XVII: sea level change

XVII: sea level change

CU NSIDC

Larsen B Ice Shelf collapseMarch 2002

3250 km2 ice loss in less than1 mo.

attributed to recent warming ofthe Antarctic Peninsula

News Flash!: Antarctica is warming1957-81:

1979-’03:

Antarctic is warming, most dramatically at lower elevationsSteig et al. ‘09

Larsen B Ice Shelf collapse• an “ice shelf” is the floating extension of glacier ice on land

(and, unlike sea-ice, is thick enough to flow under its ownweight)

• the sudden collapse of the ice shelf in March 2002 involvedan area larger than the state of Rhode Island

• this collapse is the most dramatic of a recent series ofmajor ice loss events in the region, all considered aresponse to recent warming of the Antarctic Peninsula

• since shelf ice is already floating, its collapse and eventualmelting have little direct effect on sea level

• however, the ice shelves act to hold back the seaward flowof adjacent land-based glaciers and the loss of ice shelveshas led to measurable increases in the flow of land-ice tothe sea, which does contribute to global sea level change

review• the cryosphere is responding to warming not just

in high latitudes but globally• this response includes (but is not restricted to) loss

of Arctic sea-ice, melting and thinning of themargin of the Greenland ice sheet, increasing meltof low lying portions of Antarctic ice sheet, andmelting of globally distributed “small” glaciers

• a temperature - sea ice - ocean heat flux feedbackamplifies temperature change in the Arctic

• sliding and thinning of the Greenland ice sheetmargin may lead to irreversible ice loss

• melting of any ice that is not already floatingcontributes to rising sea level (today’s topic)

clicker questionThe temperature melting of large ice sheets is a

relatively slow process, but if meltwater canreach the bed of an ice sheet then...

a) the friction at the bed of the ice sheet isreduced

b) the ice sheet margin may begin slidingc) the ice sheet profile may begin to collapsed) all of the abovee) none of the above

ice sheet sliding

• this possibility is the single biggestuncertainty in our ability to predict sealevel and poses the greatest potentialrisk……..

• IPCC AR4 (2007) punted on this!

today’s outline

• ice age sea level change (from welldated corals)

• recent sea level change from tidegauges and satellites

• contributions to recent sea level change• projected sea level change• uncertainties and consequences

ice age temperatures

-15

-10

-5

0

5

-15

-10

-5

0

5

0 50 100 150 200 250 300 350 400age kyr

-15

-10

-5

0

5

tem

pera

ture

cha

nge

(°C

)

Vostok, Antarctica

Greenland after prolonged cooling, iceage ice sheets reached theirmaximum size ~21 kyr ago

(water stored on land as ice)

global temperatures during this last glacial maximum periodwere ~ 6 °C below present

(can see “polar amplification” present during ice age)

ice age (de)glaciation, NHfollowing the last glacialmaximum ~21 kyr ago,warming resulted in theprogressive melting of

large ice sheets that hadgrown in the NH (andexpanded over Ant-

arctica) and water thathad been trapped in

land ice was transferredback to the oceans......

(these melt rates wereabout 10 times fasterthan the rates of ice

sheet growth leading tothe ice sheet maximum)

sea level rise since the last ice age

based on radiometric dating of fossil shallow water coralsenough water returned to ocean to raise sea level ~130 meterssea level rise flattened out after melting ceased ~8,000 yr ago

last 9000 years

very large changes associated with rapid melting ofice sheets largely ceased by 5000 yr ago...

the last century......

note change in scale

during the last century sea level rose ~ 2mm/yr

the last decade +.......

during the last decade sea level rose ~3.1 mm/yr

from TOPEX & Jason

satellites

note change in scale

contributions to recent sea level rise

• thermal expansion of sea water!

• ice sheet melting

• small glacier melting

warm water expands!• sea water expands by, roughly, ~0.33%

for every K (or °C) of warming

• so for every K of warming in upper 1 kmof ocean, level would rise ~3.3 m due tothermal expansion

• precise measurements of oceantemperature change indicate ~0.05Kwarming of upper few km over last ~50 yrdue to global warming

• this would explain much of sea rise of~20 cm observed fr. tide gauges

IPCC AR4 estimated contributions(mm/yr)

3.1 ± 0.71.8 ± 0.5observed2.8 ± 0.71.1 ± 0.5sum0.21 ± 0.350.14 ± 0.41Antarctic IS0.21 ± 0.070.05 ± 0.12Greenland IS0.77 ± 0.220.50 ± 0.18“small” glaciers

