Global Sea Level Budget 1993-Present

1 1 2 3 4 5 6 7 8 9 10 11 Global Sea Level Budget 1993-Present 12 13 14 WCRP Global Sea Level Budget Group* 15 16 *A full list of authors and their affiliations appears at the end of the paper 17 18 19 20 Revised version 21 23 July 2018 22 23 24 25 26 27 Corresponding author: Anny Cazenave, LEGOS, 18 Avenue Edouard Belin, 31401 Toulouse, 28 Cedex 9, France; [email protected] 29 30 1 2 31 Abstract 32 33 Global mean sea level is an integral of changes occurring in the climate system in response to 34 unforced climate variability as well as natural and anthropogenic forcing factors. Its temporal 35 evolution allows detecting changes (e.g., acceleration) in one or more components. Study of 36 the sea level budget provides constraints on missing or poorly known contributions, such as 37 the unsurveyed deep ocean or the still uncertain land water component. In the context of the 38 World Climate Research Programme Grand Challenge entitled “Regional Sea Level and 39 Coastal Impacts”, an international effort involving the sea level community worldwide has 40 been recently initiated with the objective of assessing the various data sets used to estimate 41 components of the sea level budget during the altimetry era (1993 to present). These data sets 42 are based on the combination of a broad range of space-based and in situ observations, model 43 estimates and algorithms. Evaluating their quality, quantifying uncertainties and identifying 44 sources of discrepancies between component estimates is extremely useful for various 45 applications in climate research. This effort involves several tens of scientists from about fifty 46 research teams/institutions worldwide (www.wcrp-climate.org/grand-challenges/gc-sea- 47 level). The results presented in this paper are a synthesis of the first assessment performed 48 during 2017-2018. We present estimates of the altimetry-based global mean sea level (average 49 rate of 3.1 +/- 0.3 mm/yr and acceleration of 0.1 mm/yr2 over 1993-present), as well as of the 50 different components of the sea level budget (http://doi.org/10.17882/54854). We further 51 examine closure of the sea level budget, comparing the observed global mean sea level with 52 the sum of components. Ocean thermal expansion, glaciers, Greenland and Antarctica 53 contribute by 42%, 21%, 15% and 8% to the global mean sea level over the 1993-present. We 54 also study the sea level budget over 2005-present, using GRACE-based ocean mass estimates 55 instead of sum of individual mass components. Our results demonstrate that the global mean 56 sea level can be closed to within 0.3 mm/yr (one sigma). Substantial uncertainty remains for 57 the land water storage component, as shown in examining individual mass contributions to 58 sea level. 59 60 61 62 63 64 65 2 3 66 1. Introduction 67 68 Global warming has already several visible consequences, in particular increase of the Earth’s 69 mean surface temperature and ocean heat content (Rhein et al., 2013, Stocker et al., 2013), 70 melting of sea ice, loss of mass of glaciers (Gardner et al., 2013), and ice mass loss from the 71 Greenland and Antarctica ice sheets (Rignot et al., 2011, Shepherd et al., 2012). On average 72 over the last 50 years, about 93% of heat excess accumulated in the climate system because of 73 greenhouse gas emissions has been stored in the ocean, and the remaining 7% has been 74 warming the atmosphere and continents, and melting sea and land ice (von Schuckmann et al., 75 2016). Because of ocean warming and land ice mass loss, sea level rises. Since the end of the 76 last deglaciation about 3000 years ago, sea level remained nearly constant (e.g., Lambeck et 77 al., 2010, Kemp et al., 2011, Kopp et al. 2014). However, direct observations from in situ tide 78 gauges available since the mid-to-late 19th century show that the 20th century global mean sea 79 level has started to rise again at a rate of 1.2 mm/yr to 1.9 mm/yr (Church and White, 2011, 80 Jevrejeva et al., 2014a, Hay et al., 2015, Dangendorf et al., 2017). Since the early 1990s sea 81 level rise is measured by high-precision altimeter satellites and the rate has increased to ~3 82 mm/yr on average (Legeais et al., 2018, Nerem et al., 2018). 