental Panel on Climate Change (IPCC). This result could be interpreted to mean that Global Sea Level Rise (GSLR) underwent a substantial acceleration in the 1990’s. Alternatively, the lower bound of the IPCC assessment could represent an extreme underestimate. To help sort through these possibilities, we examine the characteristic strengths and weaknesses of traditional in-situ observations sea level versus satellite based observations of sea level. First, we present an analysis of tide gauge measurements combined with observations of temperature and salinity in the Pacific and Atlantic Oceans close to the gauges. We find that gauge-determined rates of sea level rise, which encompass both mass and volume changes, are two to three times higher than the rates due to volume change derived from temperature and salinity data. This result supports earlier studies that put the twentieth-century value of GSLR close to 2 mm/yr, i.e. the upper bound of the most recent IPCC estimate, but more surprisingly it demonstrates that the melting of continental ice sheets and glaciers plays a more important role in GSLR than ocean warming. Next, we examine the satellite altimeter records, taking advantage of the fact that the different missions have different capabilities, but that all provide far better spatial sampling than traditional in-situ techniques. As we learn more about those limitations, and correct for errors, the records are beginning to become consistent with each other. However, another 5 to 10 years of additional observations may be needed to obtain a stable long term rate from altimetry. I. Introduction HE rate of twentieth-century global sea level rise and its causes are the subjects of intense controversy 1-7 . Most direct estimates from tide gauges measurements made over the past 50 to 100 years give about 2 mm/yr, while indirect estimates based on the two processes responsible for sea level rise, namely mass and volume change, fall far below this figure. Estimates of the volume increase due to ocean warming give a rate of about 0.5 mm/yr and the rate due to mass increase, primarily from the melting of continental ice, is thought to be even smaller. To further complicate matters, estimates of global sea level rise (GSLR) based on measurements made by TOPEX/Poseidon, Jason-1 and four other satellite altimeter missions over the past decade indicate a rate of about 3 mm/yr. To help sort through these conflicting results, we present an analysis of tide gauge and density (temperature and salinity) observations in the Pacific and Atlantic Oceans close to the gauge sites. This analysis suggests that the generally accepted value for the rate of mass increase is too low by a factor of 2 to 3 and that 2 mm/yr is a reasonable figure for the rate of 20 th century GSLR 8 . These results help resolve a controversy that emerged between the second and third IPCC assessments. At the time of the second assessment in 1995 9 , there seemed to be little dispute regarding GSLR. Most of the tide gauge estimates fell in the range of 1.5 to 2.0 mm/yr. Most of this rise was thought to result from ocean warming, with the rest due to the melting of continental ice. However, by the third IPCC assessment in 2001 1 , this consensus view had collapsed. New and better estimates of ocean warming had