The Terrestrial Ice Sheets: dynamics, stability and sea level rise Dr Hamish Pritchard.

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Presentation transcript:

The Terrestrial Ice Sheets: dynamics, stability and sea level rise Dr Hamish Pritchard

Polar ice sheets Antarctica over 4000 m thick 70 m of sea level Greenland 7 m of sea level east west The ice sheets today

How ice sheets work snow ice flow melt icebergs …ice gain and loss are balanced by ice flow. All of these can change. Are the ice sheets changing? What does it mean for the rest of the world? Greenland Antarctica

The ice sheets and the wider world

Temperature (Deuterium/Hydrogen ratio of Dome C ice) Siegenthaler et al., Science (2005) Petit et al., Nature (1999) CO 2 concentration in Dome C ice CO 2 concentration in Vostok ice CO 2 (ppm) Temperature (Deuterium/Hydrogen ratio of Dome C ice) 600,000400,000200,000100,000300,000500,000700,000Present years ago  D (per mil) Glacial Interglacial Today 392 CO 2 and temperature over the ice ages – a record from Antarctic ice cores “Returning to levels of CO 2 not seen since before the Antarctic ice sheet” DeConto and Pollard, 2003

Temperature and sea level over the last ice age Can we predict sea level rise? (now: +3 cm per decade) Sea level rise closely matches temperature rise. Last interglacial sea level was nearly 20 m higher than now.

SCAR Report 2009 (Scientific Committee on Antarctic Research – 35 leading international institutes) with glacier dynamics predict up to 1.4 m sea level rise by 2100 ~10 % world’s population forcibly displaced Shanghai, Alexandria, Boston, New York, Venice would all be “on the brink of submersion”, Maldives and Tuvalu lost. IPCC ‘gross underestimate’ of ice sheet melting. Why?

20 million people 3 million people 9 million people 72 million people The impact of a one metre sea-level rise – displaced people in Asia Altitude above current sea level

Flood return statistics at Thames Barrier Source: Dawson et al. (2005), Jones (2001), Environment Agency

A one-off event? Storm surges: what impact on cities? New Orleans, August Category 3 hurricane, 5 m surge. City well prepared (90% evacuated), but: 1,500 deaths, widespread looting. Neighbourhoods destroyed Cost: $81 billion New Orleans, 2008

So what is happening to the ice sheets today? observations

Height change rates Strong thinning on fast-flowing glaciers. Something is happening to the dynamics.

Glacier flow rate km / yr Glacier acceleration is driving Greenland ice loss (“dynamic thinning”) Greenland dynamic thinning

But what about snowfall and melt?

Satellite gravity measurements - weighing the whole ice sheet Velicogna, 2010 Antarctica Greenland summer/winter cycle

What is causing this ice loss? observations

Temperature trend (degrees per decade) Antarctic and Greenland temperature trends Greenland summer temperatures Southern Greenland: significant warming after (significantly more melt, also increased snowfall). Hanna et al Steig et al BUT Antarctic summers are still cold! What else is happening?

Holland et al Arrival of warm sea water Does this explain the Antarctic ice loss?

So we know what’s happening (ice sheet losses) and we have an idea why (warm air and warm oceans). Ice shelf basal melt Removed – map of ice shelf melt, subject to publication. Removed – map of ice shelf melt and its link to glacier thinning, subject to publication.

Can we predict the future? 1)Computer models 2)Ice-sheet past

Modelling the ice sheets “Marine ice sheet instability” West Antarctica more snow advance less snow or more melt

Areas of unstable ice sheet But: Models are still experimental - none represent the real world properly. If we had the perfect model now, we wouldn’t know which one it was.

Predicting the future by looking at the past

3 ice stream tributaries 70 km >120 km Includes NBP, BAS and AWI Datasets. Can map ice sheet extent in previous climate. But: we can’t yet date the inorganic sediment.

Ice sheet thins and mountains emerge Above the waves – dating is more successful Rocks get exposed to cosmic rays -steadily produce isotopes - these can be counted But: only a few sites dated so far

Summary - Sea level rise a profound threat - Ice sheets are losing ice by glacier acceleration (and some melt) - Ocean and atmosphere driven - Threat of ice sheet collapse in West Antarctica But sea level rise difficult to predict: Models still in development Observations are short-term History barely known – don’t even know if it collapsed before

Grand Challenges observations, forecasting, thresholds continue to observe change (altimetry, gravimetry and flow measurement) Vital to: push forward with models to reduce uncertainty in sea level rise (e.g. from ±40 cm to ±20 cm over next decade) date the last collapse of West Antarctica (date sediment from below ice) Are we approaching a threshold for collapse? - did the ice sheet disappear in the last interglacial?