Editing IPCC:AR6/SROCC/Chapter-4 (section)

=== 4.1.2 Future Sea level Rise and Implications for Responses ===

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For understanding responses to climate-change induced SLR, two aspects of sea level are important to note initially:

# Climate-change induced GMSL rise is caused by thermal expansion of ocean water and ocean mass gain, the latter primarily due to a decrease in land-ice mass. However, responses to SLR are local and hence always based on RSL experienced at a particular location. GMSL is modified regionally by climate processes and locally by a variety of factors, some driven or influenced by human activity. Of particular relevance for responding to SLR is anthropogenic subsidence, which can lead to rates of RSL rise that exceed those of climate-induced SLR by an order of magnitude, specifically in delta regions and near cities (4.2.2.4). In these subsiding regions, one available response to prepare for future climate-induced SLR is to manage and reduce anthropogenic subsidence (4.4.2).
# The combination of gradual change of mean sea level with ESL events such as tides, surges and waves causes coastal impacts (4.2.3). ESL events at the coast that are rare today will become more frequent in the future, which means that for many locations, the main starting point for coastal planning and decision making is information on current and future ESL events. One important response for preparing for future SLR is to improve observational systems (tide gauges, wave buoys and remote sensing techniques), because in many places around the world current frequencies and intensities of ESL events are not well understood due to a lack of observational data (4.2.3.4.1).

After an increase of sea level from 1–2 mm yr –1 in most regions over the past century, rates of 3–4 mm yr –1 are now being experienced that will further increase to 4–9 mm yr –1 under RCP2.6 and 10–20 mm yr –1 at the end of the century under RCP8.5. Nevertheless, up to 2050, uncertainty in climate change-driven future sea level is relatively small, which provides a robust basis for short-term ( ≤ 30 years) adaptation planning. GMSL will rise between 0.24 m (0.17–0.32 m, ''likely'' range) under RCP2.6 and 0.32 m (0.23–0.40 m, ''likely'' range) under RCP8.5 ( ''medium confidence'' ; 4.2.3). The combined effect of mean and extreme sea levels results in events which are rare in the historical context (return period of 100 years or larger; probability &lt;0.01 yr –1 ) occurring yearly at some locations by the middle of this century under all emission scenarios (4.2.3.4.1; ''high confidence'' ). This includes, for instance, those parts of the intertropical low-lying coasts that are currently exposed to storm surges only infrequently. Hence, additional adaptation is needed irrespective of the uncertainties in future global GHG emissions and the Antarctic contribution to SLR.

Beyond 2050, uncertainty in climate change induced SLR increases substantially due to uncertainties in emission scenarios and the associated climate changes, and the response of the AIS in a warmer world. Combining process-model based studies in which there is ''medium confidence'' , it is found that GMSL is projected to rise between 0.43 m (0.29–0.59 m, ''likely'' range) under RCP 2.6 and 0.84 m (0.61–1.10 m, ''likely'' range) under RCP 8.5 by 2100 (Figure 4.3). The range that needs to be considered for planning and implementing coastal responses depends on the risk tolerance of stakeholders (i.e., those deciding and those affected by a decision; 4.4.4.3.2). Stakeholders that are risk tolerant (e.g., those planning for investments that can be very easily adapted to unforeseen conditions) may prefer to use the ''likely'' ranges of RCP2.6 and RCP8.5 for long-term adaptation planning. Stakeholders with a low risk tolerance (e.g., those planning for coastal safety in cities and long term investment in critical infrastructure) may also consider SLR above this range, because there is a 17% chance that GMSL will exceed 0.59 m under RCP2.6 and 1.10 m under RCP8.5 in 2100. Process-model based studies cannot yet provide this information, but expert elicitation studies show that a GMSL of 2 m in 2100 cannot be ruled out (4.2.3).

