Ocean Fingerprints

So did you get round to watching the talk? I hope you enjoyed it if you did but don’t worry if you didn’t have time. My previous post highlighted the 3 key arguments that sceptics make and I shall try to explain how Mitrovica attempts to show why he believes each is false.


1) 2 mm/year is not anomalous – sea level has been rising at this rate for thousands of years.

Geological evidence - Equatorial corals, which are about 3m above sea level, have been found to be 5000 years old. If the sea had been rising at a rate of 2 mm/year these would be 10m underwater.

Archaeological evidence - Fish tanks (piscinae) in the mediterranean carved into the rock during the 2nd Century and early 1st Century BC were built at a precise height relative to sea level to enable the movement of water but not fish. Measuring current elevation relative to high tide (and correcting for vertical tectonic movement) leaves a rise of about 1 metre which equates to a 0.5 mm/year rise. Correcting for ice age effects, there has been very little change at all over the past 2000 years (Lambeck et al. 2004)! If it had been rising at 2 mm/year these tanks would be 4 metres under.

Ancient eclipse record evidence -
Stephenson and Morrison’s (1995) study suggests the Earth’s rotation is slowing down due to water waves breaking and causing a dissipation. Taking into account this effect, the Earth’s current rotation rate matches up with ancient eclipse records. If ice sheets had been melting during this time, large amounts of water would have been added meaning slower rotation. Consequently, ice sheets could not have been melting for the past 2000 years!

- Therefore, 2 mm/yr in 20th C was anomalous. nothing has been seen like it during the past 10,000 years. Bjork (2011: 5) certainly believes this is the case as recent trends have been ‘triggered by anthropogenically forced alterations of the carbon cycle in the general global environment’.


2) Sea level change varies dramatically from place to place – melting ice sheets cannot be the culprit.
They do indeed vary. By thinking like this and taking the average of different tide gauge records around the globe you are assuming the ‘Bathtub Model’ (see below).

Ice Sheet melt and resultant sea level change

The problem with this is that when an ice sheet melts the sea doesn’t rise uniformly (Mitrovica et al. 2009). Instead, because ice sheets have mass, they exert a gravitational pull on the nearby water. When a sheet melts, the pull on nearby water is relaxed and more water is added. The result of these two factors near an ice sheet is a drop in sea level (see above). The implications for this are global differences in sea level. There is a hinge point around 2000km away from the melt where there is observable sea level rise. 

Global differences in sea level change

‘Sea level fingerprints’ is the name given to the pattern of sea level change and this can change depending on which ice sheets melt . This does mean though that fingerprint analysis can be carried out to predict the outcomes and potential risks of different ice sheets melting separately, or even in combinations.

3) Even so, 2 mm/year is small and stable.
Well, this rate already has changed! Satellite data from altimeter satellites has been measuring sea level in recent years and this has found that the rate is closer to 3.3 mm/year in the last decade (Ablain et al. 2006).



References

Ablain, M., A. Cazenave, G. Valladeau and S. Guinehut (2009) ‘A new assessment of the error budget of global mean sea level rate estimated by satellite altimetry over 1993–2008’, Ocean Science, 5, 193-201.

Bjork (2011) ‘Current global warming appears anomalous in relation to the climate of the last 20,000 years’, Climate Research, 48, 5-11.

Lambeck, K., M. Anzidei, F. Antonioli, A. Benini, A. Esposito (2004) ‘Sea level in Roman time in the Central Mediterranean and implications for recent change’, Earth and Planetary Science Letters, 224, 563-575.

Mitrovica, J. X., N. Gomez and P. U. Clark (2009) ‘The sea-level fingerprint of West Antarctic collapse’, Science, 323, 5915, 753.

Stephenson, F. R. and L. V. Morrison (1995) ‘Long term fluctuations in the Earth’s rotation: 700 BC to AD 1990’, Philosophical Transactions: Physical Sciences and Engineering, 351, 1695, 165-202.