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

eastwest

The ice sheets today

How ice sheets work

snow

ice flow

mel

ticebergs

…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)

CO2 concentration in Dome C ice

CO2 concentration in Vostok ice

160

320

240

220

200

180

CO

2 (

ppm

)

300

280

260

Temperature (Deuterium/Hydrogen ratio of Dome C ice)

600,000 400,000 200,000 100,000300,000500,000700,000 Presentyears ago

-460

-360

-380

-400

-420

-440

D (

per

mil)

Glacial

Interglacial

Today 392

CO2 and temperature over the ice ages – a record from Antarctic ice cores

“Returning to levels of CO2 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 d

ynamics

• 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 millionpeople

3 millionpeople 9 million

people

72 millionpeople

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

1 10 100 10001 10 100 10001 10

A one-off event?

Storm surges: what impact on cities?

New Orleans, August 2005.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 2003-2007

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

Gla

cie

r fl

ow

ra

tekm

/ y

r

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, 2010Velicogna, 2010

Antarctica

Greenland

summer/winter cycle

What is causing this ice loss?

observations

1957-2007

Temperature trend (degrees per decade)

Antarctic and Greenland temperature trends

Greenland summer temperatures

Southern Greenland: significant warming after 1990.

(significantly more melt, also increased snowfall).

Hanna et al. 2008

Steig et al. 2009

BUT Antarctic summers are still cold!What else is happening?

1982-2004

Holland et al. 2008

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 kmIncludes 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 Challengesobservations, 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?