Contributions of the Greenland Ice Sheet to Rising Sea Level

Robert H Thomas

(SIGMA/NASA Wallops, VA)

NASA Sea-Level Workshop, 2-4 Nov., 2009

Austin, Texas

Estimated sea-level rise (SLR) from ocean expansion and different ice masses

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Greenland Antarctica

SLR (1961-2003)SLR (1993-2003)

Ocean expansion

Relative Areas, SLE, net snowfall, and SLR of different ice masses

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• How do we measure ice-sheet mass balance? • Recent contributions of the Greenland Ice Sheet

to sea-level rise (SLR).• What are the causes for recent mass losses from

the ice sheets?• How realistic are predictions of SLR > 1 meter

by 2100?• Program to detect/monitor future changes, and

to understand them sufficiently to develop and validate reliable models for predicting ice-sheet responses to a warming climate.

How do we measure ice-sheet mass balance?

• Mass balance is the rate of change in ice-sheet mass• It varies both spatially and temporally over the ice sheet• Our goal here is to measure the mass balance of an entire

ice sheet over a period of several years• There are three main approaches:

– Mass budget, comparing total mass added with total lost– Monitoring ice-sheet mass from time series of global gravity

surveys– Volume balance, monitoring ice-sheet volume using altimeters

Mass budget estimates

INPUT:snowfall – melt in the catchment basin (±10 to 30 %)OUTFLOW:Flux-gate thickness (±10m to ±80m) times speed (± 5m/yr) MASS BALANCE:INPUT - OUTFLOW (±5 to 30%)

From Rignot, AGU 2008

Ice mass changes from observed gravity

changes dM/dt = (dGo/dt , dGf/dt , dGa/dt , dGb/dt)

ice mass observed far field atmosphere bed

There are uncertainties in all terms and in procedures used to infer dGo/dt. Errors in dGb/dt are difficult to quantify because of poor knowledge of vertical crustal motion beneath the ice, amplified by density difference between ice and rock. However, dGb/dt changes little with time, so errors in d2M/dt2 should be far smaller.

Volume Balance

dM/dt = (dZs/dt – dZc/dt– dZb/dt)

Surface Compaction Bed

Prime causes for uncertainty in dZs/dt are interpolation and, for radar altimetry, time-variable radar penetration plus topography effects.

~ 600 + 300 kg/m3 implies an additional + 50% dM/dt uncertainty, but this can be reduced substantially using ancillary information.

Monitoring ice-sheet mass balance from satellite altimeter surveys is

appealing, and is the approach selected by both NASA with ICESat, and ESA

with CRYOSat. But there are problems with this approach.

Rates of elevation change over Greenland (dS/dt) from radar-altimeter minus dS/dt from laser altimeter, averaged within 500-m elevation bands, and plotted against surface

elevation: blue in the north; red in the south.

Effects of time-variable radar penetration into surface snow

RADAR OVER-ESTIMATES

GREENLAND THICKENING.

ERS wavefront over Jakobshavn

The radar does not

“see” into the

valleys where

thinning rates are highest

EFFECTS OF TIME-VARIABLE RADAR PENETRATION SHOULD BE LESS IN ANTARCTICA, WHERE THERE IS LITTLE SURFACE MELTING. BUT WILL PROBABLY BECOME MORE SEVERE AS WARMING INCREASES.

TOPOGRAPHY EFFECTS SHOULD ALSO BE LESS, BECAUSE ANTARCTIC GLACIER VALLEYS ARE MORE GENTLY

SLOPING.

Reconstructed annual meanAntarctic temperature

anomaliesJanuary 1957 to December 2006,from Automatic Weather Stations(dashed lines) and satellite thermalinfrared data (solid black lines).Red lines show average trends.Grey shading, 95% confidencelimits. (Steig et al., 2009)

EastAntarctica

West Antarctica

WESTANTARCTICA

Regions of surfacemelting derived fromsatellite scatterometer

data

Nghiem et al, 2006

>0.1oC/decade

LASER OVERCOMES THE RADAR PROBLEMS: NO PENETRATION, AND ITS SMALL FOOTPRINT

AVOIDS TOPOGRAPHY PROBLEMS.

