Working Group I: The Scientific Basis

Other reports in this collection Earth rotation constraints on recent sea level rise

Changes in the Earth’s ice sheets introduce a time-dependency in the Earth’s inertia tensor whose consequences are observed both in the planet’s rotation (as an acceleration in its rotation rate and as a shift in the position of the rotation axis) and in the acceleration of the rotation of satellite orbits about the Earth (Wu and Peltier, 1984; Lambeck, 1988). Model estimates of these changes are functions of mass shifts within and on the Earth and are dependent, therefore, on the past ice sheet geometries, on the Earth’s rheology, and on the recent past and present rates of melting of the residual ice sheets. Other geophysical processes also contribute to the time-dependence of the rotational and dynamical parameters (e.g. Steinberger and O’Connell, 1997). Hence, unique estimates of recent melting cannot be inferred from the observations.

Some constraints on the present rates of change of the ice sheets have, nevertheless, been obtained, in particular through a combination of the rotational observations with geological and tide gauge estimates of sea level change (Wahr et al., 1993; Mitrovica and Milne, 1998; Peltier, 1998; Johnston and Lambeck, 1999). Results obtained so far are preliminary because observational records of the change in satellite orbits are relatively short (Nerem and Klosko, 1996; Cheng et al., 1997) but they will become important as the length of the record increases. Peltier (1998) has argued that if the polar ice sheets contributed, for example, 0.5 mm/yr to the global sea level rise, then the rotational constraints would require that most of this melting derived from Greenland. Johnston and Lambeck (1999) concluded that a solution consistent with geological evidence, including constraints on sea level for the past 6,000 years (Section 11.3.1), is for a non-steric sea level rise (i.e., not resulting from ocean density changes) of 1.0 ± 0.5 mm/yr for the past 100 years, with 5 to 30% originating from Greenland melting. However, all such estimates are based on a number of still uncertain assumptions such that the inferences are more indicative of the potential of the methodology than of actual quantitative conclusions.

Table 11.8: Estimates of terrestrial storage terms. The values given are those of Gornitz et al. (1997) and Sahagian (2000). The estimates used in this report are the maximum and minimum values from these two studies. The average rates over the period 1910 to 1990 are obtained by integrating the decadal averages using the time history of contributions estimated by Gornitz et al. (1997).
  Rate of sea level rise for 1990 (mm/yr) Average rate 1910 to 1990 (mm/yr)
Gornitz et al. (1997)
Sahagian (2000)
This assessment
min max min max min max min max
Groundwater mining +0.1 +0.4 +0.17 +0.1 +1.0 0.0 0.5
Lakes 0.0 +0.2 0.0 +0.2 0.0 0.1
Impoundment in reservoirs –0.38 –0.30 –0.70 –0.35 –0.7 –0.3 –0.4 –0.2
Infiltration from reservoirs –0.75 –0.61 –0.84 –0.42 –0.8 –0.4 –0.5 –0.2
Evapotranspiration –0.15 –0.14 0.0 –0.1 0.0 –0.1 0.0
Infiltration from irrigation –0.48 –0.40 0.0 –0.5 0.0 –0.2 0.0
Runoff from urbanisation +0.35 +0.41 0.0 0.0 +0.4 0.0 0.1
Deforestation +0.1 +0.14 +0.1 0.14 0.1 0.1
Total   –1.9 +1.0 –1.1 0.4

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