Correction of GRACE Measurements of the Earth’s Moment of Inertia (MOI)
The widely used 15-year Gravity Recovery and Climate Experiment (GRACE) data sets do not conserve global total mass. They have a spurious decreasing trend of ~ 280 Gt/year. The primary aim of this study is to quantify the total Earth MOI contributions from its various components. Identifying the persistent contributors during this 15-year period assists in the estimation of the climate-driven MOI evolution, in the transient climate change period. Various regions contribute differently to the global total mass loss error, with the Greenland Ice Sheet (GrIS) generating ~ 10% of the error alone. Atmospheric parameters from reanalysis datasets drive a well-tested ice model to generate mass variation time series over the GrIS for 2002–2015. Because shorter timescale spikes of ~ 10–30 Gt in GRACE measurements are physically based, only the overall trend of ~ 30 Gt/year requires correcting. A more accurate mass loss rate estimate for 2002–2015 is ~ 120 Gt/year, considerably below previous estimates. With the water redistribution to lower latitudes and other effects of a warming climate, the nontidal Earth moment of inertia (MOI) also increases.
After rectification, the GRACE measured mass redistribution shows a steady, statistically robust (passed a two-tailed t-test at p = 0.04 for dof = 15) rate of MOI increase reaching ~ 10.1 × 1027 kg m2/year, equivalent to a 10.91 μs/year increase in the length of a day, during 2002–2017. In the foreseeable future, this is the dominant term for MOI changes, as the climate warms.
The primary aim of this study is to quantify the total Earth MOI contributions from its various components.
Identifying the persistent contributors during this 15-year period assists in the estimation of the climate-driven MOI evolution, in the transient climate change period. It was found that there is a statistically significant (p = 0.039) total increase in the Earth’s MOI, during the GRACE 15-year period. This steady increase in MOI has five major contributors. The two leading contributions are from the AIS and GrIS, with trends in both passing two tailed t-tests at p = 0.035 (dof = 15). Considered jointly, the linear trend of the polar ice sheets reaches 8.7 × 1027kg m2/year (equivalent to a 9.4 μs/year increase in the length of a day). Among the climate warming contributors, only the changes in rainfall redistribution have a statistically non-significant trend (p = 0.604). The mass redistribution caused by the hydrological cycle also is the most dynamic, contributing to fluctuation spikes in the global MOI. The ice sheet contribution is steady and is a persistent trend in the recent MOI evolution during the GRACE observational period. Compensating mechanisms, such as the GIA, operate on a much longer time scale. The increasing MOI trend is likely to continue in the transient climate warming period (the upcoming several hundred years), before the GIA and other possible negative factors dominate. Except for the wavier hydrological cycles, other contributing components are responses to a warming climate in synergy, rather than opposing each other. The Earth’s MOI variations during the 15-year GRACE period of reliable observational data form a basis for identifying climate change impacts on the Earth’s MOI and provide more precise insights into fluid and solid mass transport near the Earth’s surface.