Abstract
Abstract. We combine satellite observations and numerical models to show that Earth lost 28 trillion tonnes of ice between 1994 and 2017. Arctic sea ice (7.6 trillion tonnes), Antarctic ice shelves (6.5 trillion tonnes), mountain glaciers (6.1 trillion tonnes), the Greenland ice sheet (3.8 trillion tonnes), the Antarctic ice sheet (2.5 trillion tonnes), and Southern Ocean sea ice (0.9 trillion tonnes) have all decreased in mass. Just over half (58 %) of the ice loss was from the Northern Hemisphere, and the remainder (42 %) was from the Southern Hemisphere. The rate of ice loss has risen by 57 % since the 1990s – from 0.8 to 1.2 trillion tonnes per year – owing to increased losses from mountain glaciers, Antarctica, Greenland and from Antarctic ice shelves. During the same period, the loss of grounded ice from the Antarctic and Greenland ice sheets and mountain glaciers raised the global sea level by 34.6 ± 3.1 mm. The majority of all ice losses were driven by atmospheric melting (68 % from Arctic sea ice, mountain glaciers ice shelf calving and ice sheet surface mass balance), with the remaining losses (32 % from ice sheet discharge and ice shelf thinning) being driven by oceanic melting. Altogether, these elements of the cryosphere have taken up 3.2 % of the global energy imbalance.
Highlights
Fluctuations in Earth’s ice cover have been driven by changes in the planetary radiative forcing (Vaughan et al, 2013), affecting global sea level (The IMBIE Team, 2018, 2020; Zemp et al, 2019a), oceanic conditions (Rahmstorf et al, 2015), atmospheric circulation (Francis and Vavrus, 2012; Vellinga and Wood, 2002) and freshwater resources (Huss and Hock, 2018; Immerzeel et al, 2020)
We estimated trends in the mass of Arctic sea ice using a combination of sea-ice–ocean modelling and satellite measurements of thickness change: between 1980 and 2011 we used the Pan-Arctic Ice-Ocean Modelling and Assimilation System (PIOMAS), a coupled sea-ice–ocean model forced with atmospheric reanalyses (Zhang and Rothrock, 2003); from 2011, we used CryoSat-2 satellite radar altimetry measurements of sea ice volume (Tilling et al, 2018)
We gridded Global Ice-Ocean Modeling and Assimilation System (GIOMAS) sea ice thickness data onto 0.2 ◦×0.5 ◦ grids, multiplied by cell area to retrieve total volume and used a density of 917 kg m−3 to convert to mass (as in PIOMAS, this is the density used to attribute a volume to the simulated sea ice growth in GIOMAS (Jinlun Zhang, personal communication, 2020))
Summary
Fluctuations in Earth’s ice cover have been driven by changes in the planetary radiative forcing (Vaughan et al, 2013), affecting global sea level (The IMBIE Team, 2018, 2020; Zemp et al, 2019a), oceanic conditions (Rahmstorf et al, 2015), atmospheric circulation (Francis and Vavrus, 2012; Vellinga and Wood, 2002) and freshwater resources (Huss and Hock, 2018; Immerzeel et al, 2020). The polar ice sheets store more than 99 % (30 million km3) of Earth’s freshwater ice on land (Fretwell et al, 2013; Morlighem et al, 2017), and even modest losses raise the global sea level (The IMBIE Team, 2018, 2020), increase coastal flooding (Vitousek et al, 2017) and disturb oceanic currents (Golledge et al, 2019). To date, these losses have tracked the upper range of climate warming scenarios forecast by the Intergovernmental Panel on Climate Change, which predict an ice sheet sea level contribution of up to 42 cm by 2100 (Slater et al, 2020). These elements of the cryosphere have experienced considerable change over recent decades: for example, it is estimated that the quantity of snow on land has decreased by 49 ± 49 Gt per decade in the Northern Hemisphere since 1980 (Pulliainen et al, 2020), that permafrost (perennially frozen ground) has warmed globally by 0.29 ± 0.12 ◦C during the past decade (Biskaborn et al, 2019), and that the duration of river and lake ice cover has shortened by 12 d per century in the Northern Hemisphere over the last 200 years (Magnuson et al, 2000)
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