Ch. 2.2 The Cryosphere

1. Ch. 2.2 The Cryosphere • Text, Ch. 2.1.2, • Supplementary reading: Curry and Webster’s • thermodynamics of Ocean and atmosphere, Reading assignment: (graduate students read both and undergraduate read one of the two) • Bamber et al. 2009 • Alley et al et al. 1997

2. 2.1.2 Cryosphere • What is the roles of cryosphere in climate and abrupt climate change? • What is the distribution of different components of cryosphere? • How has cryosphere changed in recent decades? How would it affect climate in the 21st century? • What control formation of sea ice and land ice/snow?

3. What is cryosphere? • Cryo (frozen), a component of the earth’s climate system comprised of water in its solid state. It consists of • glaciers & ice sheets, • snow, • permafrost (continuous and discontinuous) • sea ice (perennial and seasonal). • Largest fresh water reservoir on earth

4. Cryospheric component Area (% of earth surface) Mass (103 kg/m2) Antarctic ice sheet 2.7 53 Greenland ice sheet 0.35 5 Alpine glaciers 0.01 0.2 Sea-ice (in season of maximal extent) 7 0.01 Seasonal snow cover 9 <0.01 Permafrost 5 1 What can we infer from ice mass listed above? • Surface surface area: 5.1X1014 m2, total land area: 1.45X1014 m2 • 103 kg/m2: equivalent to depth of liquid water in meter per unit area. • If Antarctic ice sheet melted, it would create 53 m deep water layer over entire earth. • How much would sea-level rise? 76M = 53mX5.1/(5.1-1.45)

5. Role in climate system: • Largest fresh water storage: • Influence sea-level rise • Water resources • Influence ocean circulation • Regular earth’s albedo change, • Reduce turbulent transport of heat, water and momentum • Change ocean buoyancy flux, S and T • Glacial runoff from Antarctic is a major source of fresh water for southern ocean. • Regular regional-global climate

6. How do we estimate water in snow and ice? • Snow (ice) equivalent depth: snow is porous and its porosity depends on temperature and age of the snow. A measure of liquid water contained in snow is water equivalent depth, hm: hm=s/w·hs s,w: density of the snow and water, respectively. hs: depth of the snow/ice layer hm: The depth of water that will resulted from complete melt of snow/ice. Snow relative density, s/w ranges from 0.15-0.4

8. Snow/ice albedo (whiteness): • Albedo: ratio of the reflected vs. incident radiative flux. It is a function of wavelength. Surface Typical Albedo Fresh asphalt 0.04 Conifer forest (Summer) 0.08, 0.09 to 0.15 Worn asphalt 0.12 Deciduous trees 0.15 to 0.18 Bare soil 0.17 Green grass 0.25 Desert sand 0.40 New concrete 0.55 Fresh snow 0.80–0.90

9. Snow: Distribution and variations: • Seasonal snow covers ~12.5% of the global surface, mainly in high latitudes and high altitudes; • Snow cover varies strongly (50%), seasonally (8-16.5%), weekly, interannually, decadally;

10. Impact on climate: • In NH high latitudes, snow can increase surface albedo from <0.2 to 0.5-0.8. • Good isolator for surface: Thermal conductivity k is a measure of a material’s ability to transfer heat. A high value of k is a good conductor while a low value of k is a good insulator. k has units of Wm-1K-1 Fresh Snow 0.03 (better than fiberglass insulation!) Old Snow 0.4 Ice 2.1

11. Formation of the snow, ice and glaciers: Surface snow: • Newly fallen snow: r=50-150 kgm-3 • Snow flakes break due to winds and weight of snow above, settle at rate of 1%/hr to r=250 kgm-3 • Small ice crystals have large es, sublimate first, supplies water vapor to large round ice crystals, so snow layer become icy. • Formation of crust: lower snow layer is insolated from cold temperature, but water vapor from warmer lower layer goes to upper colder layer, freeze and cement the snow top layer. • Melting of smaller icy crystals, grow of large grains. • http://www.sciencelearn.org.nz/Contexts/Icy-Ecosystems/Sci-Media/Animations-and-Interactives/Snow-to-ice-animation

12. Glacier: • when snow/ice deposit reaches 50 m, pressure compaction and melt-freeze cycles cause ice r=550 kgm-3, i.e., firn. As firn r increases to 820 kgm-3, air no longer can circulate within ice and glacier ice forms. • As the thickness of glacier ice continues to increase and pressure at the base of the glacier increase, melting at the glacier base occurs. Glacier behavior as a 3D lattice, and deform and slip along slop of underlying topography at speed upto to 1-10 km/yr.

13. P S Satellite image of the Antarctic ice sheet and the rate of creep of the ice (m/yr) on a logarithmic scale. Antarctic Ice Sheet: • Creep rate: near zero at the divides of the ice sheet, and >10 m/yr at the periphery; Why? • Creep rate is especially high in the W. Antarctic. • Collapse of W. Antarctic can cause abrupt sea-level rise.

