Editing IPCC:AR6/SROCC/Cross-Chapter-Box-9 (section)

== D Drivers of Impacts and Risks ==

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'''''Climate-related hazards''''' —LLIC are subject to the same climate-related hazards as other islands and coasts (overview in Wong et al., 2014), for both extreme events, for example, marine heat waves, tropical and extratropical storms, associated storm surges, and heavy precipitation; and slow onset changes, for example, retreat of glaciers and ice sheets, sea ice and permafrost thaw, sea level rise, and ocean warming and acidification (Sections 1.4, 2.2, 3.2 to 3.4, 4.2, 5.2, 6.2 to 6.6, Box 6.1). Table CB9.1 summarises the SROCC updates of these hazards, which often combine to explain part of observed climate impacts and projected risks. For example, accelerating sea level rise will combine with storm surges, tides and waves to generate to extreme sea level events that affect flooding (Section 4.3.3.2), shoreline changes (Section 4.3.3.3) and salinisation of soils, groundwater and surface waters (Section 4.3.3.4). Sea level rise will also combine with ocean warming to accelerate permafrost thawing in the Arctic (Sections 3.4.1.2, 3.4.2.2). Ocean acidification will combine with ocean warming and deoxygenation to impact benthic and pelagic organisms, associated ecosystems (e.g., coral reefs, oyster beds) and top predators, with subsequent impacts on species’ abundance and distribution, and the ecosystem services benefiting human societies (Sections 4.3.3.5, 5.2.2, 5.3.1 to 5.3.6, 5.4.1, 6.4.2, 6.5.2, 6.6.2, 6.7.2, 6.8.2). Importantly, LLIC are at risk for multi-metre sea level rise projected post-2100 under Representative Concentration Pathway (RCP)8.5 and restricted to 1–2 m in 2300 under RCP2.6 (Section 4.2.3.5)

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'''Table CB9.1'''

Summary information on the critical climate-related drivers for low-lying islands and coasts, their trends due to climate change, and their main physical and ecosystem effects. Based on SROCC chapters and IPCC 5th Assessment Report (AR5). MSL is mean sea level, RCP is Representative Concentration Pathway, TC is tropical cyclone, ETC is extratropical cyclone, SLR is sea level rise, SST is sea surface temperature.

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{| class="wikitable"
|-
| '''Climate-related driver'''
| '''Physical/chemical effects'''
| '''Observed trends'''
| '''Projections'''
| '''SROCC section'''
|-
| Global mean sea level (MSL)
| rowspan="3"| Submergence, flood damage, erosion; saltwater intrusion; rising water tables/impeded drainage; ecosystem loss (and change)
| Tide gauge records:
''very likely'' increase of

1.5 (1.1–1.9) mm yr <sup>–1</sup> (1902–2010) and a total sea level rise of 0.16 (0.12–0.21) m

Acceleration: with ''high confidence''

(–0.002–0.019) mm yr <sup>–2</sup> over (1902–2010)

Satellite altimetry:

Global MSL of 3.0 mm yr <sup>–1</sup> (2.4–3.6) over (1993–2015)

Acceleration: with ''high confidence''

0.084 (0.059–0.090) mm yr <sup>–2</sup> over (1993–2015)

| RCP2.6 (2046–2065): 0.24 (0.17–0.32) m
RCP2.6 (2081–2100): 0.39 (0.26–0.53) m

RCP2.6 (2100): 0.43 (0.29–0.59) m

Rate of sea level rise (SLR) 4 (2–6) mm yr <sup>–1</sup> in 2100

RCP4.5 (2046–2065): 0.26 (0.19–0.34) m

RCP4.5 (2081–2100): 0.49 (0.34–0.64) m

RCP4.5. (2100): 0.55 (0.39–0.72) m

Rate of SLR 7 (4-9) mm yr <sup>–1</sup> in 2100

RCP8.5 (2046–2065): 0.32 (0.23–0.40) m

RCP8.5 (2081–2100): 0.71 (0.51–0.92) m

RCP8.5 (2100): 0.84 (0.61–1.10) m

Rate of SLR 15 (10–20) mm yr <sup>–1</sup> in 2100

| 4.2.2.2
4.2.3.2

|-
| Regional sea level
| Substantial regional variability at decadal at multi-decadal time scales due to changing winds, air-sea heat and freshwater fluxes and altered ocean circulation
| Increased regional relative sea level with respect to AR5 nearly everywhere for RCP8.5 because of the increased Antarctic contribution (Figure 4.8)
| 4.2.2.4
4.2.3.2

