Weeks 6 & 7 - Sea level change Flashcards
What is coastal squeeze?
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
what are the terrestrial factors that influence sea level rise
terrestrial water storage, extraction of groundwater, building of reservoirs, changes in runoff, and seepage into aquifers
what are the factors that influence sea level rise at the shore
subsidence in river delta region, land movements, tectonic displacements
what are the factors that influence sea level rise in the ocean
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
what is ocean thermal expansion
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.
how does Greenland and Antarctica contribute to sea level rise
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.
where can we get data on sea level rise?
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
describe the general trend in SLR across the late Holocene period
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)
why is SLR a risk to infrastructure?
so much of the worlds population located around the coastline
majority of large mega-cities are at the coast
why are cities drawn to the coast?
international travel possible
exchange of goods via shipping
lots of flat land for agriculture or industry development
fisheries
tourism
what paper suggests ‘realistic’ statistics of the % of world population affected by SLR?
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
what happens as a result of using sea defences in terms of coastal squeeze?
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.
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?
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
what is Reverse Uniformitarianism?
where we look into the past to provide context for what can happen in the future
what are tide gauges?
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
what is a pressure gauge?
sits below the water level and measures how much water is above it
from that we can get the tidal level
what is a radar gauge?
sits above the water level and takes measurements of how close the tidal level is to the instrument
what is an advantage of using tidal gauges?
give us extremely accurate measurements of sea level across a period of time
what are the limitations of using tidal gauges?
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)
data from tidal gauges in ports around the world show variability. what causes this variability?
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
what paper shows evidence of accelerated sea level rise using tide gauges?
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
what paper shows an inconsistency in SLR from tidal gauge data?
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)
what is Satellite Altimetry?
satellites can measure the level of the sea without experiencing problems from land level movements (as they are not tethered to the land)
what is the difference between data of SLR from tidal gauges and satellites?
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
