finding 2.4 : key-message-2-4

Global average sea level has risen by about 7–8 inches (16–21 cm) since 1900, with almost half this rise occurring since 1993 as oceans have warmed and land-based ice has melted (very high confidence). Relative to the year 2000, sea level is very likely to rise 1 to 4 feet (0.3 to 1.3 m) by the end of the century (medium confidence). Emerging science regarding Antarctic ice sheet stability suggests that, for higher scenarios, a rise exceeding 8 feet (2.4 m) by 2100 is physically possible, although the probability of such an extreme outcome cannot currently be assessed.

This finding is from chapter 2 of Impacts, Risks, and Adaptation in the United States: The Fourth National Climate Assessment, Volume II.

Process for developing key messages:

This chapter is based on the collective effort of 32 authors, 3 review editors, and 18 contributing authors comprising the writing team for the Climate Science Special Report (CSSR),75cf1c0b-cc62-4ca4-96a7-082afdfe2ab1 a featured U.S. Global Change Research Project (USGCRP) deliverable and Volume I of the Fourth National Climate Assessment (NCA4). An open call for technical contributors took place in March 2016, and a federal science steering committee appointed the CSSR team. CSSR underwent three rounds of technical federal review, external peer review by the National Academies of Sciences, Engineering, and Medicine, and a review that was open to public comment. Three in-person Lead Authors Meetings were conducted at various stages of the development cycle to evaluate comments received, assign drafting responsibilities, and ensure cross-chapter coordination and consistency in capturing the state of climate science in the United States. In October 2016, an 11-member core writing team was tasked with capturing the most important CSSR key findings and generating an Executive Summary. The final draft of this summary and the underlying chapters was compiled in June 2017.

The NCA4 Chapter 2 author team was pulled exclusively from CSSR experts tasked with leading chapters and/or serving on the Executive Summary core writing team, thus representing a comprehensive cross-section of climate science disciplines and supplying the breadth necessary to synthesize CSSR content. NCA4 Chapter 2 authors are leading experts in climate science trends and projections, detection and attribution, temperature and precipitation change, severe weather and extreme events, sea level rise and ocean processes, mitigation, and risk analysis. The chapter was developed through technical discussions first promulgated by the literature assessments, prior efforts of USGCRP,75cf1c0b-cc62-4ca4-96a7-082afdfe2ab1 e-mail exchanges, and phone consultations conducted to craft this chapter and subsequent deliberations via phone and e-mail exchanges to hone content for the current application. The team placed particular emphasis on the state of science, what was covered in USGCRP,75cf1c0b-cc62-4ca4-96a7-082afdfe2ab1 and what is new since the release of the Third NCA in 2014.dd5b893d-4462-4bb3-9205-67b532919566

Description of evidence base:

Multiple researchers, using different statistical approaches, have integrated tide gauge records to estimate global mean sea level (GMSL) rise since the late 19th century (e.g., Church and White 2006, 2011; Hay et al. 2015; Jevrejeva et al. 20091295b731-1d4c-44e2-b877-74df46d8e58d,7c318710-b8fb-4e09-9982-546f2b60be67,94a8514e-063e-45ef-b893-11c82b49a597,f935f0bf-548c-4e70-a69e-b1f2a310664c). The most recent published rate estimates are 1.2 ± 0.2 mm/year7c318710-b8fb-4e09-9982-546f2b60be67 or 1.5 ± 0.2 mm/year94a8514e-063e-45ef-b893-11c82b49a597 over 1901–1990. Thus, these results indicate about 4–5 inches (11–14 cm) of GMSL rise from 1901 to 1990. Tide gauge analyses indicate that GMSL rose at a considerably faster rate of about 0.12 inches/year (3 mm/year) since 1993,7c318710-b8fb-4e09-9982-546f2b60be67,94a8514e-063e-45ef-b893-11c82b49a597 a result supported by satellite data indicating a trend of 0.13 inches/year (3.4 ± 0.4 mm/year) over 1993–2015 (update to Nerem et al. 2010;7b7ffcb0-766c-43b3-ac22-db29fbffef71 see also Sweet et al. 2017,3bae2310-7572-47e2-99a4-9e4276764934 Figure 12.3a). These results indicate an additional GMSL rise of about 3 inches (7 cm) since 1990. Thus, total GMSL rise since 1900 is about 7–8 inches (18–21 cm).

