ellisbrown/BDML25SP_hw1_processed_data

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few model studies available addressing time scales of centuries to millennia indicate multi-metre (2.3–5.4 m) rise in sea level for RCP8.5 (low conﬁdence ). There is low conﬁdence in threshold temperatures
for ice sheet instabilities and the rates of GMSL rise they can produce. {Cross-Chapter Box 5 in Chapter 1, Cross-Chapter Box 8 in Chapter 3, and Sections 4.1, 4.2.3.1.1, 4.2.3.1.2, 4.2.3.6}
Sea level rise is not globally uniform and varies regionally.
Thermal expansion, ocean dynamics and land ice loss
contributions will generate regional departures of about ±30%
around the GMSL rise. Differences from the global mean can be greater than ±30% in areas of rapid vertical land movements, including those caused by local anthropogenic factors such as
groundwater extraction ( high conﬁdence ). Subsidence caused by
human activities is currently the most important cause of relative sea
level rise (RSL) change in many delta regions. While the comparative
importance of climate-driven RSL rise will increase over time, these
ﬁndings on anthropogenic subsidence imply that a consideration of
local processes is critical for projections of sea level impacts at local
scales ( high conﬁdence ). {4.2.1.6, 4.2.2.4}
Due to projected GMSL rise, ESLs that are historically rare (for
example, today’s hundred-year event) will become common
by 2100 under all RCPs ( high conﬁdence ). Many low-lying cities
and small islands at most latitudes will experience such events
annually by 2050. Greenhouse gas (GHG) mitigation envisioned in
low-emission scenarios (e.g., RCP2.6) is expected to sharply reduce
but not eliminate risk to low-lying coasts and islands from SLR and
ESL events. Low-emission scenarios lead to slower rates of SLR and allow for a wider range of adaptation options. For the ﬁrst half of the
21st century differences in ESL events among the scenarios are small, facilitating adaptation planning. {4.2.2.5, 4.2.3.4, Figure TS.6}
Non-climatic anthropogenic drivers will continue to increase
the exposure and vulnerability of coastal communities to future
SLR and ESL events in the absence of major adaptation efforts
compared to today ( high conﬁdence ). {4.3.4, Cross-Chapter Box 9}
The expected impacts of SLR on coastal ecosystems over
the course of the century include habitat contraction, loss
of functionality and biodiversity, and lateral and inland
migration. Impacts will be exacerbated in cases of land
reclamation and where anthropogenic barriers prevent inland
migration of marshes and mangroves and limit the availability
and relocation of sediment ( high conﬁdence ). Under favourable
conditions, marshes and mangroves have been found to keep pace with fast rates of SLR (e.g., >10 mm yr
–1), but this capacity varies
signiﬁcantly depending on factors such as wave exposure of the
location, tidal range, sediment trapping, overall sediment availability and coastal squeeze ( high conﬁdence ). {4.3.3.5.1}
In the absence of adaptation, more intense and frequent ESL
events, together with trends in coastal development will
increase expected annual ﬂood damages by 2–3 orders of
magnitude by 2100 ( high conﬁdence ). However, well designed
coastal protection is very effective in reducing expected
damages and cost efﬁcient for urban and densely populated
regions, but generally unaffordable for rural and poorer areas
(high conﬁdence ). Effective protection requires investments on the
order of tens to several hundreds of billions of USD yr
–1 globally ( high
conﬁdence ). While investments are generally cost efﬁcient for densely
populated and urban areas ( high conﬁdence ), rural and poorer areas
will be challenged to afford such investments with relative annual
costs for some small island states amounting to several percent of
GDP ( high conﬁdence ). Even with well-designed hard protection, the
risk of possibly disastrous consequences in the event of failure of
defences remains. {4.3.4, 4.4.2.2, 4.4.3.2, Cross-Chapter Box 9}
Risk related to SLR (including erosion, ﬂooding and
salinisation) is expected to signiﬁcantly increase by the end of
this century along all low-lying coasts in the absence of major
additional adaptation efforts ( very high conﬁdence ). While
only urban atoll islands and some Arctic communities are expected
to experience moderate to high risk relative to today in a low
emission pathway, almost high to very high risks are expected in all
low-lying coastal settings at the upper end of the likely range for high
emission pathways ( medium conﬁdence ). However, the transition
from moderate to high and from high to very high risk will vary from one coastal setting to another ( high conﬁdence ). While a slower
rate of SLR enables greater opportunities for adapting, adaptation
beneﬁts are also expected to vary between coastal settings. Although
ambitious adaptation will not necessarily eradicate end-century SLR
risk ( medium conﬁdence ), it will help to buy time in many locations
and therefore help to lay a robust foundation for adaptation beyond
2100. {4.1.3, 4.3.4, Box 4.1, SM4.2}

