Weeks 6 & 7 - Sea level change

Question 1

What is coastal squeeze?

Answer 1

the loss of natural habitats or deterioration of their quality arising from anthropogenic structures or actions, preventing the landward transgression of those habitats that would otherwise naturally occur in response to sea level rise in conjunction with other coastal processes

Question 2

what are the terrestrial factors that influence sea level rise

Answer 2

terrestrial water storage, extraction of groundwater, building of reservoirs, changes in runoff, and seepage into aquifers

Question 3

what are the factors that influence sea level rise at the shore

Answer 3

subsidence in river delta region, land movements, tectonic displacements

Question 4

what are the factors that influence sea level rise in the ocean

Answer 4

surface and deep ocean circulation changes, storm surges
as the ocean warms, the water expands
exchange of the water stored on land by glaciers and ice sheets with ocean water

Question 5

what is ocean thermal expansion

Answer 5

The warming of Earth is primarily due to accumulation of heat-trapping greenhouse gases, and more than 90 percent of this trapped heat is absorbed by the oceans. As this heat is absorbed, ocean temperatures rise and water expands.

Question 6

how does Greenland and Antarctica contribute to sea level rise

Answer 6

Greenland and Antarctica contain giant ice sheets that are also considered glaciers. As temperatures rise, glaciers melt faster than they accumulate new snow. As these ice sheets and glaciers melt, the water eventually runs into the ocean, causing sea level to rise.

Question 7

where can we get data on sea level rise?

Answer 7

instrumental data - tidal gauges which are located at the shoreline which measure the average level of the sea each year
proxy records - e.g., sedimentary records of coastal change and sea level from which we can get quantitative info on sea level
more recent data from satellites - geostationary satellites which are able to regularly measure the level of the sea
future projections - from global atmospheric models and global climate models where we have looked at how temperature leads to ice melt and thermal expansion of the oceans

Question 8

describe the general trend in SLR across the late Holocene period

Answer 8

up to about the mid 1800s, SL was oscillating - it was variable but not increasing significantly (less than 1mm per year)
mid 1800s - present day: acceleration from about 1mm per year to about 3mm per year
future projections: potential for an acceleration in the rate of rising even further (4-6mm per year). there are IPCC projections for very high emissions (RCP8.5 scenario) and very low emissions (RCP2.6 scenario)

Question 9

why is SLR a risk to infrastructure?

Answer 9

so much of the worlds population located around the coastline
majority of large mega-cities are at the coast

Question 10

why are cities drawn to the coast?

Answer 10

international travel possible
exchange of goods via shipping
lots of flat land for agriculture or industry development
fisheries
tourism

Question 11

what paper suggests ‘realistic’ statistics of the % of world population affected by SLR?

Answer 11

Lichter et al. (2011):
12/13% of worlds population would be effected by SLR and increased storm surges
however, w/ todays population numbers, that is still over 1 billion people

Question 12

what happens as a result of using sea defences in terms of coastal squeeze?

Answer 12

as sea level rises, our salt marshes and mangroves start to disappear as a result of the increasing hydroperiod that takes place
normal process would be: there would be inland migration of these environments therefore no net loss of biodiversity (enables the shoreline to be dynamic)
however, by protecting low lying infrastructure using sea defences: lose marine wetlands, reed swamps, salt marshes - left w mud flats and sand flats.

Question 13

hurricanes cause storm surges of around 5/6 m whereas predictions of SLR show an increase of 4-6mm per year. why is SLR seen as such as big problem if these numbers are small in comparison to storm surges?

Answer 13

SLR is a permanent phenomena that will lead to permanent change whereas storms, whilst still damaging, are only temporary
therefore coastlines have a recovery time in between storms
but no such recovery time in terms of SLR

Question 14

what is Reverse Uniformitarianism?

Answer 14

where we look into the past to provide context for what can happen in the future

Question 15

what are tide gauges?

Answer 15

instruments that are located at the shoreline
made up of a vertical tube which sits between the range of the high tide and the low tide
within the tube, the water level can rise and fall w the tide, and there is a float which is connected to a recorder
as the tide rises and falls, the level of the sea is recorded
average sea level per year
can plot yearly trends

Question 16

what is a pressure gauge?

Answer 16

sits below the water level and measures how much water is above it
from that we can get the tidal level

Question 17

what is a radar gauge?