14. Greenland Ice Sheet: • Lower latitudes and smaller than the Antarctic ice sheet, • S. Greenland is highly vulnerable to climate change because summer temperature reaches melting point (-5C). IV

15. P T to form glacier for given P Air T Alpine glaciers: • Alpine glaciers, smaller ice sheets, can exist at any latitudes although their altitudes increase from < 1 km in high latitudes to 4-6 km in tropics; Why? • Alpine glacier retreat has been observed globally.

16. Permafrost: • Structure and distribution of the permafrost:

17. Permafrost: • The top few meters of soil thaws during summer and freezes in winter. • Below a few meters, the soil temperature remains constant around 0˚C. It would takes hundreds of years for the permafrost to adjust to air temperature; • Carbon locked up in the permafrost > carbon stored in global vegetation.

18. Sea Ice: • Sea ice in arctic covers maximumly 3% of the earth and in Antarctic covers maximumly 4% of the earth’ surface, and about 1-3 m thick (not much mass, 0.01 m) • Sea ice cover in Antarctic varies seasonally from 2 to 14 X1012 m2, and in Arctic varies from 4 - 11 X1012 m2. • Why does sea ice varies more in Antarctic than in Arctic?

19. Formation of sea ice: • Maximum density of pure water, rw.max=1000 kgm-3 at Tp=3.98oC • Maximum of pure ice, ri,max=916.4 kgm-3 at Tp=0oC • Temperature of maximum r and freezing decrease with S. • For earth ocean, S~ 34-36 psu, thus, Tf>Tp, freeze occurs before rw reaches maximum. • Ice initially forms disks, as size of ice disks increase, it becomes dendritic to increases surface/volume ratio to dissipate heat and solute. • Frazil ice: arms of dendrites break and form secondary ice formation, 2-4 mm ice grains • Pancake ice: 0.3-3.0: frazil ice bonding into larger solid ice.

20. Formation of sea ice transition and columnar zones • Formation of sea ice release heat and salt below. Because heat transfer is faster than salt transfer, act temperature below sea ice can be lower than freezing temperature, i.e., supercooling. • Supercooling leads to downward growth of platelets into brine layer, brine trapped between platelets form long and narrow brine pockets.

21. Fridtjof Nansen (1861-1930) • Sea ice is a fractal field comprised of ice floes. • A new pack of ice is formed by freezing of water in newly formed leads in region where wind drag pack ice away from shore; after reach 1 m thick, it is formed by collisions of ice floes; • Sea ice moves with transpolar drift stream. floes leads Floes streaming southward off the east coast of Greenland

22. Important to North Atlantic Deep Water (NADW) formation: • Ice is formed by fresh water and concentrated salt water is left behind as brine; • Brine mixed with sounding water. This heavy saline surface water sinks in Arctic to form NADW. • Why is sea-ice only a few meters thick?

23. Influence of ice sheet and sea-ice melting on the ocean thermohaline circulation: • Melting of Greenland Ice Sheet and Arctic sea ice increase fresh water discharge to the source region of North Atlantic Deep Water, reduce surface water density and weaken the deep ocean convection; • Strong melting of the Greenland Ice Sheet in future could weaken the NADW formation and thermohaline circulation.

24. Summary: • What is cryosphere? • Cryo (frozen), a component of the earth’s climate system comprised of water in its solid state. It consists of • glaciers & ice sheets, • snow, • permafrost (continuous and discontinuous) • sea ice (perennial and seasonal). • What is the distribution of different components of cryosphere? • Largest mass in Antarctic and Greenland, 58 m deep of water globally if they melt completely; • Sea ice and land snow cover 8-16% of the earth’s surface • Greenland and W. Antarctic ice sheet, Arctic sea ice and alpine glaciers have retreated rapidly in recent decades. • What is the roles of cryosphere in climate system? • Largest storage of global surface fresh water • Contribute to the thermal inertial of the earth’s climate • Contribute to albedo of the earth • Controls fresh water flux in the polar region, thus influence oceanic thermohaline circulation; • Store more carbon than that by global vegetation

25. Exercise-3: • If the Greenland ice sheet were completely melted, how much heat it would absorb and how high would global sea-level rise?

26. Exercise-3: • If the Greenland ice sheet were completely melted, how much heat it would absorb and how high would global sea-level rise? • ~ 8.5X1023 J, or 78 days of total solar radiation received by the earth system. 7 m Complete melt of Greenland ice sheet absorbs: Q=ViceXL=5Kg/m2X5.1X1017m2 X 3.34X105J/kg =8.5X1023 J of heat It takes ~ 77.7 days of total solar radiation received by the earth system (assuming 30% of earth’s albedo): t=Q/S=8.5X1023 J/(1.8X1017J/s)=4.7X106s/(2.4X3.6X104s/day)=77.7 days meters of sea level rise for complete melt of Greenland ice: 5 mX100%/72%~7 m

27. Exercise-3: • Consider a two-layer system consisting of a slab of snow 30 cm thick overlying a 3 m thick slab of sea ice. The temperature at the bottom of the sea ice is -1.7C and the temperature at the top of the snow is -40C. Assuming a steady-state diffusive process, find • a) temperature at the ice/snow interface, • b) temperature gradient in ice and snow, respectively. • Assume that density of the snow is 250 kgm-3 and ice is 500kgm-3. Specific heat of ice 2113 J kg-1 K-1. Thermal conductivity of snow/ice is