57Technical Summary
TS
Choosing and Implementing Responses
All types of responses to SLR, including protection,
accommodation, EbA, advance and retreat, have important
and synergistic roles to play in an integrated and sequenced
response to SLR ( high conﬁdence ). Hard protection and advance
(building into the sea) are economically efﬁcient in most urban contexts facing land scarcity ( high conﬁdence ), but can lead to
increased exposure in the long term. Where sufﬁcient space is available, EbA can both reduce coastal risks and provide multiple other beneﬁts ( medium conﬁdence ). Accommodation such as ﬂood
prooﬁng buildings and EWS for ESL events are often both low-cost and
highly cost-efﬁcient in all contexts ( high conﬁdence ). Where coastal risks are already high, and population size and density are low, or in
the aftermath of a coastal disaster, retreat may be especially effective,
albeit socially, culturally and politically challenging. {4.4.2.2, 4.4.2.3, 4.4.2.4, 4.4.2.5, 4.4.2.6, 4.4.3}
Technical limits to hard protection are expected to be reached
under high emission scenarios (RCP8.5) beyond 2100 ( high
conﬁdence ) and biophysical limits to EbA may arise during
the 21st century, but economic and social barriers arise well
before the end of the century ( medium conﬁdence ). Economic
challenges to hard protection increase with higher sea levels and will
make adaptation unaffordable before technical limits are reached
(high conﬁdence ). Drivers other than SLR are expected to contribute 1/month1/year1/decade1/century
1/month1/year1/decade1/century
recent past futuremean sea levelmean sea level
sea
level
rise
2000Year
2020 2040 2060 2080 2100TimeSea level height and recurrence frequencyHCEHistorical Centennial extreme sea level
Events (HCEs) become more common due to sea level rise

few model studies available addressing time scales of centuries to millennia indicate multi-metre (2.3–5.4 m) rise in sea level for RCP8.5 (low conﬁdence ). There is low conﬁdence in threshold temperatures

for ice sheet instabilities and the rates of GMSL rise they can produce. {Cross-Chapter Box 5 in Chapter 1, Cross-Chapter Box 8 in Chapter 3, and Sections 4.1, 4.2.3.1.1, 4.2.3.1.2, 4.2.3.6}

Sea level rise is not globally uniform and varies regionally.

Thermal expansion, ocean dynamics and land ice loss

contributions will generate regional departures of about ±30%

around the GMSL rise. Differences from the global mean can be greater than ±30% in areas of rapid vertical land movements, including those caused by local anthropogenic factors such as

groundwater extraction ( high conﬁdence ). Subsidence caused by

human activities is currently the most important cause of relative sea

level rise (RSL) change in many delta regions. While the comparative

importance of climate-driven RSL rise will increase over time, these

ﬁndings on anthropogenic subsidence imply that a consideration of

local processes is critical for projections of sea level impacts at local

scales ( high conﬁdence ). {4.2.1.6, 4.2.2.4}

Due to projected GMSL rise, ESLs that are historically rare (for

example, today’s hundred-year event) will become common

by 2100 under all RCPs ( high conﬁdence ). Many low-lying cities

and small islands at most latitudes will experience such events

annually by 2050. Greenhouse gas (GHG) mitigation envisioned in

low-emission scenarios (e.g., RCP2.6) is expected to sharply reduce

but not eliminate risk to low-lying coasts and islands from SLR and

ESL events. Low-emission scenarios lead to slower rates of SLR and allow for a wider range of adaptation options. For the ﬁrst half of the

21st century differences in ESL events among the scenarios are small, facilitating adaptation planning. {4.2.2.5, 4.2.3.4, Figure TS.6}

