Week 20: Walking on thin ice? Flashcards
Ice Age Earth =
T variations ~10’C every 100,000 years
The last deglaciation
Took around 10,000 years
Max @27ka BP
- SL+1230m, sometimes in rapid pulses (4m/century)
Since the last deglaciation
Currently Greenhouse World but 65m of SL currently locked up in ice sheets
~10% Earth’s surface remains glaciated
Current glacier statistics
Pfeffer et al 2014
198,000 glaciers
15,861,766 km2
33,000,000 km3
Distribution of mountain glaciers/ice caps (excluding Greenland and Antarctica)
50cm of SLE locked up in them i.e. we could cope without them
Ice cap =
less than 50,000km2
Ice sheet =
bigger than 50,000km2, covers whole island/continent e.g. Greenland/EAIS/WAIS
Distribution of all glaciers (including ice sheets in Greenland and Antarctica)
6m of SLE in Greenland
58m of SLE in Antarctica
i.e. most ice locked up in big continental scale ice sheets, majority Antarctica
SLE =
Sea Level Equivalent
Recent climate change
Controversial hockey stick graph
We should have been going into another ice age!
BUT anthropogenic reasons = rapid increase in T instead
Recent climate change statistics
10 warmest years all since 1998
2014 officially hottest, beaten in 2015 and then in 2016
2017 still in top 3 warmest
Source: NASA GISS showing global mean surface T change from 1880-2017 relative to 1951-1980 mean
WARMING AND WETTING
Last 3 decades = warmest in last 1400 years
Recent climate change quotes
IPCC 2001
“Most observed warming last 50 years LIKELY due to increase in GHG concs”
IPCC 2007
“Most observed increase in globally averages Ts since mid-20th century VERY LIKELY due to observed increase in antropogenic GHG concs”
IPCC 2013
“It is EXTREMELY LIKELY that human influence has been the dominant cause of the observed warming since mid-20th century”
Have any areas cooled from global warming?
UK - one of few
Perhaps due to changes in ocean circulation
Recent change in mountain glaciers and ice caps
-ve mass balance
Largely due to increase in summer Ts
Although N.B. Sparse field measurements (only 50-60 glaciers of 198,000 regularly monitored for MB)
Potential issue with estimation of volume losses for large numbers of glaciers?
Different techniques = different answers
Methods of estimating volume losses for large number of glaciers
- Map area changes then estimate with area-volume scaling relationship (graph)
- Satellite measurements of changes in ice elevation/flow rate e.g. radar/laser altimetry
- Detection of changes in gravity fields caused by glacier mass gain/loss (GRACE)
Kaser et al 2006
= one of most famous papers over last couple of decades
Glaciological method = global mass balance estimates for glaciers/ice caps
~below 0 1970 and increasingly -ve trend since then
- 1961-1990 = -219+/-112km/m2/a2
- 2001-2004 = -510+/-101km/m2/a2
Kaser et al 2006 Scandinavian anomaly
Mountain glaciers/landscape +0.77mm/yr SL 2001-2004
Snowy winters in 80s/90s = changes in N Atlantic oscillation = didn’t decrease as much
Jacob et al 2012
= GRACE
0.41+/-0.08mm/yr SL
30% smaller than Kaser et al
Gardner et al 2013
Compilation of methods
Showed general decline
Contributing 0.5-1.5mm/yr to SL depending on methodological approach
How do ice sheets lose mass?
- Melting at ice sheet surface with runoff into ocean (margin)
- Increase discharge from marine-terminating outlet glaciers
Discharge…
the “dynamic component” of mass loss
90% of Antarctica’s margin is in the ocean = losing mass all the time from flow into ocean –> melts –> SL
How does Greenland lose its mass?
50% melting 50% increase in marine terminating glaciers (discharge)
Marginal melt zone migrating higher and higher (fluctuations)
Not getting increased snowfall in winter to compensate
= increasingly -ve MB
How could positive feedback work? Debate?
Zwally et al 2005
Poorly understood
Supraglacial lake (several 100km across)
Weight fractures through ice thickness to bed = inject water
= increase lubrication = increase ice velocity
BUT Tedstone et al 2005 = speed up is transient
Repeated = increases drainage system efficiency = slows
What happens if you increase dynamic discharge?
