finding 12.2 : key-finding-12-2

Relative to the year 2000, GMSL is very likely to rise by 0.3–0.6 feet (9–18 cm) by 2030, 0.5–1.2 feet (15–38 cm) by 2050, and 1.0–4.3 feet (30–130 cm) by 2100 (very high confidence in lower bounds; medium confidence in upper bounds for 2030 and 2050; low confidence in upper bounds for 2100). Future pathways have little effect on projected GMSL rise in the first half of the century, but significantly affect projections for the second half of the century (high confidence). Emerging science regarding Antarctic ice sheet stability suggests that, for high emission scenarios, a GMSL rise exceeding 8 feet (2.4 m) by 2100 is physically possible, although the probability of such an extreme outcome cannot currently be assessed. Regardless of pathway, it is extremely likely that GMSL rise will continue beyond 2100 (high 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 methods for estimating the probability of future sea level change and on new modeling results regarding the stability of marine based ice in Antarctica.

Description of evidence base: 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 upon estimates for the higher scenario (RCP8.5) from three studies producing fully probabilistic projections across multiple RCPs. Kopp et al. 201438924fa0-a0dd-44c9-a2a0-366ca610b280 fused multiple sources of information accounting for the different individual process contributing to GMSL rise. Kopp et al. 2016a0130167-b319-493d-bedc-7cab8f8fe9d9 constructed a semi-empirical sea level model calibrated to the Common Era sea level reconstruction. Mengel et al.94117a50-acc5-4dbf-8029-368aa3fc9680 constructed a set of semi-empirical models of the different contributing processes. All three studies show negligible RCP dependence in the first half of this century, becoming more prominent 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.be9f25a7-6fb1-4599-b971-47aeb2abf967 and by Jackson and Jevrejeva,c748bd06-bc78-4b9c-b511-7dab1974211e used frameworks similar to Kopp et al. 2016a0130167-b319-493d-bedc-7cab8f8fe9d9 but incorporated directly an expert elicitation study on ice sheet stability.86851f34-1534-4feb-aa11-8e0d7eeb0b11 (This study was incorporated in Kopp et al. 2014’s38924fa0-a0dd-44c9-a2a0-366ca610b280 main results with adjustments for consistency with Church et al. 2013da0fddf2-c9c9-40d0-8e33-a86342d8b864). These studies extend the very likely range for the higher scenario (RCP8.5) as high as 160–180 cm (5–6 feet) (Kopp et al. 2014,38924fa0-a0dd-44c9-a2a0-366ca610b280 sensitivity study).be9f25a7-6fb1-4599-b971-47aeb2abf967 c748bd06-bc78-4b9c-b511-7dab1974211e

To estimate the effect of incorporating the DeConto and Pollardae82c8a3-3033-4103-91e9-926a27d1fa18 projections of Antarctic ice sheet melt, we note that Kopp et al. (2014)’s38924fa0-a0dd-44c9-a2a0-366ca610b280 median projection of Antarctic melt in 2100 is 4 cm (1.6 inches) (RCP2.6), 5 cm (2 inches) (RCP4.5), or 6 cm (2.4 inches) (RCP8.5). By contrast, DeConto and Pollard’sae82c8a3-3033-4103-91e9-926a27d1fa18 ensemble mean projections are (varying the assumptions for the size of Pliocene mass loss and the bias correction in the Amundsen Sea) 2–14 cm (0.1–0.5 foot) for an even lower scenario (RCP2.6), 26–58 cm (0.9–1.9 feet) for a lower scenario (RCP4.5), and 64–114 cm (2.1–3.7 ft) for the higher scenario (RCP8.5). Thus, we conclude that DeConto and Pollard’sae82c8a3-3033-4103-91e9-926a27d1fa18 projection would lead to a –10 cm (−0.1–0.3 ft) increase in median RCP2.6 projections, a 21–53 cm (0.7–1.7 feet) increase in median RCP4.5 projections, and a 58–108 cm (1.9–3.5 feet) increase in median RCP8.5 projections.

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 Mengel et al. (2016)94117a50-acc5-4dbf-8029-368aa3fc9680
RCP8.5 11–18 (0.4–0.6) 8–15 (0.3–0.5) 7–12 (0.2–0.4)
RCP4.5 10–18 (0.3–0.6) 8–15 (0.3–0.5) 7–12 (0.2–0.4)
RCP2.6 10–18 (0.3–0.6) 8–15 (0.3–0.5) 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 Mengel et al. (2016)94117a50-acc5-4dbf-8029-368aa3fc9680
RCP8.5 21–38 (0.7–1.2) 16–34 (0.5–1.1) 15–28 (0.5–0.9)
RCP4.5 18–35 (0.6–1.1) 15–31 (0.5–1.0) 14–25 (0.5–0.8)
RCP2.6 18–33 (0.6–1.1) 14–29 (0.5–1.0) 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 Mengel et al. (2016)94117a50-acc5-4dbf-8029-368aa3fc9680
RCP8.5 55–121 (1.8–4.0) 52–131 (1.7–4.3) 57–131 (1.9–4.3)
RCP4.5 36–93 (1.2–3.1) 33–85 (1.1–2.8) 37–77 (1.2–2.5)
RCP2.6 29–82 (1.0–2.7) 24–61 (0.8–2.0) 28–56 (0.9–1.8)

New information and remaining uncertainties: 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: There is very high confidence that future 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 emission scenarios are small before 2050 but significant beyond 2050.

This finding was derived from figure -.2: Confidence / Likelihood

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