Past Assessments

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Our ability to quantify changes in the mass of ice sheets has been revolutionised over recent decades by satellite geodetic techniques, which provide measurements at spatial scales and frequencies that cannot be achieved by traditional methods.

Between 1989, when the first estimate of ice sheet mass balance was published, and 2011, 30 further  estimates were published based on satellite altimetry, gravimetry, or the input-output (mass budget) method (e.g. Zwally et al., 1989; Wingham et al., 1998; Rignot and Kanagaratnam, 2006; Velicogna and Wahr, 2006; Zwally et al., 2011).

Agreement between results was however poor, with the estimates plus uncertainties reflecting a combined Greenland and Antarctic ice sheet mass imbalance of somewhere between -676 and + 69 Gt/yr.

 

Past estimates of ice sheet mass balance from radar and laser altimetry, gravimetry and the input-output method. See Table S1.1 in Shepherd et al., (2012) for references.

Past estimates of ice sheet mass balance from radar and laser altimetry, gravimetry and the input-output method. See Table S1.1 in Shepherd et al., (2012) for references.

Such a large range limited confidence in estimates of ice sheet contribution to sea level, whilst even the most advanced climate models struggled to resolve detailed patterns of ice sheet imbalance.

It was clear that a better understanding of the processes and mechanisms controlling ice sheet mass imbalance was required.  This in turn depended on having detailed and accurate observations of changes to the ice sheets themselves.

The result was the first IMBIE exercise (IMBIE 2012) which aimed to produce a single reconciled estimate of ice sheet mass balance using satellite altimetry, gravimetry and the input-output method (Shepherd et al., 2012).