1. Introduction to Global Warming
Cryosphere (including sea level) and its modelling
Ralf GREVE
Institute of Low Temperature Science Hokkaido University
Sapporo,

2. Cryosphere (1) Part of the climate system related to ice. Consists of:
(Inland-) Ice sheets ® large land-based ice masses (Antarctica, Greenland).
Ice shelves ® floating ice masses, connected to an ice sheet (Antarctica).
Glaciers ® small land-based ice masses in mountanous regions.
Sea ice ® frozen ocean water.
Ground ice ® frozen ground, permafrost.
Snow.

3. Cryosphere (2) Time-scales Snow cover: Days, weeks.
Sea ice: Months – 10 years.
Glaciers: 10 – 100 years.
Ice shelves: 102 – 103 years.
Ice sheets: 103 – 105 years.

4. Climate and cryosphere (1)
Global warming
Mean global surface temperature: Increase of 0.6 ± 0.2°C during the 20th century.
Main cause: Emission of greenhouse gases (CO2, CH4, N2O, HC = halogenated carbohydrates, e.g. CFC) ® anthropogenic greenhouse effect.

5. Climate and cryosphere (2)
Natural and anthropogenic greenhouse effect
Without greenhouse effect: - Mean surface temperature of the Earth: –18°C.
Natural greenhouse effect: - True mean surface temperature: +15°C Therefore warming of the Earth‘s surface of approx. 33°C Contributions: Water vapour ~ 62% (!), CO2 ~ 22%, O3 ~ 7%, N2O ~ 4%, CH4 ~ 2,5%.
Anthropogenic greenhouse effect: - Cause: Anthropogenic emissions of greenhouse gases Contributions: CO2 ~ 60%, CH4 ~ 20%, N2O ~ 6%, HC ~ 14%.

6. Climate and cryosphere (3)
Ice sheets as climate archive
Atmospheric CO2
Pre-industrial concentration (1750): [CO2] ~ 280 ppm.
Today (2000): [CO2] ~ 370 ppm.
Present emissions: ca. 22 Gt/a (due to usage of fossil fuels).
Of this amount ~ 50% release to the atmosphere, ~ 50% absorption by oceans and vegetation (forests).
Data:
Resulting increase at present: d[CO2]/dt ~ 1.8 ppm/a.

7. Climate and cryosphere (4)
Natural variability during the last 420 ka (Vostok measurements):
Ice sheets as climate archive
Correlation with atmospheric temperature: - Interglacial maxima: [CO2] ~ 280 ppm Glacial minima: [CO2] ~ 200 ppm.
Present value and rate of increase never occured.

8. Climate and cryosphere (5)
Projections of the IPCC (2001)
CO2 concentration in the year 2100: ppm.
Mean global surface temperature (2100 – 1990): Increase by °C.
Mean global sea level (2100 – 1990): Increase by 9-88 cm.
Ice-sheet + glacier melt
Increase of extreme weather events: Tropical cyclons, strong precipitation events.
Increased aridity of several regions of the Earth, and therefore Increase of drought risk.

9. Climate and cryosphere (6)
Difficulties with the predictions:
Future emissions of greenhouse gases uncertain.
Influence of aerosols (airborne particles).
Numerous positive and negative feedbacks, e.g.: - Increasing cloud cover (negative) Decreasing snow cover (positive) Decreasing sea-ice extent (positive) Smaller solubility of CO2 in the ocean (positive).
Regional details, e.g. gulfstream weakening/shutdown.
Meltwater from the Greenland ice sheet

10. Ice sheets in the climate system (1)
Large potential for sea-level rise (~ 70 m).
Response time 1-10 ka ® internal dynamics negligible on time-scales < 100 a.
Interactions with atmosphere, ocean, lithosphere.

11. Ice sheets in the climate system (2)
Antarctic ice sheet
Ice volume: 26 ´ 106 km3 (2% ice shelves).
Sea-level equivalent: c. 61 m.
Ice-covered area: 13.5 ´ 106 km2 (8.5% ice shelves).
Tertiary origin, ³ 30 Ma BP.
Present mass loss: 1% melting, 99% calving.
Larsen
WAIS
® little susceptible to temperature rise of ~ 5°C.
However: potential for “irregular behaviour”:
- Rapid ice-shelf disintegration (Larsen!).
- Instability of the West-Antarctic ice sheet.

