27.11.2007

PRESS RELEASE

Arctic Centre, University of Lapland, Finland

Embargo: 27.11.2007, 01.00 CET

Volcanic eruptions raise global sea level

Major volcanic eruption’s effect on the global water cycle and sea level changes equal to El Niño’s, researchers say.

Large volcanic eruptions inject aerosols into the stratosphere, and these

aerosols reflect sunlight causing global dimming and thus lower temperatures

at the earth surface. The cooling of the ocean surface causes less

evaporation. As water flux from terrestrial reservoirs and river discharge

continue, the reduced evaporation leads to a global sea level rise of 6-12

millimeters during the first year following an eruption.

This results from research by Professor John Moore, Dr. Aslak Grinsted and

Dr. Svetlana Jevrejeva, published in Proceedings of the National Academy of

Sciences of the United States of America, November 26-30, 2007.

After approximately one year, stratospheric aerosols have been removed and

evaporation reaches normal values. However, now the river discharge is

reduced due to the low precipitation in the preceding year and sea level

therefore drops by 4-10 millimeters 2-3 years after the eruption.

The magnitude of this phenomena is similar to the impact of El Niño -

Southern Oscillation, which causes heavy rainfall in Americas, drought in

the eastern Pacific and global changes in weather.

To quantify the impact of eruptions, Prof. Moore, Dr. Grinsted and Dr.

Jevrejeva used records from 830 tide gauges across the globe, and five major

eruptions between 1890 and the Pinatubo eruption of 1991. The effects of

climate change and other disturbances such as earthquakes and uneven

distribution of tide gauges were eliminated from the data by using novel

statistics.

No other previous sea level reconstruction has been accurate enough to show

either drop or rise caused by volcanic eruptions. Climate models have only

showed the drop.

For more information, please contact:

Research Professor, John Moore, Arctic Centre, University of Lapland,

Finland/Thule Institute, University of Oulu, Finland

Mobile: +358 400194850

Email: john.moore (at) ulapland.fi

Researcher, Dr. Aslak Grinsted Arctic Centre, University of Lapland, Finland

Mobile: +358 405121481

Email: aslak.grinsted (at) ulapland.fi

www.arcticcentre.org

Researcher, Dr. Svetlana Jevrejeva, Proudman Oceanographic Laboratory,

Liverpool, UK

Switchboard: +44 (0) 151 795 4800

Email: sveta (at) pol.ac.uk

Further alternative text:
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 largest 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.
4.      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.