INSTEAD, HOWEVER, RESULTS ARE STRONGLY AFFECTED BY THE NEED TO INTERPOLATE

BETWEEN TIME/SPACE-SEPARATED ORBITS.

THIS PROBLEM IS LARGELY AVOIDED BY AIRCRAFT SURVEYS OF INDIVIDUAL CATCHMENT

BASINS, WHICH CAN BE MADE WITHIN SHORT PERIODS ALONG FLIGHT TRACKS DICTATED BY

THE GLACIER ORIENTATION.

Examples of elevation changes at ICESat orbit crossing points, binned into 50-km grids

Orbit spacing and cloud cover result in very sparse coverage at lower latitudes, particularly near the coast, where clouds are more common

Spatial resolution of satellite laser-altimeter data is limited by orbit separation and cloud cover

Total Volume change within different elevation bands in the NE quadrant since 2003.2

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Approximately 90% of the volume loss in the NE comes from parts of the ice sheet below 1500 m, where estimates have the biggest

uncertainty!!

Volume change for entire ice sheet, from different analyses

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Sum of GLAS - GLAS 2A: loss of 224 cu km/yr

Sum of GLAS - ATM 93/4: loss of 137 cu km/yr

Sum of GLAS - ATM 98/9: loss of 355 cu km/yr

These very different estimates were all derived from time series of laser-altimeter data, with the only differences being their

spatial distribution.

All mass-balance estimates reflect natural variability plus long-term

trends. Most fail to include all potential errors, particularly biases, in

uncertainty estimates.

FORTUNATELY, RESULTS FROM THE THREE APPROACHES GIVE

INDEPENDENT ESTIMATES OF THE MASS BALANCE THAT PROVIDE

A CONSISTENCY CHECK.

Satellite radar altimetryA/C Laser altimetry & GLASMass budget (INSAR etc)GRACE gravity changes

-360 Gt/year ~ 1mm/yr SLR

Velicogna:

2007-09

Total loss between 2000 and 2009 was almost 1000 sq. km, or 11 times the area of Manhattan. In total these glaciers lost 106 sq. km per year (from J. Box, U Colorado)

Time series of satellite imagery shows Greenland glaciers to be losing area at a roughly constant rate

MASS LOSS FROM BOTH ICE SHEETS HAS INCREASED RAPIDLY SINCE THE

MID 1990s, TO 200-400 GT/YEAR FOR 2005-8, ENOUGH TO CONTRIBUTE AS MUCH AS 1 MM/YEAR TO SEA-LEVEL

RISE.

WHY?

Drawdown of Kangerdlugssuaq, East Greenland.

Glaciers are thinning…

Acceleration of Jakobshavn Isbrae, W. Greenland (Joughin et al., JGR, 2009)

…accelerating...

Recent retreat of

the calving front of Helheim Glacier,

East Greenland

…and retreating.

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2003

The graph shows the total melt area 1979 to 2007 for the Greenland Ice Sheet derived from satellite passive microwave data. The map inserts display the area of melt for 1996, 1998, and the record year 2007 (from K. Steffen, CIRES, University of Colorado).

Area and intensity of summer melting is increasing….

Is melt water lubricating glacier sliding?

Surface melt water lubricating glacier flow

probably explains measured summer acceleration of 8-10% in slower-moving ice, but this is small compared to observed acceleration of

fast-moving glaciers by 100% or more.

Greenland ice-sheet thinning from airborne laser altimetry

Krabill et al., 2006

Airborne Topographic Mapper

(ATM)

Observed thinning is far too rapid to be caused by increased melting.

The three most rapidly thinning glaciers all flow along very deep

troughs into the ocean. Jakobshavn thinning and

acceleration started very soon after breakup of its floating ice tongue.

dS/dt

(m/y)

Distance (km)

Retreat of Jakobshavn Isbrae, W. Greenland

Retreat of the Jakobshavn ice front since the Little Ice Age paused in the 1960s, until 2000, when it began retreating rapidly as its floating ice tongue

thinned and finally broke up, and velocity doubled to 13 km/yr

WHAT TRIGGERED THE RETREAT?

