This Report assesses the scientific basis for projections of future sea level rise-Voir le texte complet sous ce titre

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6. U.S. coastal sea level change . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
6.1 Overview of local U.S. sea level change
6.2 Vertical land motion
6.2.1 U.S. west coast
6.2.2 U.S. mid- and north-Atlantic coast
6.2.3 Summary of local vertical land motion
6.3 Absolute local sea level rise

6.4 Pacific coast
6.4.1 King County
6.4.2 San Francisco Bay area
6.4.3 San Diego
6.5 Gulf coast
6.5.1 Galveston-Houston
6.5.2 New Orleans and the Mississippi Delta
6.5.3 St. Petersburg and Tampa Bay
6.6 Atlantic coast

6.6.1 Rhode Island
6.6.2 New York City
6.6.3 Baltimore and Chesapeake Bay
6.6.4 Miami and South Florida
6.7 Local projections for the 21st century

7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

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Executive Summary

This Report assesses the scientific basis for projections of future sea level rise. The Report
evaluates the projections from the Intergovernmental Panel on Climate Change (IPCC)
and recent national assessments regarding sea level rise. The uncertainties and challenges
at the knowledge frontier are assessed in the context of recent research, particularly with
regards to natural variability. The following four issues frame this Report:

1. Whether recent global sea level rise is unusual.
At least in some regions, sea level was higher than present around 5000 to 7000 years ago.
After several centuries of sea level decline following the Medieval Warm Period, sea
levels began to rise in the mid 19th century. Rates of global mean sea level rise between
1920 and 1950 were comparable to recent rates. It is concluded that recent change is
within the range of natural sea level variability over the past several thousand years.

2. The extent to which recent global sea level rise is caused by human-caused global
warming, relative to natural causes of global sea level rise.
The slow emergence of fossil fuel emissions prior to 1950 did not contribute significantly
to 19th and early 20th century sea level rise. Identifying a potential human fingerprint on
recent sea level rise is confounded by the large magnitude of natural internal variability
associated with ocean circulation patterns. There is not yet any convincing evidence of
such a fingerprint on sea level rise associated with human-caused global warming.
3. The extent to which local sea level rise is influenced by the global sea level rise,

relative to local vertical land motion and local land use practices.
In many of the most vulnerable coastal locations, the dominant causes of local sea level
rise problems are natural oceanic and geologic processes and land use practices. Land use
and coastal engineering in the major coastal cities have brought on many of the worst local
problems, notably landfilling in coastal wetland areas and groundwater extraction.
4. The amount of sea level rise (global and local) projected for the 21st century.
Local sea level in many regions will continue to rise in the 21st century – independent of
global climate change. There are numerous reasons to think that projections of 21st century
sea level rise from human-caused global warming are too high, and some of the worst-case
scenarios strain credulity.

Understanding climate change and sea level rise involves incomplete information from a
fast-moving and irreducibly uncertain science. The challenges of understanding the causes
of sea level rise and projecting future climate change and sea level rise are well-
recognized by the international community of climate and sea level researchers, as
summarized in the World Climate Research Programme (WCRP) Grand Challenges. 5
1. The alarm over sea level rise

The public discourse on the threat of sea level rise is typified by these dire statements
from climate scientists:

“That’s the big thing – sea-level rise – the planet could become ungovernable.”1
– Dr. James Hansen, former Director, NASA GISS

“We’re talking about literally giving up on our coastal cities of the world and
moving inland.”2 – Dr. Michael Mann, Penn State University

The alarm over sea level rise is not so much about the 7-8 inches or so that global sea
level has risen since 1900. Rather, it is about projections of 21st century sea level rise
from human-caused global warming.

This Report refers extensively to the Assessment Reports prepared by the
Intergovernmental Panel on Climate Change (IPCC AR4, 2007; IPCC AR5, 2013), since
these Reports are used to guide policies developed by the UN Framework Convention on
Climate Change, including the 2015 Paris Agreement.

According to the IPCC, the projected 21st century sea level rise depends on the amount of
greenhouse gas emissions. The likely range of projected sea level rise by the end of the
21st century is from 0.26 to 0.82 m [10 to 32 inches], depending on the emissions
scenario.

The primary concern over future sea level rise is related to the potential collapse of the
West Antarctic Ice Sheet, which could cause global mean sea level to rise substantially
above the IPCC’s likely range in the 21st century. The IPCC AR5 has medium confidence
that this additional contribution from the West Antarctic ice sheet would not exceed
several tenths of a meter [less than a foot] during the 21st century.

