The Prominent Role of GRACE in the Series of IMBIE Studies: Future Plans and Prominent Issues
<p>The Ice Mass Balance Intercomparison Exercize &#160;(IMBIE) was initiated in 2011 with the intent of better reconciling the various reports&#160; on the Greenland ice sheet (GrIS) &#160;and Antarctic ice sheet (AIS) mass balance during the 2000&#8217;s. The focused study was funded and promoted by both ESA and NASA to better understand the origins of &#160;contradictory results using space observations for a 20 year-long period: 1990-2010. Here we review some of the main results of phase I and II of IMBIE and the strength of the GRACE mission results. &#160;For 20-year long trends (2002-2021) trends are influenced by glacial isostatic adjustment (GIA) in Greenland, but with more profound consequence for Antarctica. IMBIE-I determined a mass balance trend for 1992-2011: -142 &#177; 49 and -71 &#177; 83 Gt/yr, for GrIS and AIS, respectively. &#160;IMBIE-II was open to a wider sampling of international &#160;investigative teams and the results for GrIS over 1992-2018 changed to -150 &#177; 13 Gt/yr. Most notably the 1-sigma formal errors reported in IMBIE-II were 25% of those reported in the earlier IMBIE-I study for GrIS. For Antarctica the most notable contrast in results was the total value of the trend over 1992-2017 (IMBIE-II) in contrast 1992-2011 (IMBIE-I) (-109 &#177; 56 vs -71 &#177; 83 Gt/yr, respectively). The loss estimate for AIS rose by 67% and the error also reduced by about 33%. Glacial isostatic adjustment (GIA) estimates for Antarctica cluster around + 54 Gt/yr (meaning their correction adds to the negativity of the mass balance result for GRACE and GRACE-FO). &#160;The East Antarctica Ice Sheet (EAIS) has trend errors for the estimate 1992-2017 (IMBIE-II) that continue to dwarf the uncertainty: +5 &#177; 46 Gt/yr. Beneath EAIS, GIA is also most uncertain and models have the greatest spread. We discuss the general plan for IMBIE-III that is currently forming.</p>