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finding 12.1 : key-finding-12-1
Global mean sea level (GMSL) has risen by about 7–8 inches (about 16–21 cm) since 1900, with about 3 of those inches (about 7 cm) occurring since 1993 (very high confidence). Human-caused climate change has made a substantial contribution to GMSL rise since 1900 (high confidence), contributing to a rate of rise that is greater than during any preceding century in at least 2,800 years (medium confidence).
This finding is from chapter 12 of Climate Science Special Report: The Fourth National Climate Assessment: Volume I.
Process for developing key messages: This key finding is based upon multiple analyses of tide gauge and satellite altimetry records, on a meta-analysis of multiple geological proxies for pre-instrumental sea level change, and on both statistical and physical analyses of the human contribution to GMSL rise since 1900.
Description of evidence base: Multiple researchers, using different statistical approaches, have integrated tide gauge records to estimate GMSL rise since the late nineteenth century (e.g., Church and White 2006,f935f0bf-548c-4e70-a69e-b1f2a310664c 2011;94a8514e-063e-45ef-b893-11c82b49a597 Hay et al. 2015;7c318710-b8fb-4e09-9982-546f2b60be67 Jevrejeva et al. 2009).1295b731-1d4c-44e2-b877-74df46d8e58d The most recent published rate estimates are 1.2 ± 0.27c318710-b8fb-4e09-9982-546f2b60be67 or 1.5 ± 0.294a8514e-063e-45ef-b893-11c82b49a597 mm/year over 1901–1990. Thus, these results indicate about 11–14 cm (4–5 inches) of GMSL rise from 1901 to 1990. Tide gauge analyses indicate that GMSL rose at a considerably faster rate of about 3 mm/year (0.12 inches/year) since 1993,7c318710-b8fb-4e09-9982-546f2b60be67 94a8514e-063e-45ef-b893-11c82b49a597 a result supported by satellite data indicating a trend of 3.4 ± 0.4 mm/year (0.13 inches/year) over 1993–2015 (update to Nerem et al. 20107b7ffcb0-766c-43b3-ac22-db29fbffef71) (Figure 12.3a). These results indicate an additional GMSL rise of about 7 cm (about 3 inches) rise since 1990. Thus, total GMSL rise since 1900 is about 16–21 cm (about 7–8 inches).
The finding regarding the historical context of the 20th century change is based upon Kopp et al.a0130167-b319-493d-bedc-7cab8f8fe9d9, who conducted a meta-analysis of geological RSL reconstructions spanning the last 3,000 years from 24 locations around the world as well as tide gauge data from 66 sites and the tide gauge based GMSL reconstruction of Hay et al.7c318710-b8fb-4e09-9982-546f2b60be67 By constructing a spatio-temporal statistical model of these data sets, they identified the common global sea level signal over the last three millennia and its uncertainties. They found a 95% probability that the average rate of GMSL change over 1900–2000 was greater than during any preceding century in at least 2,800 years.
The finding regarding the substantial human contribution is based upon several lines of evidence. Kopp et al.,a0130167-b319-493d-bedc-7cab8f8fe9d9 based on the long term historical relationship between temperature and the rate of sea level change, found that it is extremely likely that GMSL rise would have been <59% of observed in the absence of 20th century global warming, and that it is very likely that GMSL has been higher since 1960 than it would have been without 20th century global warming. Using a variety of models for individual components, Slangen et al.dde395ae-d68c-4fdd-b3c8-d1ccbee85102 found that 69% ± 31% out of the 87% ± 20% of GMSL rise over 1970–2005 that their models simulated was attributable to anthropogenic forcing, and that 37% ± 38% out of 74% ± 22% simulated was attributable over 1900–2005. Jevrejeva et al.,1295b731-1d4c-44e2-b877-74df46d8e58d using the relationship between forcing and GMSL over 1850 and 2001 and CMIP3 models, found that ~75% of GMSL rise in the 20th century is attributable to anthropogenic forcing. Marcos and Amores,4b421615-26ce-4f22-827a-db9598e1ea82 using CMIP5 models, found that ~87% of ocean heat uptake since 1970 in the top 700 m of the ocean has been due to anthropogenic forcing. Slangen et al.,9a5f3738-4283-4df2-adb6-8a0cac785d22 using CMIP5, found that anthropogenic forcing was required to explain observed thermosteric SLR over 1957–2005. Marzeion et al.dc9d8bb2-9388-4e4a-8976-88c6b825fafc found that 25% ± 35% of glacial loss over 1851–2010, and 69% ± 24% over 1991–2010, was attributable to anthropogenic forcing. Dangendorf et al.,e1472969-f184-4ec2-9a71-9d876c55ba41 based on time series analysis, found that >45% of observed GMSL trend since 1900 cannot (with 99% probability) be explained by multi-decadal natural variability. Becker et al.,680e0305-eeef-42b9-a6b5-d840bfec9be8 based on time series analysis, found a 99% probability that at least 1.0 or 1.3 mm/year of GMSL rise over 1880–2010 is anthropogenic.
New information and remaining uncertainties: Uncertainties in reconstructed GMSL change relate to the sparsity of tide gauge records, particularly before the middle of the twentieth century, and to different statistical approaches for estimating GMSL change from these sparse records. Uncertainties in reconstructed GMSL change before the twentieth century also relate to the sparsity of geological proxies for sea level change, the interpretation of these proxies, and the dating of these proxies. Uncertainty in attribution relates to the reconstruction of past changes and the magnitude of unforced variability.
Assessment of confidence based on evidence: Confidence is very high in the rate of GMSL rise since 1900, based on multiple different approaches to estimating GMSL rise from tide gauges and satellite altimetry. Confidence is high in the substantial human contribution to GMSL rise since 1900, based on both statistical and physical modeling evidence. It is medium that the magnitude of the observed rise since 1900 is unprecedented in the context of the previous 2,800 years, based on meta-analysis of geological proxy records.
ProvenanceThis finding was derived from figure -.2: Confidence / Likelihood
- Anthropogenic forcing dominates sea level rise since 1850 (1295b731)
- Quantifying anthropogenic and natural contributions to thermosteric sea level rise (4b421615)
- Long-term sea level trends: Natural or anthropogenic? (680e0305)
- Estimating Mean Sea Level Change from the TOPEX and Jason Altimeter Missions (7b7ffcb0)
- Probabilistic reanalysis of twentieth-century sea-level rise (7c318710)
- Sea-Level Rise from the Late 19th to the Early 21st Century (94a8514e)
- Detection and attribution of global mean thermosteric sea level change (9a5f3738)
- Temperature-driven global sea-level variability in the Common Era (a0130167)
- Attribution of global glacier mass loss to anthropogenic and natural causes (dc9d8bb2)
- Anthropogenic forcing dominates global mean sea-level rise since 1970 (dde395ae)
- Detecting anthropogenic footprints in sea level rise (e1472969)
- A 20th century acceleration in global sea-level rise (f935f0bf)
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