|-
| Extreme sea levels
| It is ''very likely'' that flood return period in low-lying areas has decreased over the past 20th century
| ''High confidence'' in more frequently or yearly extreme sea level events which are currently rare (e.g., return period of 100 years) as a consequence of sea level rise at many locations for RCP8.5 by the end of the century (Figure 4.10).
Even earlier and for RCP2.6 in locations where historical sea level variability (tides and storm surges) is small compared to projected sea level rise

| 4.2.3.4.1
4.2.3.4.3

|-
| Storms: tropical cyclones (TCs), extratropical cyclones (ETCs)
| Storm surges and storm waves, coastal flooding, erosion; saltwater intrusion; rising water tables/impeded drainage; wetland loss (and change); coastal infrastructure damage and flood defense failure
| TCs: Decreasing frequency of severe TCs in eastern Australia since the late 1800s; increase in frequency of moderately large US storm surge events since 1923; recent increase of extremely severe cyclonic storms over the Arabian Sea and intense TCs that make landfall in East and Southeast Asia in recent decades; increase in annual global proportion of hurricanes reaching Category 4 or 5 intensity in recent decades

ETCs: ''likely'' poleward movement of circulation features but ''low confidence'' in intensity changes (AR5)

| TCs: SLR will lead to higher storm surge levels for the TCs that do occur, assuming all other factors are unchanged ( ''high confidence'' ).
''Medium confidence'' that the proportion of TCs that reach Category 4 or 5 levels will increase, that the average intensity of TCs will increase (by roughly 1–10%, assuming a 2 <sup>o</sup> C global temperature rise), and that average tropical cyclone precipitation rates (for a given storm) will increase by at least 7% per degree Celsius (SST) warming. ''Low confidence'' in how global TC frequency will change, although most studies project some decrease in global TC frequency

ETCs: ''Low confidence'' in future changes in blocking and storm tracks in the northern hemisphere. The storm track projections for the southern hemisphere indicate an observed poleward contraction and a continued strengthening and southward contraction of storm tracks in the future ( ''medium confidence'' )

| 6.3.1.1
6.3.1.2

|-
| Waves
| Coastal erosion, overtopping and coastal flooding
| Small increases in significant wave height globally and larger increases (5%) in extreme wave height, especially in the Southern Ocean ( ''medium confidence'' ). Global wave power has increased over the last six decades with marked spatial changes by oceans and long-term correlations with sea surface temperature ( ''low confidence'' )
| ''High confidence'' for projected increase of the mean significant wave height across the Southern Ocean, tropical eastern Pacific and Baltic Sea and for projected decrease of significant wave height over the North Atlantic and Mediterranean Sea. ''Low confidence'' in projections of significant wave height over the eastern north Pacific and Southern Indian and Atlantic Oceans. ''Low confidence'' in projected extreme significant wave height everywhere, except for the Southern Ocean (increase) and North Atlantic (decrease) ( ''high confidence'' ). Limited knowledge on projected wave period and direction.
| 4.2.3.4.2
6.3.1.3

|-
| Sea surface temperature (SST)

| rowspan="2"| Changes to stratification and circulation; reduced incidence of sea ice at higher latitudes; increased coral bleaching and mortality, poleward species migration; increased algal blooms
| The ocean has warmed unabated, continuing the clear multi-decadal ocean warming trends documented in AR5. The 0−700 m layer of the ocean has warmed at rate of 5.31 ZJ yr <sup>–1</sup> from 2005 to 2017. The long-term trend for 0–700 m layer has warmed 4.35 ZJ yr <sup>–1</sup> from 1970 to 2017
| For RCP8.5, the 0–2000 m layers of the ocean are projected to warm by a further 2150 ZJ ( ''very likely'' range 1710 to 2790 ZJ) between 2017 and 2100
For RCP2.6, the 0–2000 m layers are projected to warm by 900 ZJ ( ''very likely'' range 650 to 1340 ZJ) by 2100

(*) ZJ is Zettajoule

|  5.2.2.2.1
|-
| Marine heat waves
| Have ''very likely'' doubled since 1980s
| ''Very likely'' increase in frequency, duration, spatial extent and intensity, even under future low levels of warming
| 6.4.1
|-
| Freshwater inputs
| Altered flood risk in coastal lowlands; altered water quality/salinity; altered fluvial sediment supply; altered circulation and nutrient supply
| ''Medium confidence'' in a net declining trend in annual volume of freshwater input
| ''Medium confidence'' for general increase in high latitude and wet tropics and decrease in other tropical regions
| AR5
|-
| Ocean acidity
| Increased CO <sub>2</sub> fertilization; decreased seawater pH and carbonate ion concentration (or ‘ocean acidification’)
| ''Virtually certain'' that ocean surface water pH is declining by a ''very likely'' range 0.017 to 0.027 pH units per decade, since 1980, everywhere individual time-series observations exist
| ''High confidence'' that the ocean will experience pH drops of between 0.1 (RCP2.6) or 0.3 (RCP8.5) pH
units by 2100, with regional and local variability, exacerbated in polar regions