Temperatures at nearby Ilulissat Airport show steady increase, but no

major change in the mid 1990s

Scharling et al., 2006

But measurements by Demersal Fisheries show a considerable warming of deep waters since the early 1990s, with > 1oC increase between 1996

and 1997

Wieland & Kanneworff, 2002

Holland et al (2008) concluded that a change in wind patterns over the subpolar gyre of the North Atlantic in 1995-1996 precipitated a chain of

events that ultimately led to flooding of the Jakobshavn fjord with warm, subsurface water that caused a massive increase in melt rates from the base of the floating

ice tongue. This reduced buttressing forces acting on the

glacier, resulting in the rapid glacier acceleration and ice

thinning that began after 1997.

Temperature oC

Bed profile from Gogineni/KU

Floating tongue until 2000

Sea water?

Thinning and retreat of Jakobshavn Isbrae

Sill

Slow thickening until 1997…..

…. followed by rapid thinning.

Ice-shelf breakup and glacier un-grounding?

Ice shelves and floating glacier tongues exert a “back pressure” on tributary glaciers by upstream transmission of stresses caused by shear between the floating ice and its sides and/or locally grounded “pinning

points”. Weakening or breakup of floating/lightly-grounded ice reduces this back pressure, allowing the glaciers to accelerate, rather like loosening the

cork in a tilted bottle of wine.

What we don’t know is how far the bottle is

tilted – how far inland the glacier “feels” the

effect of ice-shelf breakup.

Area, and average rate of ice loss, within the 1 m/yr thinning-rate contour

1700 sq km 4600 sq km >8200 sq km

6 cu km/yr 15 cu km/yr >24 cu km/yr

RAPID INLAND MIGRATION OF THE THINNING ZONE SUGGESTS THE BOTTLE IS TILTED QUITE STEEPLY!

Global warming and the ice sheets

o The warming atmosphere carries more moisture, which should result in increased snowfall over the cold ice sheets. This is happening at higher elevations over Greenland and the Antarctic Peninsula, where warming is pronounced, but not over most of Antarctica, where warming is small.

o Area and intensity of summer melting and melt-water runoff into the ocean are all increasing as air temperatures rise, causing 50% or more of recent Greenland ice losses, but little of those from Antarctica.

o Some of the increased melt water drains to the ice-sheet bed, where it lubricates basal sliding. Although this appears to have little effect on the speed of already fast-moving outlet glaciers, it may cause appreciable acceleration of slower ice that flows into the outlet glaciers.

o Warming ocean waters cause substantially increased basal melting from some floating glacier tongues and ice shelves, some of which have broken up. Most ice draining from Antarctica flows into ice shelves, and their weakening or breakup has allowed tributary glaciers to accelerate, in some cases by more than 100%.

A history of sea-level predictions

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Thomas

IPCC

Rahmstorf

Model-predicted SLR decreased with time, until leveling off after mid nineties, while

estimated uncertainties decreased. Rahmstorf estimate, based on extrapolation of past SLR

into a warmer climate, shows a return to higher predicted SLR – approximately double

the most recent IPCC estimate.

IF RAHMSTORF IS CORRECT, WHAT DID THE IPCC MISS?

Early estimates of ice-sheet contributions to sea-level rise used simple models that assumed ice-stream discharge was

affected by the ice shelves into which they flow. But the real modelers later proved this to be unlikely or even impossible! Instead, the IPCC used elaborate 3-D models which included

assumptions that prevented outlet glaciers from changing their behaviour very quickly. Impacts of climate warming were then more or less reduced to changes in surface melting and snowfall, which could be predicted with reasonable accuracy

and progressively smaller error bounds.

However, observations increasingly show that glaciers can change extremely rapidly, so these

modeling exercises were rather like predicting the water level in a leaky bucket by ignoring the holes.

Rahmstorf (2006)

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WHAT WOULD IT TAKE TO RAISE IPCC TO RAHMSTORF?

Contributions to sea-level rise by 2100 (cm)

Thermal expansion

Glaciers/

ice caps

Greenland

SMB

Antarctica

SMB

Ice-sheet dynamics

Total

IPCC-07 10 to 41 7 to 18 1 to 13 -14 to -2 2 to 12 21 to 71

Rahmstorf 50 to 140

IPCC adjusted to 100 cm

30 14 10 - 4 50 100

SLR of 1 m by 2100 would require a dynamic loss rate in 2100 enough to raise sea level by 10 mm/y,

assuming linear increase from zero in 2000

CAN THE ICE SHEETS DO THIS?

