2B.7C Contemporary Sea Level Change
Contemporary sea level change from global warming or tectonic activity is a risk to some coastlines.
Climatic warming leads to eustatic sea level rise.
Past Change
Since 1870
What coastlines are at risk?
Climatic warming leads to eustatic sea level rise.
- Warming leads to the melting of mountain glaciers (Alps, Himalaya) and polar ice sheets increasing the amount of water in the ocean store.
- Melting of sea ice has no effect on sea level as the ice was already displacing the equivalent water volume to that produced by melting.
- IPCC attributes 50% of sea level rise 1990-2010 to ice sheets melting (Greenland ice sheet 15%, Antarctic ice sheet 10%)
- Warming also leads to the thermal expansion of existing ocean water as its temperature rises.
- 94% of increased heat energy in the climate system is transferred to oceans.
- IPCC attributes 40% of sea level rise 1990-2010 to this
- Tectonic activity caused the other 10% of sea level rise
- Emission of geothermal heat into oceans by underwater volcanic activity can cause thermal expansion of ocean water
- Rising magma at constructive plate boundaries produces a doming upwards of crust along mid-ocean ridges reducing the ocean basin volume
- At destructive margins:
- folding of plates increases ocean basin volume lowering sea levels
- earthquakes along boundary can allow rebound of non-subducting margin - uplift of sea floor reduces ocean volume raising sea levels
- 2004 Boxing Day tsunami with moment magnitude 9.3 lifted sections of the Indian Ocean bed raising sea levels by 0.1 mm
- It can also cause isostatic change:
- faulting can uplift sections of crust, lowering sea levels (or vice versa), sometimes by up to 2 m
- Turakirae Head near Wellington on New Zealand's North Island uplifted 6 m in 1855 earthquake
- Sea floor spreading transports volcanic islands away from the uplifted crustal zone along constructive boundaries or hotspots - to places where the ocean floor is colder, denser and lower lying - islands sink
Past Change
- Sea levels have risen by 125 m since the Devensian Glacial
- Sea level rise was on average 10 mm p.a. in the early Holocene Interglacial (18,000 - 6,000 BP)
- The UN's Intergovernmental Panel on Climate Change (IPCC) 5th Assessment Report suggests that sea level rise was only 0.5 mm p.a. from 6,000 BP to 1860.
Since 1870
- The rate of sea level rise has increased.
- The IPCC attributes this to global warming due to anthropogenic forcing through greenhouse gas emissions
- Contemporary sea level rise has accelerated since 1940 reaching 3 mm p.a. between 1990 and 2000.
- IPCC predicts sea level rise of 18-59 cm by 2100. (28-98 in 2013?)
- US National Research Council predicts 56-200 cm
- This wide variation in prediction is due to:
- uncertainties in science of relationship between GHG increase and climatic warming due to complex feedback effects
- uncertainties in science of relationship between climatic warming and rate of ice melting
- uncertainties about rate of population growth and economic growth impacting on rate of GHG emission
- uncertainty about future political commitment to introducing new measures to reduce GHG emissions
- Complete melting of Greenland ice sheet would raise global sea levels by 7 m
- Complete melting of Antarctic ice sheet would raise sea levels by 50 m
- However complete melting of ice sheets would take many centuries even by the most rapid estimates
What coastlines are at risk?
- low lying ones - coastal flooding through marine trangression
- low lying volcanic islands or coral atolls set atop submerged volcanic guyots e.g. Maldives in the Indian Ocean, Kiribati Islands in the Pacific Ocean - at risk of complete disappearance
- Volcanic islands at risk from both global warming and tectonic activity