The finding regarding the historical context of the 20th-century change is based upon Kopp et al. (2016),a0130167-b319-493d-bedc-7cab8f8fe9d9 who conducted a meta-analysis of geological regional sea level (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. (2015).7c318710-b8fb-4e09-9982-546f2b60be67 By constructing a spatiotemporal statistical model of these datasets, 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 lower bound of the very likely range is based on a continuation of the observed, approximately 3 mm/year rate of GMSL rise. The upper end of the very likely range is based on estimates for a higher scenario (RCP8.5) from three studies producing fully probabilistic projections across multiple RCPs. Kopp et al.(2014)38924fa0-a0dd-44c9-a2a0-366ca610b280 fused multiple sources of information accounting for the different individual process contributing to GMSL rise. Kopp et al. (2016)a0130167-b319-493d-bedc-7cab8f8fe9d9 constructed a semi-empirical sea level model calibrated to the Common Era sea level reconstruction. Mengel et al. (2016)94117a50-acc5-4dbf-8029-368aa3fc9680 constructed a set of semi-empirical models of the different contributing processes. All three studies show negligible scenario dependence in the first half of this century but increasing in prominence in the second half of the century. A sensitivity study by Kopp et al. (2014),38924fa0-a0dd-44c9-a2a0-366ca610b280 as well as studies by Jevrejeva et al. (2014)be9f25a7-6fb1-4599-b971-47aeb2abf967 and by Jackson and Jevrejeva (2016),c748bd06-bc78-4b9c-b511-7dab1974211e used frameworks similar to Kopp et al. (2016)a0130167-b319-493d-bedc-7cab8f8fe9d9 but incorporated an expert elicitation study on ice sheet stability.86851f34-1534-4feb-aa11-8e0d7eeb0b11 (This study was incorporated in the main results of Kopp et al. 201438924fa0-a0dd-44c9-a2a0-366ca610b280 with adjustments for consistency with Church et al. 2013.da0fddf2-c9c9-40d0-8e33-a86342d8b864) These studies extend the very likely range for RCP8.5 as high as 5–6 feet (160–180 cm; see Kopp et al. 2014, sensitivity study; Jevrejeva et al. 2014; Jackson and Jevrejeva 2016).38924fa0-a0dd-44c9-a2a0-366ca610b280,be9f25a7-6fb1-4599-b971-47aeb2abf967,c748bd06-bc78-4b9c-b511-7dab1974211e

As described in Sweet et al. (2017),3bae2310-7572-47e2-99a4-9e4276764934 Miller et al. (2013),b58704d1-b4ec-46d0-9dd5-e7573523951e and Kopp et al. (2017),38924fa0-a0dd-44c9-a2a0-366ca610b280 several lines of arguments exist that support a plausible worst-case GMSL rise scenario in the range of 2.0 m to 2.7 m by 2100. Pfeffer et al. (2008)bfa425f2-e044-44c2-8bdb-5f8491c577de constructed a “worst-case” 2.0 m scenario, based on acceleration of mass loss from Greenland, that assumed a 30 cm GMSL contribution from thermal expansion. However, Sriver et al. (2012)b15cbb81-a2ac-4201-a184-a361bbd238d6 find a physically plausible upper bound from thermal expansion exceeding 50 cm (an additional ~20-cm increase). The ~60 cm maximum contribution by 2100 from Antarctica in Pfeffer et al. (2008)bfa425f2-e044-44c2-8bdb-5f8491c577de could be exceeded by ~30 cm, assuming the 95th percentile for Antarctic melt rate (~22 mm/year) of the Bamber and Aspinall (2013)86851f34-1534-4feb-aa11-8e0d7eeb0b11 expert elicitation study is achieved by 2100 through a linear growth in melt rate. The Pfeffer et al. (2008)bfa425f2-e044-44c2-8bdb-5f8491c577de study did not include the possibility of a net decrease in land-water storage due to groundwater withdrawal; Church et al. (2013)da0fddf2-c9c9-40d0-8e33-a86342d8b864 find a likely land-water storage contribution to 21st century GMSL rise of −1 cm to +11 cm. These arguments all point to the physical plausibility of GMSL rise in excess of 8 feet (240 cm).

Additional arguments come from model results examining the effects of marine ice-cliff collapse and ice-shelf hydro-fracturing on Antarctic loss rates.ae82c8a3-3033-4103-91e9-926a27d1fa18 To estimate the effect of incorporating the DeConto and Pollard (2016)ae82c8a3-3033-4103-91e9-926a27d1fa18 projections of Antarctic ice sheet melt, Kopp et al. (2017)387b7906-07c3-431f-a441-5a103220a974 substituted the bias-corrected ensemble of DeConto and Pollardae82c8a3-3033-4103-91e9-926a27d1fa18 into the Kopp et al. (2014)38924fa0-a0dd-44c9-a2a0-366ca610b280 framework. This elevates the projections for 2100 to 3.1–8.9 feet (93–243 cm) for RCP8.5, 1.6–5.2 feet (50–158 cm) for RCP4.5, and 0.9–3.2 feet (26–98 cm) for RCP2.6. DeConto and Pollardae82c8a3-3033-4103-91e9-926a27d1fa18 is just one study, not designed in a manner intended to produce probabilistic projections, and so these results cannot be used to ascribe probability; they do, however, support the physical plausibility of GMSL rise in excess of 8 feet.