Accelerating sea-level rise (SLR) from the polar ice sheets

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Based on GRACE measurements (Velicogna, 2009)

Retreat to here?

with glacier speeds exceeding 20 km/yr

SLR ~ 0.3 mm/yr

Ice loss from Greenland by 2100 will probably be limited to surface melting and

comparatively rapid retreat of outlet glaciers up to the heads of deeper fjords, with

associated draw-down of surrounding ice. But this dynamic retreat will be limited by

the fringe of coastal mountains. A preliminary, very approximate estimate of total resulting sea-level rise is 15-25 cm.

But large parts of the Antarctic Ice Sheet lack such a protective fringe of coastal mountains.

Pine Island Glacier, Antarctica

ICE SHELF

ICE PLAIN

DISTANCE FROM 2002 GROUNDING LINE (km)

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Sill

dS/dt (m/y)

Has a profile similar to Jakobshavn, but is far larger. It also appears to be floating free from its sill, with its ice shelf still intact

PIG

The Future?2013: Jakobshavn calving front has retreated into its deep trough with velocity

increasing to > 25 km/y; other glaciers in SE Greenland continue sporadic retreat and acceleration

2020: Larsen-C ice shelf shows clear signs of weakening; Greenland southern dome is shrinking irreversibly

2030: Rapid thinning and acceleration of N Greenland outlet glaciers while many southern glaciers slow down as grounding lines retreat to heads of fjord troughs; most Antarctic Peninsula ice shelves have collapsed, with big increases in tributary-glacier velocities; Amundsen Sea ice shelves are breaking up, PIG velocity exceeds 10 km/y, and local glaciers have accelerated sufficiently to raise sea level by > 3 mm/y

2100: West Antarctic Ice Sheet is rapidly losing mass along its entire north coast, coastal parts of the East Antarctic Ice Sheet with deep beds are also losing mass, and Antarctic Peninsula ice cover is rapidly shrinking. Most Greenland losses are by summer melting, with ever-increasing ablation as surface lowering enhances effects of warming atmosphere. Total ice-sheet contributions to SLR exceed 1 cm/y, and total SLR since 2000 is close to one metre, with worse to come at progressively increasing rates

This is conjecture, but even a small possibility that it is correct must surely prompt urgent efforts to improve our understanding of the

recent ice-sheet changes sufficiently to allow us to make more reliable predictions

HOW?

Satellite SAR, GRACE, Altimetry, and ice-thickness measurements PROS and CONS

• Satellite SAR– All-weather capability to measure ice velocity and grounding-line migration over very

large regions at high spatial resolution– Provides only short-period velocity estimates, and needs surface mass balance and

glacier ice thickness for mass-budget estimates• GRACE

– Provides estimates of mass changes (dM/dt) integrated over very large regions or entire ice sheets

– Results are biased by errors caused by crustal vertical motion; but errors are smaller for d2M/dt2

• Altimetry– Radar

• All-weather capability to detect changes in ice-surface conditions, such as elevation, wetness, ice layering etc

• Interpretation problems resulting from time-variable radar penetration into the snow surface and the effects of surface topography

– Laser• Provides accurate measurements of surface elevation within laser footprints• Poor spatial and/or temporal coverage because of orbit/aircraft-track separation

and cloud coverage• Aircraft surveys provide information at any desired spatial/temporal resolution

along specific glaciers, and provide the opportunity for simultaneous measurement of ice thickness

• Ice-thickness measurements– Essential for mass-budget estimates, and for understanding of glacier changes– Require extensive airborne surveys over all major outlet glaciers, and this is difficult

over fast glacier trunks

Ice-sheet mass balance: key tools

Change detection: INSAR; hi-resolution imagery; GRACE; SRALT; ICESat; passive/active microwave; Automatic Weather Stations

Volume change at higher elevations: ICESat, Model simulations of surface mass balance

Focused high-resolution surveys of regions undergoing change: Aircraft laser altimeter and ice-sounding radars, photogrammetry, satellite INSAR, and field measurements