Subsequent to the IPCC AR5, there has been a focus on the possible worst-case scenario
for global sea level rise. Estimates of the maximum possible global sea level rise by the
end of the 21st century range from 1.6 to 3 meters [5-10 feet], and even higher. These
extreme values of possible sea level rise are regarded as extremely unlikely or so
unlikely that we cannot even assign a probability. Nevertheless, these extreme, barely
possible values of sea level rise are becoming anchored as outcomes that are driving
local adaptation plans.3


1 http://nymag.com/intelligencer/2017/07/scientist-jim-hansen-the-planet-could-become-
ungovernable.html?gtm=top&gtm=bottom 2 https://www.sciencefriday.com/segments/hurricane-harvey-and-the-new-normal/ 3 https://www.scientificamerican.com/article/prepare-for-10-feet-of-sea-level-rise-california-commission-
tells-coastal-cities/ 6
Is the alarm over sea level rise a false alarm, or not? The following four issues frame
this report:
1. Whether recent global sea level rise is unusual in context of the historical and
geological record.
2. The extent to which recent global sea level rise is caused by human-caused
global warming, relative to natural causes of global sea level rise.
3. The extent to which local sea level rise is influenced by the global sea level rise,

relative to local vertical land motion and land use practices.
4. Projections of sea level rise (global and local) for the 21st century, from all
causes.

This Report critically evaluates the assessment and conclusions from the IPCC and other
recent assessment reports regarding sea level rise, and includes an assessment of recent
research and the knowledge frontiers. The IPCC and other assessment reports have been
framed around assessing support for the hypothesis of human-caused climate change. As
a result, natural processes of climate variability have been relatively neglected in these
assessments. Arguments are presented here supporting the important and even dominant
role that natural processes play in global and regional sea level variations and change.

Understanding and predicting sea level rise is a vibrant and active area of research. The
challenges and uncertainties are well recognized by international scientific community,

as formulated by the World Climate Research Programme (WCRP) Grand Challenge on
Regional Sea Level Change and Coastal Impacts (WCRP, 2017a).

2. Mechanisms of sea level variations

Changes in sea level occur over a broad range of temporal and spatial scales. As context
for the concerns about future sea level rise from human-caused global warming, it is
important to understand the broad range of factors that influence sea level. Rovere et al
(2016) provides a helpful overview of the definitions of sea level, why it varies and how

it is measured.

An understanding of sea-level change requires maintaining a clear distinction
between global (or eustatic) sea-level and local relative sea-level. Sea level changes can
be driven by either variations in the masses or volume of the oceans (‘eustatic’), or by
changes of the sea surface relative to the land (‘relative’).

2.1 Global
Eustatic change (as opposed to local change) results in an alteration to the global sea
levels due to changes in either the volume of water in the world’s oceans or net changes
in the volume of the ocean basins. Determination and interpretation of sea level rise is
complicated by the fact that both mean sea level and the solid earth surface move

vertically with respect to each other. This movement in effect changes the shape of the
‘bathtub’.
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The primary contributors to global, eustatic sea level change are the expansion of the
ocean as it warms and the transfer to the ocean of water from melting ice locked in
mountain glaciers and continental ice sheets. Additional contributions are from changes
in land-water storage and the shape of the ocean basins. Figure 2.1 provides a schematic
of these processes.

Figure 2.1: Climate sensitive processes and components that influence global sea level.
The term ‘ocean properties’ refers to ocean temperature, salinity and density, which
influence and are dependent on ocean circulation. [IPCC AR5 WGI, Chapter 13]

Sea level also changes in response to changing volume of the ocean basins. Some land
movements occur because of isostatic adjustment of the Earth’s mantle since the end of
the last ice age. The weight of the ice sheet depresses the underlying land, and when the
ice melts away the land slowly rebounds. This is referred to as glacial isostatic
adjustment (GIA). The Earth’s ocean basins have been expanding slightly since the end
of the last ice age via geological processes including converging and diverging plate
tectonics, uplift of the collision margin, basin subsidence of the extensional crust,

volcanic activities in the oceanic region, delta buildup, ocean floor height change and
sub-marine mass avalanche. Hence, the amount of water added to the oceans is changing
sea level less than it would if the size of the ocean basins was fixed.

Melting of glaciers and ice sheets since the end of the last ice age has transferred water
mass from the land to the oceans. This mass transfer contributes to present-day sea level
change due to the ongoing deformation of the Earth and the corresponding changes of
the ocean floor height and gravity (glacial isostatic adjustment). Ice sheets have long
response times and continue to respond to past climate change, while responding to
current climate conditions as well.

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Melting of sea ice has no impact on sea level – water or ice that is already floating does
not change the sea level by melting/freezing. Ice on a continent that melts and runs into
the ocean increases sea level due to increasing the amount of ocean water. Antarctic ice
shelves are in the ocean but are supported by the continent of Antarctica – melting these
ice sheets will increase sea level.

The global hydrological cycle influences the amount of rainfall over land versus ocean.
Interannual variations in global sea level are caused by different rainfall patterns in El
Niño versus La Niña years.

Land-based water management practices such as water impoundment in dams and
reservoirs, irrigation and ground water extraction influence the amount of water stored
in the ground or on its surface. This storage influences the amount of surface water

runoff into the ocean, and so influences sea level.

2.2 Regional

Figure 2.2 shows a map of sea level trends over the period 1993-2014. Complex spatial
patterns result from ocean and atmospheric circulation systems, vertical movements of
the sea floor, and changes in gravity due to water mass redistribution. As a result, the
locally observed sea level behavior can differ significantly from the global average.

