What have we learnt from ICESat on greenland ice sheet change and what to expect from current ICESat-2

Ice-sheet mass balance and ice behaviour have been effectively monitored remotely by space-borne laser ranging technology, i.e. satellite laser altimetry, and/or satellite gravimetry. ICES atmission launched in 2003 has pioneered laser altimetry providing a large amount of elevation data related to ice sheet change with high spatial and temporal resolution. ICESat-2, the successor to the ICESat mission, was launched in 2018, continuing the legacy of its predecessor. This paper presents an overview of the satellite laser altimetry and a review of Greenland ice sheet change estimated from ICESat data and compared against estimates derived from satellite gravimetry, i.e. changes of the Earth’s gravity field obtained from the GRACE data. In addition to that, it provides an insight into the characteristics and possibilities of ice sheet monitoring with renewed mission ICESat-2, which was compared against ICESat for the examination of ice height changes on the Jakobshavn glacier. ICESat comparison (2004–2008) shows that an average elevation change in different areas on Greenland varies up to ±0.60 m yr−1. Island’s coastal southern regions are most affected by ice loss, while inland areas record near-balance state. In the same period, gravity anomaly measurements showed negative annual mass balance trends in coastal regions ranging from a few cm up to -0.36 m yr-1 w.e. (water equivalent), while inland records show slightly positive trends. According to GRACE observations, in the following years (2009–2017), negative annual mass balance trends on the coast continued. Obseg ledenikov in spremembe v njihovi masi je mogoče učinkovito spremljati s tehnologijo laserskega satelitskega daljinskega zaznavanja, to je s satelitskim laserskim višinomerstvom, in/ali satelitsko gravimetrijo. ICESat, ki je bil izstreljen leta 2003, je prvi satelit za lasersko višinomerstvo, s katerim je zbranih mnogo podatkov o višinah na površju Zemlje z visoko časovno in prostorsko ločljivostjo, kar se uporablja tudi za spremljanje ledenikov. ICESat-2 je bil izstreljen leta 2018. Na primeru Grenlandije podajamo oceno o spreminjanju njenega ledenega pokrova na podlagi podatkov ICESat, kar primerjamo z ocenami o spremembi ledeniške mase na podlagi sprememb težnostnega polja na tem območju z uporabo podatkov satelita za spremljanje težnostnega polja Zemlje GRACE. Analiza podatkov ICESat za obdobje 2004–2008 kaže, da je povprečna sprememba višine ledu na Grenlandiji ± 0,60 m na leto. Večje izgube mase ledu so zaznavne na južnih obalnih predelih otoka, v notranjosti otoka v tem obdobju spremembe v masi ledenega pokrova skoraj niso nezaznavne. Za isto obdobje so tudi gravitacijski satelitski podatki pokazali, da ustrezajo spremembe težnostnega polja spremembi v masi ledenega pokrova na južni obali Grenlandije od nekaj centimetrov do –0,36 metrov vodnega ekvivalenta na leto (angl. water equivalent per year), medtem ko spremembe v masni bilanci v notranjosti otoka kažejo pozitivni trend. Na podlagi podatkov GRACE se tudi v obdobju 2009–2017 kaže negativni trend letne masne bilance ledenikov na obalnih območjih.