Download Cryosphere

Cryosphere
© Andy Mahoney, NSIDC
D. Farinotti
Klimasysteme – SS 2017
[email protected]
What is the “cryosphere”?
What does the cryosphere include?
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© Bob Lapis
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Aletsch region, Switzerland
© ISS013E77377
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Myrdalsjökull ice cap, Iceland
© Michael Studinger, NASA
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Ice cap or ice sheet?
South west Greenland
© Jesse Allen, NASA
The former if the area is < 50,000 km2
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© A. Morel, mrwallpaper.com
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North east Antarctic Peninsula
© John Sonntag, NASA
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Beaufort Sea
© Alek Petty, NASA
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Alaska North Slope, USA
© NASA/JPL-Caltech
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Permafrost
-10°
Definition:
Ground (soil or rock
and included ice or
organic material) that
remains at or below
0°C for at least two
consecutive years.
0°
+10° C
Active layer
Annual minimum
temperature
Permafrost table
Annual maximum
temperature
Upper limit of
seasonally invariant
temperature
[International Permafrost Association]
Seasonally invariant
temperature gradient
Isothermal
permafrost
Base of permafrost
Frost-free soil
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Where is permafrost occurring?
Brown, J., O.J. Ferrians, J.A. Heginbottom, and E.S. Melnikov, eds. (1997). Circum-Arctic map of
permafrost and ground-ice conditions. Washington, DC: U.S. Geological Survey in Cooperation with
the Circum-Pacific Council for Energy and Mineral Resources. Circum-Pacific Map Series CP-45
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Which cryosphere components
are the most important?
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How much is 24.7 106 km3 of ice?
Sea Level Equivalent (SLE):
Amount (in meters) the global oceans would rise, if the given
volume would be equally distributed over the Earth surface.
Vice ⋅ ρ ice ρ water
SLE =
S oceans
24.7 ⋅106 ⋅ 917 1028 km3
=
≅ 60 m
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2
3.62 ⋅10 km
Note: The above is conceptional, neglects relative SL changes (we speak
about eustatic SL change), neglects steric effects (effects due to
changes in temperature and salinity), and does not account for ice that is
already grounded below sea level.
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How is sea level changing at the moment?
IPCC AR5 WG1, Fig. 4.25
Present rate of sea level change : ca. +1.4 mm yr -1
Of which: ca. + 0.3 mm yr -1 from Antarctica
ca. + 0.4 mm yr -1 from Greenland
ca. + 0.7 mm yr -1 from Glaciers and Ice Caps
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How do we know?
Space gravimetry (Gravity Recovery and Climate Experiment)
Satellite altimetry (Ice, Clouds, and Land Elevation Satellite)
And modelling…
IPCC AR5 WG1, Figs 4.13+4.14
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Average trend:
+0.45 °C/decade
-4.5 cm/decade
IPCC AR5 WG1, Fig. 4.22
Permafrost is warming.
Active layer thickness anomaly (m)
Mean annual temperature at 10-20m depth (°C)
Changes in permafrost
IPCC AR5 WG1, Fig. 4.24
Active layer is thickening.
Little to no information about area and volume changes.
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Changes in snow cover extent (SCE)
IPCC AR5 WG1, Fig. 4.19
SCE is decreasing.
Trend (1967-2012): - 1.6%/decade
- 4.5% / °C
IPCC AR5 WG1, Fig. 4.19
Why is the correlation to temperature so good?
Great question. Did he ever notice that
snow melts when it gets warm?
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Melt as residual term in the energy balance
Surface energy balance equation:
R + H + Lv E + C + L f M = 0
with: R = net radiation
H = sensible heat flux
E = evaporation rate
M = melt rate
Average
Abs, excluding melt
79
66%
(1/3)
Ohmura, A. (2001). Physical basis for the
temperature-based melt-index method. J.
Applied Meteorology, 40 (4), 753-761
C = conductive heat flow in the subsurface
Lv= latent heat of vaporization (2.257106J kg-1)
Lf = latent heat of fusion (0.334  106 J kg-1)
-2
2%
-30
25%
-9
7%
-39
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Melt as residual term in the energy balance
Net radiation decomposed:
R = S(1 - α ) + L ↓ −σ Ts
(2/3)
L = longwave outgoing radiation
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with: S = shortwave radiation L = longwave incoming radiation
Ohmura, A. (2001). Table is cut.