12. Ice sheets in the climate system (3)
Greenland ice sheet
Ice volume: 2.9 ´ 106 km3 (no ice shelves).
Sea-level equivalent: c. 7.5 m.
Ice-covered area: 1.7 ´ 106 km2.
Quaternary origin, 2-3 Ma BP.
Present mass loss: 50% melting, 50% calving ® susceptible to temperature rise of ~ 5°C.
No “irregular behaviour” foreseeable.

13. Greenland: Paleoclimatic simulation (1)
Ice-sheet model SICOPOLIS
(“SImulation COde for POLythermal Ice Sheets”)

14. Greenland: Paleoclimatic simulation (2)
Set-up for simulation with SICOPOLIS
Model time: t = 250 ka BP (present).
Atmospheric forcing: - “Glacial index” g(t) from ice-core records (GRIP, Vostok) Surface temperature, precipitation: Interpolation between present and LGM conditions, weighed by g(t). - Surface melting: Degree-day parameterization.
Grid spacing (resolution): Dx = 20 km.
colder climate
warmer climate

15. Greenland: Paleoclimatic simulation (3)
Results: Topography
127 ka BP (Eem):
21 ka BP (LGM):
Present:
Present (data):

16. Results: Present-day surface velocity
Greenland: Paleoclimatic simulation (4)
Results: Present-day surface velocity
Main drainage systems
Jacobshavn
ice stream

17. Greenland: Greenhouse simulations (1)
Set-up for simulation with SICOPOLIS
Model time: t = year 1990 (present)
Atmospheric forcing: - Global surface temperature from “WRE scenarios” (assumed future stabilization of atmospheric CO2 at 450, 550, 650, 750 or 1000 ppm): - Temperature increase over Greenland = 2 x global temperature increase. - Precipitation: 5% increase per degree warming. - Surface melting: Degree-day parameterization.
Grid spacing (resolution): Dx = 20 km.

18. Results: Temporal evolution
Greenland: Greenhouse simulations (2)
Results: Temporal evolution
Volume change (sea-level equivalent):
Freshwater discharge (1 Sv = 106 m3/s):

19. Greenland: Greenhouse simulations (3)
Results: Topography
Present (year 1990):
Year 2350, WRE 1000:

20. Sea-level change (1)
Observation in the 20th century: Sea-level rise of 1-2 mm/a.
Contributions:

21. Sea-level change (2)
Prediction of the Greenland simulations driven by WRE 450 – 1000 for the 21st century: Sea-level increase of ~ mm/a.
Contribution of Antarctica: Likely negative (sea-level decrease!) because of very small increase in surface melt, but significantly increased precipitation.
Uncertainties involved: - Emission scenario itself. - Changes of precipitation (increase likely, acts somewhat against ice-sheet melting). - Regionally varying sea-level change due to isostatic reactions of the Earth‘s lithosphere and changes of the geoid (gravity field).

22. Sea-level change (3)
Thus, IPCC prediction for the 21st century quite uncertain: – 1990: Rise of 9-88 cm, or mm/a.
Massive threat to the living space of about 80 million people in Bangladesh in the delta area of the rivers Ganges and Brahmaputra.
Danger of flooding of entire island states in the South Seas.
Necessity to reconstruct the north-German seaports (expensive!).
Some consequences of a sea-level rise of about 1 m:

23. Appendix: Ice-sheet flow (1)
Macro-scale
~ 3 km
~ 1000 km

24. Appendix: Ice-sheet flow (2)
Micro-scale
Loose-packed lattice, packing factor only 34% (close packing of spheres 74%, body-centered cubic lattice 68%).
Strongly anisotropic monocrystals (ice Ih, hexagonal structure).
Deformation along crystallographic planes (mainly basal, to a much lesser extent prismatic and pyramidal).
Grain size ~ 1 mm … 1 cm.

25. Appendix: Ice-sheet flow (3)
Macroscopic ice rheology
Control volume contains ensemble of randomly oriented ice crystals (grains).
Isotropic, non-linear viscous fluid.

26. Appendix: Ice-sheet flow (4)
Simple shear experiment:
Ice fluidity (inverse viscosity):
effective stress
homologous temperature
Creep function: Power law f(s) = sn-1, exponent n = 3 (Glen).
Rate factor: Arrhenius law
Enhancement factor E (equal to 1 for pure isotropic ice).

27. End of Chapter “Cryosphere (including sea level) and its modelling”