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Volcanic impact on sea level and the global water cycle

posted Feb 28, 2008 7:20 AM by Aslak Grinsted   [ updated Feb 28, 2008 7:46 AM ]
Until now climate models showed that large volcanic eruptions lowered global sea level. In fact, we prove that sea level rises in the year following an eruption. This illustrates how difficult realistic climate modeling is and that unexpected surprises often occur.
 
We examined the impact of the 5 & 9 most major volcanic eruptions on global sea level. Global sea level was inferred from the observational records from 830 tide gauges all around the world. Sea level is complicated because many factors influence it. We use novel statistics and an optimum way to collate the global network of tide gauges to make the best record of sea level and its uncertainty since 1854. From this we isolate the volcanic signature which surprisingly behaves very differently than models predict.
 
The mechanism that explains the result is:
  1. Large volcanic eruption inject aerosols into the stratosphere
  2. These aerosols reflect sunlight causing a global dimming, thus lowering temperatures at the earth surface.
  3. The cooling of the ocean ‘skin’ causes less evaporation.
Less evaporation leads to less precipitation and hence less water in terrestrial reservoirs and less river discharge.
 
 
This influences sea level in two ways:
  1. Lower evaporation cause an imbalance in water fluxes to/from the ocean. -Sea level rises by 9±3 mm (dependent on amount on eruption aerosol in stratosphere). After approximately one year the stratospheric aerosol has been removed and evaporation reaches normal values. However, now the river discharge is reduced due to the low precipitation in the preceding years and sea level therefore drops accordingly.
  2. The lower ocean temperatures lead to a densification which lowers sea level by ~7±3 mm (2-3 years after the eruption).
The water cycle disturbance causes an ‘immediate’ rise in sea level which is then followed by the drop due to ocean densification. It is the only the drop that has been modeled in the past, and no other sea level reconstruction is accurate enough to show either drop or rise in the volcanic signature.
 
Reference: Grinsted, A., J. C. Moore, S. Jevrejeva (2007) Observational evidence for volcanic impact on sea level and the global water cycle. PNAS, 104, 19730-19734. pdf
 
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