α = surface albedo
σ = Stefan-Boltzmann constant
Ts = surface temperature
Average
93
23%
276
70%
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7%
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Melt as residual term in the energy balance
(3/3)
Where is the longwave radiation coming from?
70% of L is emitted from
the first 100m of the
atmosphere.
Ohmura (2001), Fig. 1
Modelled incoming longwave radiation flux
at the surface (%)
Since L= f (T), and since
the vertical profile of T is
easy to predict given T at
some height above the
surface (e.g. 2m), T turns
out to be a good predictor
for L and thus for R, and
thus for M !
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Temperature-index modelling
Temperature-index (TI) melt models take full advantage of
the former observations and calculate melt simply as:
M = DDF ⋅ T
with: M = melt T = air temperature
DDF = degree day factor
Note that DDF is dependent
on season and location, and
can vary over time.
If calibrated for a specific
site/region, TI-models can
have very high performance!
Braithwaite, R.J. (1995). Positive degree-day factors for
ablation on the Greenland ice sheet studied by energy
balance modelling. J. Glaciology, 41 (137), 153-160.
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Change in age distribution of sea ice
© NASA Scientific Visualization Studio
Older sea ice is thicker and more resistant to melt than new ice, so it protects the ice cap from summer
melting. In September 1984, 1.86  106 km2 of sea ice was 5 years or older. In September 2016, this area
was reduced to 0.110106 km2 (-94%).
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Change in age distribution of sea ice
© NASA Scientific Visualization Studio
Older sea ice is thicker and more resistant to melt than new ice, so it protects the ice cap from summer
melting. In September 1984, 1.86  106 km2 of sea ice was 5 years or older. In September 2016, this area
was reduced to 0.110106 km2 (-94%).
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Summary: The cryosphere is affecting large
part of the planet and is changing rapidly…
IPCC AR5 WG1, Fig. 4.1
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Polar amplification
(1/2)
IPCC AR5 WG1, Fig. 12.10
Arctic: Expected to warm twice as much as the global mean
Main mechanism: Albedo feedback
Warming  snow, ice, and sea ice melt  lower albedo  increased energy
uptake  more warming  etc. Holland, M.M. and C.M. Bitz (2003). Polar amplification of climate
change in coupled models. Climate Dynamics, 21 (3), 221–232.
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Polar amplification
(2/2)
IPCC AR5 WG1, Fig. 12.10
Arctic: Expected to have stronger precipitation change
Main mechanism: Sea ice change
Warming  decreasing sea ice  more open water  increased moisture
flux (sustained by increased temperature)
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Cryosphere and ocean circulation
(1/4)
Rahmstorf (2002). Ocean circulation
and climate during the past
120,000 years. Nature,
419, 207-214
Affected by
the cryosphere
Ocean circulation is mainly driven by:
- winds (upper 102 m of the sea)
- differences in temperature and salinity (thermohaline circulation)
- tides (gravitational pull of the Moon and Sun)
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Cryosphere and ocean circulation
(2/4)
https://www.nasa.gov/feature/goddard/warming-seas-and-melting-ice-sheets
NOTE: Not all is
cryosphere-driven
Decadal-scale changes can be observed but are poorly understood.
Changes in circulation are understood to be important over long time scales.
E.g. “Heinrich events” (freshening of the North Atlantic during glacial periods due to
massive iceberg calving; detectable in ocean sediments from signature of entrained minerals)
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Cryosphere and ocean circulation
1992-2010
Sea-level pressure (color) and 10m wind trend (vectors)
(3/4)
1992-2010
Ice-motion trend (vectors) and change in ice speed (color)
Holland and Kwok (2012). Wind-driven trends in Antarctic
sea-ice drift. Nature Geoscience, 5, 872–875.
Salinity changes in the Southern Ocean are prominent signals of climate
change in the global oceans.