Very likely ranges, 2030 relative to 2000 in cm (feet)
Kopp et al. (2014)38924fa0-a0dd-44c9-a2a0-366ca610b280 Kopp et al. (2016)a0130167-b319-493d-bedc-7cab8f8fe9d9 Kopp et al. (2017)387b7906-07c3-431f-a441-5a103220a974 DP16 Mengel et al. (2016)94117a50-acc5-4dbf-8029-368aa3fc9680
RCP8.5 (higher) 11–18 (0.4–0.6) 8–15 (0.3–0.5) 6–22 (0.2–0.7) 7–12 (0.2–0.4)
RCP4.5 (lower) 10–18 (0.3–0.6) 8–15 (0.3–0.5) 6–23 (0.2–0.8) 7–12 (0.2–0.4)
RCP2.6 (very low) 10–18 (0.3–0.6) 8–15 (0.3–0.5) 6–23 (0.2–0.8) 7–12 (0.2–0.4)

Very likely ranges, 2050 relative to 2000 in cm (feet)
Kopp et al. (2014)38924fa0-a0dd-44c9-a2a0-366ca610b280 Kopp et al. (2016)a0130167-b319-493d-bedc-7cab8f8fe9d9 Kopp et al. (2017)387b7906-07c3-431f-a441-5a103220a974 DP16 Mengel et al. (2016)94117a50-acc5-4dbf-8029-368aa3fc9680
RCP8.5 (higher) 21–38 (0.7–1.2) 16–34 (0.5–1.1) 17–48 (0.6–1.6) 15–28 (0.5–0.9)
RCP4.5 (lower) 18–35 (0.6–1.1) 15–31 (0.5–1.0) 14–43 (0.5–1.4) 14–25 (0.5–0.8)
RCP2.6 (very low) 18–33 (0.6–1.1) 14–29 (0.5–1.0) 12–41 (0.4–1.3) 13–23 (0.4–0.8)

Very likely ranges, 2100 relative to 2000 in cm (feet)
Kopp et al. (2014)38924fa0-a0dd-44c9-a2a0-366ca610b280 Kopp et al. (2016)a0130167-b319-493d-bedc-7cab8f8fe9d9 Kopp et al. (2017)387b7906-07c3-431f-a441-5a103220a974 DP16 Mengel et al. (2016)94117a50-acc5-4dbf-8029-368aa3fc9680
RCP8.5 (higher) 55–121 (1.8–4.0) 52–131 (1.7–4.3) 93–243 (3.1–8.0) 57–131 (1.9–4.3)
RCP4.5 (lower) 36–93 (1.2–3.1) 33–85 (1.1–2.8) 50–158 (1.6–5.2) 37–77 (1.2–2.5)
RCP2.6 (very low) 29–82 (1.0–2.7) 24–61 (0.8–2.0) 26–98 (0.9–3.2) 28–56 (0.9–1.8)

New information and remaining uncertainties:

Uncertainties in reconstructed GMSL change relate to the sparsity of tide gauge records, particularly before the middle of the 20th 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.

Since NCA3, multiple different approaches have been used to generate probabilistic projections of GMSL rise, conditional upon the RCPs. These approaches are in general agreement. However, emerging results indicate that marine-based sectors of the Antarctic ice sheet are more unstable than previous modeling indicated. The rate of ice sheet mass changes remains challenging to project.

Assessment of confidence based on evidence:

This Key Message 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.

It is also based upon multiple methods for estimating the probability of future sea level change and on new modeling results regarding the stability of marine-based ice in Antarctica.

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. There is medium confidence that the magnitude of the observed rise since 1900 is unprecedented in the context of the previous 2,700 years, based on meta-analysis of geological proxy records.

There is very high confidence that GMSL rise over the next several decades will be at least as fast as a continuation of the historical trend over the last quarter century would indicate. There is medium confidence in the upper end of very likely ranges for 2030 and 2050. Due to possibly large ice sheet contributions, there is low confidence in the upper end of very likely ranges for 2100. Based on multiple projection methods, there is high confidence that differences between scenarios are small before 2050 but significant beyond 2050.

References :

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