-ve mass balance
Outlet glacier discharge > accumulation
= THINNING, RETREAT, ACCELERATION of outlet glaciers
Outlet glaciers aka
“Arteries of fast flowing ice”
Increasing dynamic discharge; Jakobshavns Isbrae
Drains 6-7% of GrIS
Fastest flowing glacier in world
Dimensions of fjord determine rates of retreat
- 6km wide flowing at >10,000m/yr
Joughin et al 2004 = rapid accelerations early 2002
1985 = 6,700m/a
2003 = 12,600m/a
What are the recent dynamic changes in marine-terminating outlet glaciers across the Arctic likely to be linked to?
Carr et al 2013
- Increased air Ts
- Increased open sea Ts
- Reduced sea-ice concentrations
Feedbacks between increased air T and basal sliding/dynamic thinning
Increased surface meltwater availability
= thinning/inland migration of ELA
Through crevasses to bed
= enhances basal sliding
= increased ice velocities
Feedbacks between open sea Ts and grounding line retreat/thinning/calving
Warm ocean currents coming in at depth e.g. N Atlantic
= melts understand of glaciers at grounding line
Meltwater (freshwater) released = rises
= pulls in more warm water at depth
= CIRCULATION
i.e. melting and surface meltwaters = buoyant plume
Influence of sea-ice concentrations on outlet glacier calving
Sea ice = binds together surface ice bergs
Buttressing effect
Without = much more unstable system
Meland =
agglomeration of sea ice and ice bergs
Antarctica: WAIS and EAIS
Little surface melting but lots of ‘dynamic’ changes
Strong signals of retreat linked to ocean circulation changes - circumpolar deep water
WAIS = marine ice sheet = particularly vulnerable to unstable behaviour
- retreats into deeper water
- most thinning in Abundsen Sea Embayment (ASE)
EAIS = recent similar changes in deep marine basins
- Wilkes Land EAIS
- EAIS contains 5 x as much ice as WAIS (lots on land above SL)
CONCERNING!
Difference between Antarctica and Greenland
Greenland extends far south (!) = lots of snow and lots of melt
Antarctica climatically isolated from incursions of warm air = little surface melt
BUT
Similar dynamic changes i.e. thin/retreat/accelerate
Impacts of recent glaciological changes
- SEA LEVEL
- WATER RESOURCES
- HAZARDS
Impacts; SEA LEVEL
Mountain glaciers/ice caps = smaller = quicker response = largest cryospheric contribution to SL rise
Big ice sheets catching up and IPCC 2013 = may be about to overtake
Ice sheet Mass Balance Inter-comparison Exercise (IMBIE) = updated reconciled estimates of ice-sheet mass balance
1/3 of world’s population live w/in 5m of SL = SERIOUS IMPLICATION
Impacts; SL - EAIS
Period of +ve mass balance 2010-2015
- more snowfall? e.g. 2009/2010 warmer climate could hold more moisture
Much larger uncertainties
Rignot et al 2019 = one of 1st papers to quantify that EAIS is creating mass loss in a significant way (20%)
Impacts; WATER RESOURCES
1/6 Earth’s population rely on glaciers/snow packs for water supply during dry season
Hagg et al 2006 = model-generated runoff scenarios for different climates (CO2 X2) and glacierisation conditions
- initial increase in runoff
- severe decrease once glaciers gone
Impacts; HAZARDS
Clague and Evans 2000 = slope instability, debris flow, proglacial lake outbursts moraine dam failures
e. g. stagnating termini of glaciers in Bhutan Himalayas
- lakes rapidly forming on surface of debris covered glaciers worldwide during last few decades
- retreat = moraine
- melt = lake
- dammed by (little) moraine
- flood
Can happen in matter of hour with LITTLE WARNING
Future changes
IPCC 2013 = RCP (Relative Concentration Pathway)
Currently ~405ppm
Paris agreement = hit RCP scenario
- still committed to 0.5m SL rise
BUT 6 YEARS AGO???
N.B. 4’C global average could mean e.g. 12’C in other areas
How much has mean global T increased since 1880?
~1’C
