Abstract
Abstract. Global mean sea level is an integral of changes occurring in the climate system in response to unforced climate variability as well as natural and anthropogenic forcing factors. Its temporal evolution allows changes (e.g., acceleration) to be detected in one or more components. Study of the sea-level budget provides constraints on missing or poorly known contributions, such as the unsurveyed deep ocean or the still uncertain land water component. In the context of the World Climate Research Programme Grand Challenge entitled Regional Sea Level and Coastal Impacts, an international effort involving the sea-level community worldwide has been recently initiated with the objective of assessing the various datasets used to estimate components of the sea-level budget during the altimetry era (1993 to present). These datasets are based on the combination of a broad range of space-based and in situ observations, model estimates, and algorithms. Evaluating their quality, quantifying uncertainties and identifying sources of discrepancies between component estimates is extremely useful for various applications in climate research. This effort involves several tens of scientists from about 50 research teams/institutions worldwide (www.wcrp-climate.org/grand-challenges/gc-sea-level, last access: 22 August 2018). The results presented in this paper are a synthesis of the first assessment performed during 2017–2018. We present estimates of the altimetry-based global mean sea level (average rate of 3.1 ± 0.3 mm yr−1 and acceleration of 0.1 mm yr−2 over 1993–present), as well as of the different components of the sea-level budget (http://doi.org/10.17882/54854, last access: 22 August 2018). We further examine closure of the sea-level budget, comparing the observed global mean sea level with the sum of components. Ocean thermal expansion, glaciers, Greenland and Antarctica contribute 42 %, 21 %, 15 % and 8 % to the global mean sea level over the 1993–present period. We also study the sea-level budget over 2005–present, using GRACE-based ocean mass estimates instead of the sum of individual mass components. Our results demonstrate that the global mean sea level can be closed to within 0.3 mm yr−1 (1σ). Substantial uncertainty remains for the land water storage component, as shown when examining individual mass contributions to sea level.
Highlights
Global warming has already several visible consequences, in particular an increase in the Earth’s mean surface temperature and ocean heat content (Rhein et al, 2013; IPCC, 2013), melting of sea ice, loss of mass of glaciers (Gardner et al, 2013), and ice mass loss from the Greenland and Antarctica ice sheets (Rignot et al, 2011a; Shepherd et al, 2012)
There is generally broad agreement between the GRACE datasets (Fig. 10), as most of the differences between GRACE estimates are caused by differences in the glacial isostatic adjustment (GIA) correction
We find a reasonable agreement between GRACE and the Input–output method (IOM) estimates the IOM estimates indicate higher losses
Summary
Global warming has already several visible consequences, in particular an increase in the Earth’s mean surface temperature and ocean heat content (Rhein et al, 2013; IPCC, 2013), melting of sea ice, loss of mass of glaciers (Gardner et al, 2013), and ice mass loss from the Greenland and Antarctica ice sheets (Rignot et al, 2011a; Shepherd et al, 2012). On average over the last 50 years, about 93 % of heat excess accumulated in the climate system because of greenhouse gas emissions has been stored in the ocean, and the remaining 7 % has been warming the atmosphere and continents, and melting sea and land ice (von Schuckmann et al, 2016). Because of ocean warming and land ice mass loss, sea level rises. Direct observations from in situ tide gauges available since the mid-to-late 19th century show that the 20th century global mean sea level has started to rise again at a rate of 1.2 to 1.9 mm yr−1 (Church and White, 2011; Jevrejeva et al, 2014; Hay et al, 2015; Dangendorf et al, 2017). Since the early 1990s sea-level rise (SLR) is measured by high-precision altimeter satellites and the rate has Published by Copernicus Publications
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