Answer 17

sits above the water level and takes measurements of how close the tidal level is to the instrument

Question 18

what is an advantage of using tidal gauges?

Answer 18

give us extremely accurate measurements of sea level across a period of time

Question 19

what are the limitations of using tidal gauges?

Answer 19

we have lots of them to measure current SL but as we move further into the past there is less and less data because fewer were used
most tide gauge records are less than about 30-50 yrs in duration
therefore data from it is not uniformly distributed
most of the data comes from the mid latitudes because until recently that’s where most of tidal gauges where located
therefore data in inconsistent on a global scale (spatial limitation)

Question 20

data from tidal gauges in ports around the world show variability. what causes this variability?

Answer 20

tide gauge might be close to a big river - if there is a lot of rainfall that year, sea level will appear higher due to high levels of river discharge, true for the opposite (dry year = local sea level lower)
oceanographic effects - e.g., El Nino can influence sea level
storms - stormy year = low pressure systems = less atmospheric pressure pushing down on sea = higher sea level

Question 21

what paper shows evidence of accelerated sea level rise using tide gauges?

Answer 21

Woodworth et al., 2011:
evidence from ports such as Amsterdam, Liverpool, Stockholm
acceleration visible from mid-1800s
consistent w effects from the industrial revolution (increased co2 conc in atmosphere)
industrial revolution was a bit earlier (start mid-1700s) therefore this is evidence of a response time: response of sea level system to global atmospheric co2 levels and temperature rise has a delay (~30yrs) because ice melt and thermal expansion has to occur first

Question 22

what paper shows an inconsistency in SLR from tidal gauge data?

Answer 22

Wahl et al. (2013)
SL in North Sea has been rising however, Stockholm: SLR is decreasing
because: land can undergo uplift or subsidence as a result of tectonic movements or ice loading
tide gauge only gives us relative sea level - level of the sea relative to the land
land can move as well as the sea - this is what is happening here
Stockholm is undergoing uplift as a result of Last Glacial Maximum
rate of glacial isostatic rebound is faster than the rate of SLR (therefore looks like SL is falling)

Question 23

what is Satellite Altimetry?

Answer 23

satellites can measure the level of the sea without experiencing problems from land level movements (as they are not tethered to the land)

Question 24

what is the difference between data of SLR from tidal gauges and satellites?

Answer 24

Satellites give us data on eustatic SLR whereas tide gauges give us data on relative SLR which can sometimes result from isostatic land level change

Question 25

what type of change do future projections (global climate models) of SLR show?

Answer 25

eustatic

Question 26

why are tide gauges helpful when we collect eustatic data from other sources e.g., satellites?

Answer 26

eustatic data needs to be downscaled to explain what it means locally
e.g., if eustatic sea level rise is predicted to be 4-6mm per year
what we see at the coastline will also be influenced by what the land is doing
therefore, tide gauges are helpful at telling us what the land is doing relative to the sea

Question 27

what is a problem with satellite data?

Answer 27

it is only ~30yrs in duration
so if we want to use reverse uniformitarianism, we need to use other data sources

Question 28

what are deep sea sediment records?

Answer 28

things that have been deposited below the level of the sea consistently for a significant period of time

Question 29

what is morphological evidence of SLR?

Answer 29

raised beaches, raised marine terraces
these used to be at sea level but have become raised as a result of either sea level change or land level change

Question 30

where does morphological evidence of SLR come from?

Answer 30

as the coast erodes, cliff lines erode backwards and shore platforms extend in a landward direction
if sea level falls, the shore platforms are covered by beach/dune sediments above the present sea level
each shore platform can be related to a specific sea level at some time in the past
i.e., we use the elevation of these platforms and date them
dating used from: shell material (radio-carbon dating), mineral matter (luminescence dating)

Question 31

what is sedimentary evidence of SLR?

Answer 31

pattern of sediments below the surface (stratigraphy)
different layers of sediment
can see if sediments have changed from terrestrial (e.g., peat deposits) to sediment found at marine sites (e.g., mud flats)
each change in sediment represents a change in sea level

Question 32

what paper is an example of using stratigraphy as evidence of SLR?

