Earth's ice imbalance
Thomas Slater et al, The Cryosphere January 2021
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."
https://tc.copernicus.org/articles/15/233/2021/#&gid=1&pid=1"Earth's ice imbalance
To determine the global ice imbalance, we summed the mass change of each ice component computed at annual intervals and estimated the combined uncertainty as the root sum square of the individual uncertainty estimates. Between 1994 and 2017, the Earth lost 27.5 ± 2.1 Tt of ice (Fig. 4) – at an average rate of 1.2 ± 0.1 Tt per year (Table 1). Ice losses have been larger in the Northern Hemisphere, primarily owing to declining Arctic sea ice (−7559 ± 1021 Gt) followed by glacier retreat (−5148 ± 564 Gt) and Greenland ice sheet melt (−3821 ± 323 Gt). Ice in the Southern Hemisphere from the ice shelves (−6543 ± 1221 Gt), the Antarctic ice sheet (−2545 ± 554 Gt), glaciers (−965 ± 729 Gt), and sea ice in the Southern Ocean (−924 ± 674 Gt) has been lost at a total rate of −477 ± 146 Gt yr−1, which is 34 % slower than in the Northern Hemisphere (−719 ± 207 Gt yr−1). Earth's ice can be categorized into its floating and on-land components; grounded ice loss from ice sheets and glaciers raises the global sea level (The IMBIE Team, 2018, 2020; Zemp et al., 2019a) and influences oceanic circulation through freshwater input (Rahmstorf et al., 2015), and glacier retreat impacts local communities who rely on glaciers as a freshwater resource (Immerzeel et al., 2020). Grounded ice losses have raised the global mean sea level by 24.9 ± 1.8 and 9.7 ± 2.5 mm in the Northern Hemisphere and Southern Hemisphere respectively, totalling 34.6 ± 3.1 mm over the 24-year period. Although the loss of floating sea ice and ice shelves does not contribute to global sea level rise, sea ice decline increases habitat loss (Rode et al., 2014), coastal erosion (Overeem et al., 2011), and ocean circulation (Armitage et al., 2020) and may affect mid-latitude weather and climate (Blackport et al., 2019; Overland et al., 2016).
There is now widespread evidence that climate change has caused reductions in Earth's ice. On average, the planetary surface temperature has risen by 0.85 ∘C since 1880, and this signal has been amplified in the polar regions (Hartmann et al., 2013). Although this warming has led to higher snowfall in winter, it has also driven larger increases in summertime surface melting (Huss and Hock, 2018). The global oceans have warmed too (Hartmann et al., 2013), with significant impacts on tidewater glaciers (Hogg et al., 2017; Holland et al., 2008), on floating ice shelves (Shepherd et al., 2010) and on the ice streams which have relied on their buttressing (Rignot et al., 2004). Atmospheric warming – anthropogenic or otherwise – is responsible for the recent and long-term reductions in mountain glacier ice (Marzeion et al., 2014), and ocean-driven melting of outlet glaciers has caused the vast majority of the observed ice losses from Antarctica (The IMBIE Team, 2018). Elsewhere, the picture is more complicated. In Greenland, for example, roughly half of all ice losses are associated with trends in surface mass balance, and the remainder is due to accelerated ice flow triggered by ocean melting at glacier termini (The IMBIE Team, 2020). Although the retreat and collapse of ice shelves at the Antarctic Peninsula has occurred in tandem with a rapid regional atmospheric warming (Vaughan et al., 2003), warm circumpolar deep water has melted the base of ice shelves in the Amundsen and Bellingshausen seas (Jacobs et al., 2011), and this now amounts to over half of their net loss. While the progressive retreat of Arctic sea ice has been driven by radiative forcing, this has been mediated in part by the increasing presence of open water (Perovich and Richter-Menge, 2009), and broader changes in oceanic conditions are expected to play an increasingly important role (Carmack et al., 2016). Finally, although the extent of Southern Ocean sea ice has shown little overall change, there have been considerable regional variations owing to changes in both atmospheric and oceanic forcing (Hobbs et al., 2016). Attributing Arctic sea ice decline and ice shelf calving to increased radiative forcing, approximately 68 % of the recent global ice imbalance is due to atmospheric warming, and the remainder is due to ocean-driven melting. We determine the energy required to melt the total ice loss as
E=M(L+cpΔT),(1)
where M is the mass of ice, ΔT is the rise in temperature required (we assume an initial ice temperature of −20 ± 10 ∘C), L is the latent heat of fusion for water (333 J g−1) and cp is the specific heat capacity of water (2108 J kg−1 ∘C−1). Although the initial temperature is poorly constrained, the fractional energy required for warming is a small (0.7 % ∘C−1) percentage of the total energy imbalance. Altogether, the ice sheet, glacier, ice shelf and sea ice loss amounts to an 8.9 ± 0.9×1021 J sink of energy, or 3.2 ± 0.3 % of the global imbalance over the same period (von Schuckmann et al., 2020)."