83 Accurate assessment of present-day global mean sea level variations and its components 84 (ocean thermal expansion, ice sheet mass loss, glaciers mass change, changes in land water 85 storage, etc.) is important for many reasons. The global mean sea level is an integral of 86 changes occurring in the Earth’s climate system in response to unforced climate variability as 87 well as natural and anthropogenic forcing factors e.g., net contribution of ocean warming, 88 land ice mass loss, and changes in water storage in continental river basins. Temporal changes 89 of the components are directly reflected in the global mean sea level curve. If accurate 90 enough, study of the sea level budget provides constraints on missing or poorly known 91 contributions, e.g., the deep ocean or polar regions under sampled by current observing 92 systems, or still uncertain changes in water storage on land due to human activities (e.g. 93 ground water depletion in aquifers). Global mean sea level corrected for ocean mass change in 94 principle allows one to independently estimate temporal changes in total ocean heat content, 95 from which the Earth’s energy imbalance can be deduced (von Schuckmann et al., 2016). The 96 sea level and/or ocean mass budget approach can also be used to constrain models of Glacial 97 Isostatic Adjustment (GIA). The GIA phenomenon has significant impact on the 98 interpretation of GRACE-based space gravimetry data over the oceans (for ocean mass 99 change) and over Antarctica (for ice sheet mass balance). However, there is still incomplete 3 4 100 consensus on best estimates, a result of uncertainties in deglaciation models and mantle 101 viscosity structure. Finally, observed changes of the global mean sea level and its components 102 are fundamental for validating climate models used for projections. 103 In the context of the Grand Challenge entitled “Regional Sea Level and Coastal Impacts” of 104 the World Climate Research Programme (WCRP), an international effort involving the sea 105 level community worldwide has been recently initiated with the objective of assessing the sea 106 level budget during the altimetry era (1993 to present). To estimate the different components 107 of the sea level budget, different data sets are used. These are based on the combination of a 108 broad range of space-based and in situ observations. Evaluating their quality, quantifying their 109 uncertainties, and identifying the sources of discrepancies between component estimates, 110 including the altimetry-based sea level time series, are extremely useful for various 111 applications in climate research. 112 Several previous studies have addressed the sea level budget over different time spans and 113 using different data sets. For example, Munk (2002) found that the 20th century sea-level rise 114 could not be closed with the data available at that time and showed that if the missing 115 contribution were due to polar ice melt, this would be in conflict with external astronomical 116 constraints. The enigma has been resolved in two ways. Firstly, an improved theory of 117 rotational stability of the Earth (Mitrovica et al., 2006) effectively removed the constraints 118 proposed by Munk (2002), and allows a polar ice sheet contribution to 20th century sea-level 119 rise of as much as ~1.1 mm/yr, with about 0.8 mm/yr beginning in the 20th century. In 120 addition, more recent studies by Gregory et al. (2013) and Slangen et al. (2017), combining 121 observations with model estimates, showed that it was possible to effectively close the 20th 122 century sea level budget within uncertainties. For the altimetry era, many studies have 123 investigated closure of the sea level budget (e.g., Cazenave et al., 2009, Leuliette and Willis, 124 2010, Church and White, 2011, Chambers et al., 2017, Dieng et al., 2017, Chen et al., 2017, 125 Nerem et al., 2018). Assessments of the published literature have also been performed in past 126 IPCC (Intergovernmental Panel on Climate Change) reports (e.g., Church et al., 2013). 127 Building on these previous works, here we intend to provide a collective update of the global 128 mean sea level budget, involving the many groups worldwide interested in present-day sea 129 level rise and its components. We focus on observations rather than model-based estimates 130 and consider the high-precision altimetry era starting in 1993 that includes the period since 131 the mid-2000s where new observing systems, like the Argo float project (Roemmich et al., 132 2012) and the GRACE space gravimetry mission (Tapley et al., 2004) that provide improved 4 5 133 data sets of high value for such a study. Only the global mean budget is considered here. 134 Regional budget will be the focus of a future assessment. 135 Section 2 describes for each component of the sea level budget equation the different data sets 136 used to estimate the corresponding contribution to sea level, discusses associated errors and 137 provides trend estimates for the two periods. Section 3 addresses the mass and sea level 138 budgets over the study periods.

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