| 5.2.2.3
|-
| Sea ice and permafrost thaw

| More storm surges, increasing ocean swells, coastal erosion
| Permafrost temperatures have continued to increase to record high levels ( ''very high confidence'' ) Between 2007 and 2016, permafrost temperatures here increased 0.39°C ± 0.15°C in cold continuous zone permafrost and 0.20°C ± 0.10°C in warmer discontinuous zone permafrost.

It is ''very likely'' that Arctic sea ice extent continues to decline in all months of the year; the strongest reductions in September (–12.8% ± 2.3% per decade; 1979–2018) are ''likely'' unprecedented in at least 1000 years. It is ''virtually certain'' that Arctic sea ice has thinned concurrent with a shift to younger ice: since 1979, the areal proportion of thick ice at least 5 years old has declined by approximately 90%

| For stabilised global warming of 1.5°C, an approximately 1% chance of a given September being sea ice free at the end of century is projected; for stabilised warming at a 2°C increase, this rises to 10–35% ( ''high confidence'' ). The potential for reduced (further 5–10%) but stabilised Arctic autumn and spring snow extent by mid-century for RCP2.6 contrasts with continued loss under RCP8.5 (a further 15–25% reduction to end of century) ( ''high confidence'' ).

Widespread disappearance of Arctic near-surface permafrost is projected to occur this century as a result of warming ( ''high confidence'' ). Near-surface permafrost area is projected to be reduced by 2–66% for RCP2.6 and 30–99% by 2100 under RCP8.5

| 3.2.1.1
Box 3.2

3.2.2

3.3.2

3.4.1

3.4.2

|}
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'''''Anthropogenic drivers''''' —Human factors play a major role in shaping exposure and vulnerability to climate-related changes in the Arctic, in temperate and tropical small islands, and in coastal urban areas (Sections 2.5.2, 4.3.2, Cross-Chapter Box 2 in Chapter 1). In the absence of major additional adaptation efforts compared to today (i.e., neither further significant action nor new types of actions), the anthropogenic drivers’ contribution to climate change related risk will substantially increase ( ''high confidence'' ) (Section 4.3.4.2).

Highly context-specific territorial and societal dynamics have resulted in major changes at the coast, for instance the growing concentration of people and assets in risk prone coastal areas (Section 4.3.2.2), and the degradation of coastal ecosystem services such as coastal protection and healthy conditions for coastal fisheries and aquaculture (Section 4.3.2.3, 5.4.1.3, 5.4.2.2.2). Local drivers of exposure and vulnerability include, for example, coastal squeeze, inadequate land use planning, changes in construction modes, sand mining and unsustainable resource extraction (e.g., in the Comoros; Betzold and Mohamed, 2016; Ratter et al., 2016), as well as loss of Indigenous Knowledge and Local Knowledge (IK and LK; Cross-Chapter Box 4 in Chapter 1). For example, the loss of IK and LK-based practices and associated cultural heritage limits both the ability to recognise and respond to ocean and cryosphere related risk and the empowerment of local communities ( ''high confidence'' ) (Section 4.3.2.4.2). Population growth in medium-to-mega coastal cities is also of concern. For the year 2000, the Low Elevation Coastal Zones (LECZ, highest elevation up to 10 m above sea level) were estimated to host around 625 million people (Lichter et al., 2011; Neumann et al., 2015), with the vast majority (517 million) living in non-developed contexts. By 2100, the LECZ population may increase to as much as 1.14 billion under a Shared Socioeconomic Pathway (SSP) where countries focus on domestic, or even regional issues (SSP3; Jones and O’Neill, 2016). Poor planning can combine with coastal population growth and climate-related ocean change to create maladaptation (Juhola et al., 2016; Magnan et al., 2016).

Local factors drive—as well as are driven by—more regional processes such as extensive coastal urbanisation, human-induced sediment starvation (and implications on subsidence), degradation of vegetated coastal ecosystems (e.g., mangroves, coral reefs and salt-marshes), lack of long-term integrated planning, changing consumption modes, conflicting resource use and socioeconomic inequalities ( ''high confidence'' ), among others. These are vehicles of increasing exposure and vulnerability at multiple scales.

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