Despite the large uncertainty in late 21st century SLR, progress in adaptation planning and implementation is feasible today and may be economically beneficial. Many coastal decisions with time horizons of decades to over a century are made today (e.g., critical infrastructure, coastal protection works, city planning, etc.) and accounting for relative SLR can improve these decisions. Decision-analysis methods specifically targeting situations of large uncertainty are available and, combined with suitable planning, public participation and conflict resolution processes, can improve outcomes ( ''high confidence'' ; 4.4.4.2, 4.4.4.3). For example, adaptation pathway analysis recognises and enables sequenced long-term decision making in the face of dynamic coastal risk characterised by deep uncertainty ( ''medium evidence, high agreement'' ; 4.4.4.3.4). The use of these decision-analysis tools can be integrated into statutory land use or spatial planning provisions to formalise these decisions and enable effective implementation by relevant governing authorities (4.4.4.2).

Beyond 2100, sea level will continue to rise for centuries and will remain elevated for thousands of years ( ''high confidence;'' 4.2.3.5). Only a few modelling studies are available for SLR beyond 2100. However, all studies agree that the difference in GMSL between RCP2.6 and RCP8.5 increases substantially on multi-centennial and millennial time scales ( ''very high confidence'' ). On a millennial time scale, this difference is about 10 metres in some model simulations, whereas it is only several decimetres at the end of 21st century. The larger the emissions the larger the risks associated with SLR as already assessed in SR1.5. Under RCP8.5 the few available studies indicate a ''likely'' range of 2.3–5.4 m ( ''low confidence'' ) in 2300. With strong mitigation efforts (RCP2.6), SLR will be kept to a ''likely'' range of 0.6–1.1 m (Figure 4.2). Regardless, ambitious and sustained adaptation efforts are needed to reduce risks.

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'''Figure 4.1'''

<span id="figure-4.1-schematic-illustration-of-the-interconnection-of-chapter-4-themes-including-drivers-of-sea-level-rise-slr-and-extreme-sea-level-hazards-section-4.2-exposure-vulnerability-impacts-and-risk-related-to-slr-section-4.3-and-responses-associated-governance-challenges-and-practises-and-tools-for-enabling-social-choices-and-addressing-governance-challenges-section-4.4."></span>
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'''Figure 4.1 | Schematic illustration of the interconnection of Chapter 4 themes, including drivers of sea level rise (SLR) and (extreme) sea level hazards (Section 4.2), exposure, vulnerability, impacts and risk related to SLR (Section 4.3), and responses, associated governance challenges and practises and tools for enabling social choices and addressing governance challenges (Section 4.4).'''
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[[File:636c2af83c3fe98ec1d45645d066b94c IPCC-SROCC-CH_4_1-3000x1698.jpg]]

Figure 4.1 | Schematic illustration of the interconnection of Chapter 4 themes, including drivers of sea level rise (SLR) and (extreme) sea level hazards (Section 4.2), exposure, vulnerability, impacts and risk related to SLR (Section 4.3), and responses, associated governance challenges and practises and tools for enabling social choices and addressing governance challenges (Section 4.4).

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'''Figure 4.2'''

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'''Figure 4.2 | Projected sea level rise (SLR) until 2300. The inset shows an assessment of the likely range of the projections for RCP2.6 and RCP8.5 up to 2100 (medium confidence). Projections for longer time scales are highly uncertain but a range is provided (4.2.3.6; low confidence). For context, results are shown from other estimation […]'''
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[[File:9a7523b4e542c402e911b12dbb20767d IPCC-SROCC-CH_4_2-3000x1354.jpg]]

Figure 4.2 | Projected sea level rise (SLR) until 2300. The inset shows an assessment of the likely range of the projections for RCP2.6 and RCP8.5 up to 2100 (medium confidence). Projections for longer time scales are highly uncertain but a range is provided (4.2.3.6; low confidence). For context, results are shown from other estimation approaches in 2100 and 2300. The two sets of two bars labelled B19 are from an expert elicitation for the Antarctic component (Bamber et al., 2019 <sup>[[#fn:r1|1]]</sup> ), and reflect the likely range for a 2oC and 5oC temperature warming (low confidence; details section 4.2.3.3.1). The bar labelled “prob.” indicates the likely range of a set of probabilistic projections (4.2.3.2). The arrow indicated by S18 shows the result of an extensive sensitivity experiment with a numerical model for the Antarctic Ice Sheet (AIS) combined, like the results from B19 and “prob.”, with results from Church et al. (2013) <sup>[[#fn:r2|2]]</sup> for the other components of SLR. S18 also shows the likely range.

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