1.6 ± 0.50.42 ± 0.12thermalexpansion

1993-2003:1961-2003:source:

all estimated sources have increased during the last decade,consistent with observations from tide gauges and satellitessignificant increases in contributions from Greenland have

been documented since 2003

recent trends:

• ca. 50% increase in rate of sea levelrise last decade vs. preceding century

• seemingly small numbers (~3 mm/yr)stack up over time, and rates arepredicted to increase in response tocontinued warming

• let’s now look at future sea levelprojections

contributions:• thermal expansion, easiest term to predict (based on rate

of warming, expansion coefficient, and vertical mixing)

• changing mass balance (accum. - melt) of small glaciers,prediction moderately difficult– almost certain to continue net melting

• changing mass balance (accum. - melt & ice bergs) oflarge ice sheets, prediction more difficult– Greenland Ice Sheet very likely to continue net melting– most of Antarctic Ice Sheet(s) assumed to remain too cold to

permit net melting (w/ net growth possible)

• possible dynamical changes of large ice sheets (i.e.sliding), prediction not yet possible– GIS? (volume = 7 m sea level equ.)– West Antarctic Ice Sheet? (volume = 12 m sea level equ.)

a simple model projection (BAU)

assume Business As Usual burning (+1.5%/yr) no change in Land Use

D. Archer ISAM modelhttp://forecast.uchicago.edu/Projects/isam.html

a simple model projection (BAU)

-10

0

10

20

30

40

50

-10

0

10

20

30

40

50

1800 1900 2000 2100

se

a le

ve

l ch

an

ge

(cm

)

year

thermal expansionglaciersGreenlandAntarcticaTOTAL

-10

0

10

20

30

40

50

-10

0

10

20

30

40

50

1800 1900 2000 2100

se

a le

ve

l ch

an

ge

(cm

)

year

a simple model projection (BAU)

thermal expansionglaciersGreenlandAntarcticaTOTAL

-10

0

10

20

30

40

50

-10

0

10

20

30

40

50

1800 1900 2000 2100

se

a le

ve

l ch

an

ge

(cm

)

year

a simple model projection (BAU)

thermal expansionglaciersGreenlandAntarcticaTOTAL

-10

0

10

20

30

40

50

-10

0

10

20

30

40

50

1800 1900 2000 2100

se

a le

ve

l ch

an

ge

(cm

)

year

a simple model projection (BAU)

thermal expansionglaciersGreenlandAntarcticaTOTAL

note model predicts net growth of Antarctic Ice Sheet(s)

-10

0

10

20

30

40

50

-10

0

10

20

30

40

50

1800 1900 2000 2100

se

a le

ve

l ch

an

ge

(cm

)

year

a simple model projection (BAU)

thermal expansionglaciersGreenlandAntarcticaTOTAL

~ 50 cm (v. 1800) by end of century?

D. Archer ISAM modelhttp://forecast.uchicago.edu/Projects/isam.html

• our projection*: 50 cm (v. 1800) by end ofcentury, 20+ cm already realized

• built in assumptions:– net growth of Antarctic Ice Sheet(s) w/ warming– no sudden ice sheet changes (i.e. no sliding)

• our projection is thus likely to be conservative(like that of the IPCC!)

• show me the ice sheets (again)!

* given for illustrative purposes, no single projection is definitive

W & E Antarctic Ice Sheets

WAIS12 m SL equ.

EAIS>100 m SL equ.

ice volumes in meters of sea level equivalent

Ant. Peninsula <0.5 m SL equ.

very cold & stable

warming,poss.

unstable

warming & melting

clicker question:

Why might the East Antarctic Ice Sheet grow ina warming world?

a) it can’t, warming always means meltingb) warmer air holds more moisturec) most of East Antarctica is so cold melting is

unlikelyd) both b) and c)e) Antarctic climate is completely isolated from

that of the rest of the Earth

Antarctic snow fallextremely cold (~ -30 °Cin summer) polar desert

interior Antarctica would have to warm ~30 °C before melting,while water holding capacity of air still increases ~5% per °C.....

