the terrestrial ice sheets: dynamics, stability and sea level rise dr hamish pritchard
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
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?