Non-climatic anthropogenic drivers will continue to increase

the exposure and vulnerability of coastal communities to future

SLR and ESL events in the absence of major adaptation efforts

compared to today ( high conﬁdence ). {4.3.4, Cross-Chapter Box 9}

The expected impacts of SLR on coastal ecosystems over

the course of the century include habitat contraction, loss

of functionality and biodiversity, and lateral and inland

migration. Impacts will be exacerbated in cases of land

reclamation and where anthropogenic barriers prevent inland

migration of marshes and mangroves and limit the availability

and relocation of sediment ( high conﬁdence ). Under favourable

conditions, marshes and mangroves have been found to keep pace with fast rates of SLR (e.g., >10 mm yr

–1), but this capacity varies

signiﬁcantly depending on factors such as wave exposure of the

location, tidal range, sediment trapping, overall sediment availability and coastal squeeze ( high conﬁdence ). {4.3.3.5.1}

In the absence of adaptation, more intense and frequent ESL

events, together with trends in coastal development will

increase expected annual ﬂood damages by 2–3 orders of

magnitude by 2100 ( high conﬁdence ). However, well designed

coastal protection is very effective in reducing expected

damages and cost efﬁcient for urban and densely populated

regions, but generally unaffordable for rural and poorer areas

(high conﬁdence ). Effective protection requires investments on the

order of tens to several hundreds of billions of USD yr

–1 globally ( high

conﬁdence ). While investments are generally cost efﬁcient for densely

populated and urban areas ( high conﬁdence ), rural and poorer areas

will be challenged to afford such investments with relative annual

costs for some small island states amounting to several percent of

GDP ( high conﬁdence ). Even with well-designed hard protection, the

risk of possibly disastrous consequences in the event of failure of

defences remains. {4.3.4, 4.4.2.2, 4.4.3.2, Cross-Chapter Box 9}

Risk related to SLR (including erosion, ﬂooding and

salinisation) is expected to signiﬁcantly increase by the end of

this century along all low-lying coasts in the absence of major

additional adaptation efforts ( very high conﬁdence ). While

only urban atoll islands and some Arctic communities are expected

to experience moderate to high risk relative to today in a low

emission pathway, almost high to very high risks are expected in all

low-lying coastal settings at the upper end of the likely range for high

emission pathways ( medium conﬁdence ). However, the transition

from moderate to high and from high to very high risk will vary from one coastal setting to another ( high conﬁdence ). While a slower

rate of SLR enables greater opportunities for adapting, adaptation

beneﬁts are also expected to vary between coastal settings. Although

ambitious adaptation will not necessarily eradicate end-century SLR

risk ( medium conﬁdence ), it will help to buy time in many locations

and therefore help to lay a robust foundation for adaptation beyond

2100. {4.1.3, 4.3.4, Box 4.1, SM4.2}

57Technical Summary

TS

Choosing and Implementing Responses

All types of responses to SLR, including protection,

accommodation, EbA, advance and retreat, have important

and synergistic roles to play in an integrated and sequenced

response to SLR ( high conﬁdence ). Hard protection and advance

(building into the sea) are economically efﬁcient in most urban contexts facing land scarcity ( high conﬁdence ), but can lead to

increased exposure in the long term. Where sufﬁcient space is available, EbA can both reduce coastal risks and provide multiple other beneﬁts ( medium conﬁdence ). Accommodation such as ﬂood

prooﬁng buildings and EWS for ESL events are often both low-cost and

highly cost-efﬁcient in all contexts ( high conﬁdence ). Where coastal risks are already high, and population size and density are low, or in

the aftermath of a coastal disaster, retreat may be especially effective,

albeit socially, culturally and politically challenging. {4.4.2.2, 4.4.2.3, 4.4.2.4, 4.4.2.5, 4.4.2.6, 4.4.3}

Technical limits to hard protection are expected to be reached

under high emission scenarios (RCP8.5) beyond 2100 ( high

conﬁdence ) and biophysical limits to EbA may arise during

the 21st century, but economic and social barriers arise well

before the end of the century ( medium conﬁdence ). Economic

challenges to hard protection increase with higher sea levels and will

make adaptation unaffordable before technical limits are reached

(high conﬁdence ). Drivers other than SLR are expected to contribute 1/month1/year1/decade1/century

1/month1/year1/decade1/century

recent past futuremean sea levelmean sea level

sea

level

rise

2000Year

2020 2040 2060 2080 2100TimeSea level height and recurrence frequencyHCEHistorical Centennial extreme sea level

Events (HCEs) become more common due to sea level rise