Greenland Ice Sheet

GIS7 m SL equ.

ice volume in meters of sea level equivalent

gradual melting of GIS

existing forecasts consider only gradual melting in response towarming, w/ no positive feedbacks from ice dynamics

projected impacts of gradual SL rise

impacts are felt disproportionately in the developing world,projections do not consider changes in intensity of storms,coastward migration of population, or ice sheet instability

similar to ourBAU projection

what if gradual melting triggersfast sliding?

key processes are not well known andtherefore not yet incorporated into future

projections of sea level change

anomalous melting

10%more in

2007

unstable Greenland ice sheet

melt water reaches the ice sheet bed, greatly reducing frictionand permitting accelerated sliding and flow through ice

streams, increased ice berg calving and ice sheet thinning

friction

ice flow

&sliding

ice bergs

watermovesquicklythru ice(we don’t

knowexactlyhow)

deflation

ice sheet force balance

ice flow out resisted byfriction at sides and at bed

ocean ocean

equilibrium ice sheet profile is determined by balancebetween forces driving outward ice flow (gravity) and

forces resisting outward ice flow (friction)

ice flow line

ice flow out scales togravityequilibrium profile

(of ~last 10 kyr)

ice sheet force balance

ice flow out resisted byfriction at sides and at bed

ocean ocean

increased melting lubricates bed reducing bed friction,thinning reduces friction at sides, both of these reduceresistance to outward flow, leading to force imbalance

and accelerated outward flow of ice

ice flow out scales togravityequilibrium profile

(of ~last 10 kyr)

more flow

sliding

ice sheet force balance

ice flow out resisted byfriction at sides and at bed

ocean ocean

once the ice sheet starts to move toward a new equilibrium profileit cannot be stopped until force balance is restored (regardless ofany intervening climate change)- what the profile will be and how

much ice will be transferred to the sea is the big question

new profile?

ice flow out scales togravityequilibrium profile

(of ~last 10 kyr)

ice flow line

students: togglebetween these

images

growing melt area

first year of melting

first year ofmelting since

satelliteobservationsbegin in 1987

areas offloating

glacier ice(ice shelves)that buttress

inlandthese are

also meltingat the btm.!

Antarctic topography beneath ice

bed of WAIS largely below sea level(thus vulnerable to progressive attack by rel. warm sea water)

ice shelvesmelting attop andbottom!

consider impact of small fractionalloss of GIS or WAIS

1 meter SL rise would submerge:

• 18% of Bangladesh• 6% of The Netherlands• ~960 of the 1200 Maldive Islands

2 meters looks like this....

+ 5 meters, Asia

white is relativepopulation density

recall the long geologic perspective

last glaciation21 kyr ago

recent pre-industrial

period

>35 million yrs agono permanent ice

~32 million yrs ago onset AA glaciation

more…

• More on the irreversibility of warmingand sea level rise next class when weconsider the climate forecast and theconcept of “commitment warming”

key points ....• on geologic timescales sea levels vary

dramatically as continental ice grows or meltsin response to changing temperature

• over the last decade, rates of sea level risehave increased ~50% compared to thepreceding ~100 yrs (i.e. from 2 to 3 mm/yr)

• on the recent 10-100 yr timescale, thermalexpansion of sea water and melting of landice, both due to warming, have madecomparable contributions to sea level change

key points ....• future projections are (conservatively) for

another 30 cm+ rise in sea level by the end ofthe century

• changes in ice sheet dynamics may lead tosubstantially larger, faster changes

• additional sea level rise in the range of 1mwould be disastrous, even for developedcountries

• low lying countries and islands, many in thethird world, are already vulnerable

• The IPCC AR4 (2007) punted on dynamicalchanges and sliding of big ice sheets

• Observed sea levels are already rising fasterthan predicted

• A predictive understanding of ice sheetchanges is badly needed

• Sea level change is a serious consequence ofwarming

• “Manage the unavoidable but avoid theunmanageable”

• Sounds sensible, but can we really avoid theunmanageable?

next Thurs..

2nd Hour Exam!

• covers material since last exam• focus on lecture notes and “learning

goals”

lecture 17 learning goals• be able to explain the influence of continental scale

glaciation and melting (such as during and after the lastglaciation) on global sea level and the nature of theevidence constraining past sea levels

• be able to describe the different factors that havecontributed to recent sea level rise and which are easiestto quantify or predict and which are most difficult...

• be able to explain why some models forecast growth of theEast Antarctic Ice Sheet in response to global warming

• outline some consequences of accelerated melting andsliding (dynamic instability) of large ice sheets in responseto global warming (is the difference between 1/2 meterand several meters of sea level change significant?)

• be able to describe the broad relationship betweenatmospheric CO2 conc. and sea level on long geologictimescales