Changes in wind strength affect sea ice transport…
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Cryosphere and ocean circulation
(4/4)
Haumann et al. (2016). Sea-ice transport driving
Southern Ocean salinity and its recent trends.
Nature, 537, 89–92.
changes in sea ice transport affect freshwater transport (+20% during 1982-2008)…
changes in freshwater transport affect salinity (-0.2 g kg-1 during 1982-2008)…
changes in salinity affects ocean circulation, temperature, and winds…
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Ocean circulation and ice shelf stability
Hanna, E., et al. (2013). Icesheet mass balance and climate
change. Nature, 498, 51–59
Circumpolar Deep Water (1-2 °C warm)
Subaqueous melt is (very) important for the mass budget of ice shelves.
On retrograde beds (= subglacial topography sloping downwards from the margin to the interior),
ice shelf thinning can lead to a positive feedback mechanism: thinner ice
 migration of the groundling line ice flow speed up  thinner ice
This has again an effect on ocean circulation…
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Ice shelf stability and ice discharge
(1/4)
50km
Ice shelf
disintegration
has recently
been observed
on the Antarctic
Peninsula,
notably the
Larsen A and
Larsen B ice
shelves.
Larsen A
disintegrated 1995
Larsen B
© Helmut Rott
disintegrated 2002
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Ice shelf stability and ice discharge
(2/4)
Larsen B disintegration
© Peter Kuipers-Munneke, IMAU
Surface melt is
understood to
critically affect ice
shelf stability.
© Ted Scambos, NASA
Note how melt ponds “disappear”
short before the collapse.
As air temperature warms and causes melt, melt ponds can form. The
water “breaks it’s way” though the ice and causes the ice shelf to break.
The mechanism is known as hydrofracturing.
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Larsen B
collapse
(3/4)
Scambos et al. (2004). Glacier acceleration and thinning
after ice shelf collapse in the Larsen B embayment,
Antarctica. Geophysical Research Letters, 31, L18402.
Ice shelf stability and ice discharge
Ice shelves have
behind
them.is
have aabuttressing
buttressingeffect
effecton
onthe
theland
landiceice
behind
them. If this
buttressing is lost… ice flow speed up  thinner ice  migration of the
groundling line  ice flow speed up (cf. some slide ago)
Such a speedup was indeed observed after the disintegration of Larsen B.
Note: Unlike ice shelf melting, increased ice discharge contributes to SLR!
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Ice shelf stability and ice discharge
(4/4)
Rift over Larsen C (10.10.2016) © NASA
50km
Larsen A
disintegrated 1995
Larsen B
© Helmut Rott
disintegrated 2002
Larsen C
disintegrating
soon?
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(1/2)
Shuur et al. (2008). Vulnerability of Permafrost Carbon to
Climate Change: Implications for the Global Carbon
Cycle. BioScience, 58 (8), 701-714.
Permafrost and the carbon cycle
“Thawing permafrost and the resulting microbial decomposition of previously
frozen organic carbon is one of the most significant potential feedbacks from
terrestrial ecosystems to the atmosphere in a changing climate.”
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Gruber et al. (2004). The vulnerability of the carbon cycle in
the 21st century: An assessment of carbon-climate-human
interactions. In Field and Raupach (eds.). The Global
Carbon Cycle: Integrating Humans, Climate, and the
Natural World. Island Press, Washington DC. pp. 45–76.
(1/2)
Shuur et al. (2008)
Permafrost and the carbon cycle
Once thawed, C can enter either an oxic or an anoxic ecosystem.
Decomposition in oxic soils  Mostly CO2 release
GHGs!
Decomposition in anoxic soils  Release of both CH4 and CO2
Estimate: up to 100 Pg C could be released by 2100 (Gruber et al. 2004).
 cf.: current anthropogenic emissions ≈ 10 PgC a-1
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The cryosphere
is beautiful…
Getz Ice Shelf, West Antarctica
© Jeremy Harbeck, NASA
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is changing fast…
Rift on Larsen C
© John Sonntag, NASA
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is important...
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The cryosphere
is not fully understood.
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Thank you for
your attention!
Scar Inlet
© Jesse Allen, NASA
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