Note the very large sea level rise anomalies in the tropical west Pacific, the South
Pacific and Indian Ocean, although this pattern appears to have reversed since 2011
(Hamlington et al., 2016).
Figure 2.2: Regional mean sea level trends over the 1993-2014 period (Ablain et al. 2016)
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2.3 Local

Observed rates of mean global sea level rise can have little relevance for a specific
location. Multiple causes of local sea level rise were recognized by the IPCC AR5:
• “[L]ocal sea level trends are also influenced by factors such as regional
variability in ocean and atmospheric circulation, subsidence, isostatic

adjustment, coastal erosion, and coastal modification.” [IPCC AR5 WG II
Section 18.3.3]
• “Anthropogenic causes of regional sea level rise include sediment
consolidation from building loads, reduced sediment delivery to the coast,

and extraction of subsurface resources such as gas, petroleum, and
groundwater. Regional sea level rise can exceed global mean sea level rise
by an order of magnitude reaching more than 10 cm/yr.” [IPCC AR5 WG
II Section 5.3.2.2]

Human causes of local sea level rise include sediment compaction from building loads,
reduced sediment delivery to the coast, and extraction of subsurface resources such as

groundwater, gas and petroleum. Engineering practices that influence the natural flow of
sediment have a substantial influence on deltas and coastal morphology.

Marshes naturally keep pace with sea level rise by trapping sediment and growing
plants. Diminished sediment supply or human-induced sinking can reduce the
effectiveness of marshes in replenishing the coast. Coastal landfills that displace
wetlands not only stop this replenishment process, but these landfill regions can sink
through compaction under the weight of heavy building loads.

Individual locations are also subject to substantial variations in sea level from tides and
storm surges. Storm surges from hurricanes or mid-latitude cyclones can be as high as
40 feet. Local tides of magnitude up to 40 feet vary on timescales ranging from hours to
years. Extreme local sea levels can arise from combinations of astronomical tides, storm
surges, wind waves and swell, the annual cycle and interannual variability.

3. Is the recent sea level rise unusual?

To assess whether the recent sea level rise is unusual, it is important to understand the
variability of sea level and its rates of change over the historical and geologic records.
Further, it is important to understand the reliability of these data sets and the
uncertainties introduced from the analysis methods.

The IPCC AR5 WGI (2013) provided the follow summary statements:

“It is very likely that the mean rate of global averaged sea level rise was 1.7 [1.5 to
1.9] mm/year between 1901 and 2010 . . . and 3.2 [2.8 to 3.6] mm/year between 1993
and 2010. It is likely that similarly high rates occurred between 1920 and 1950.”
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“The rate of sea level rise since the mid-19th century has been larger than the mean
rate during the previous two millennia (high confidence).”

“There is very high confidence that maximum global mean sea level during the last
interglacial period (129,000 to 116,000 years ago) was, for several thousand years, at
least 5 meters higher than present, and high confidence that it did not exceed 10
meters above present.”
The recent rate of sea level rise is about 3 mm/year. For reference, 3 mm is the height of
two stacked pennies (U.S. coins). This chapter critically evaluates our knowledge base for
documenting current and past sea level rise.

3.1 The geologic record

The geologic record provides some constraints and insights into understanding how much,
and how fast, sea level might rise in the coming centuries. It also provides critical context

for understanding whether current sea levels and rates of sea level rise are unusual.

How is past sea level revealed in the geologic records? Proxy records of sea level are
preserved in a variety of marine and terrestrial settings, such as sediments and organisms
in deep ocean cores or once-submerged shorelines, and uplifted fossil reefs. Analysis of
oxygen isotopes in tiny ocean organisms, and radiometric techniques are used to interpret
and date the records (for a summary, see Horton et al., 2018).

During the past 20,000 years (the Holocene), since the end of the last glaciation, sea level
has risen by a total of about 120 meters [400 feet]. Figure 3.1 shows sea level for the last
24,000 years. Sea level was lowest between 22,000 and 18,000 years ago, rising sharply
between 15,000 and 8,000 years ago.

During the deglaciation occurring between 19,000 and 8,000 years ago, sea level rose at
extremely high rates (Cronin, 2012). At the onset of the deglaciation, a ~ 500-year long
event may have contributed as much as 10 m to sea level, with an average rate of about
20 mm/year. During the rest of the early Holocene, the rate of sea level rise varied from a

low of about 6-10 mm/year to as high as 30–60 mm/year during brief periods of
accelerated sea level rise. For reference, these values are compared to modern values of
sea level rise ranging from 1 to 3 mm/year.

On average, sea level has been relatively stable over the past 6,000 years. Of particular
interest is the so-called ‘mid-Holocene high-stand’ between about 6000 and 3000 years
ago, when sea level in some regions was up to several meters higher than present (e.g.
Prieto et al. 2016; Chiba et al. 2016; Bradley et al. 2016; Accordi and Carbone, 2016). The

sea level high-stand was associated with the so-called Climatic Optimum or the Holocene
Optimum, during 8000 to 4000 BC, when average global temperatures reached their
maximum level during the Holocene and were warmer than preindustrial

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