Answer 32

Shennan 1986:
changes in stratigraphy from terrestrial to marine
represent different tidal levels
he said: if you looked at the overall transition between the coastal muds and the coastal peats (i.e., mineral matter to organic matter), that takes place at a tidal level which is somewhere half way between the mean high water spring tide and the highest astronomical tide (M’)
so the measurement of M’ gives us the altitude of former sea level
age: take organic matter above and use radio-carbon dating or mineral matter below and use luminescence dating

Question 33

what did Fairbridge (1961) do?

Answer 33

created a ‘eustatic curve’ from sedimentary data of M’ and other types of data to show past changes in SL
important as it extended our knowledge on past SL from a few years ago to hundreds of thousands of years ago
good example of reverse uniformitarianism

Question 34

how are corals used as evidence SLR?

Answer 34

deep sea sediment records of coral fossils can be used
altitude and age of the fossil determined
collect many of these data to reconstruct past sea levels
e.g., Fairbanks (1989)

Question 35

what is a limitation of using reverse uniformitarianism of the early Holocene period to predict shoreline responses to future sea level rise?

Answer 35

past responses of the shoreline are entirely natural - shorelines can shift as a result of sea level rise
whereas now, there is significant human imprint on our coastlines
our coastlines may not have the same response to SLR because they are defended, inhabited etc

Question 36

what is a problem of using data from sources that provide reverse uniformitarianism?

Answer 36

evidence from stratigraphic and geomorphic information stops at about 3000 yrs before present
how do we connect the geological record to instrumental (present-day) record?
i.e., data is missing

Question 37

how can we fill the gap in data between the geological and instrumental record of SLR?

Answer 37

Ecological transfer functions:
come from the presence of different foram and diatom species preserved in saltmarshes
their distribution depends on:
salinity, substrate type, hydroperiod
taking a core sample of the sediment and looking at the distribution of species within the sample, we can see what type of an environment used to be there and from this can infer the tidal level
example: Mills et al (2013) - using diatoms in the Mersey estuary to reconstruct past sea levels

Question 38

how can global warming cause sea level to fall?

Answer 38

every mass has some kind of gravitational effect
if we have a lot of mass in terms of an ice sheet sitting on the coastal bit of the crust, it will cause subsidence (glacio-isostatic loading)
at the same time, that mass is attracting the ocean up towards it
so, the ocean surface is higher because of the ice mass
when the ice mass is lost, there will be a rebound response and less gravitational effect from that ice
so, sea level will fall

Question 39

in close proximity to ice masses, when the ice mass is lost - what will happen in terms of sea level?

Answer 39

loss of gravitational pull
loss of loading on crust
sea level will fall

Question 40

what paper shows different scenarios of ice sheets melting and their effect on SLR?

Answer 40

Mitrovica et al (2001):
3 scenarios - Antarctica, Greenland, melting of mountain glaciers and ice sheets
e.g., Antarctica melts: lose gravitational pull, lose glacial isostatic loading - close proximity to Antarctica = SL falls, higher rate of SLR further away
i.e., local sea level fall but SLR further away

Question 41

What do we have to take into account when we look at local SLR?

Answer 41

the land
is it experiencing subsidence or uplift?
example - North of UK is experiencing uplift whereas South is experiencing subsidence as a result of ice loading and rebound from 18,000 yrs ago
e.g., we have 3mm SLR per year but Scotland also experiencing 1mm of uplift per year = rate is 2mm (rate of SLR is reduced)
Shennan et al. (2012)

Question 42

what factors are considered in future sea level rise models?

Answer 42

ocean thermal expansion
glaciers and small ice caps
Greenland
Antarctica

Question 43

What are uncertain contributions to sea level rise?

Answer 43

Groundwater extraction vs recharge - more is being taken out than being added
Reservoirs and lakes – dams - adds to evaporative losses from the earth surface
Deforestation & Wetland loss - less capacity for water to be held on earth surface
Permafrost - changing frozen ground to water + slush which can then be added to sea level

Question 44

what is the H++ scenario?

Answer 44

the IPCC projections of climate change and sea level rise have been accused of being conservative
therefore the H++ scenario came about
it is used to put forward unlikely scenarios that could still happen (i.e., extreme but plausible)
UK: 93cm - 1.9m SLR by the end of 21st century

Question 45

what is an alternative way of projecting sea level rise instead of climate models?

Answer 45

use trends that we have observed from data (semi-empirical projections)
e.g., trends in tide gauge data can be projected
example - Grinsted et al. (2015) use semi-empirical methods for 21st century RSL projections in northern Europe