Introduction to Global Warming

Transcription

1 Introduction to Global Warming Cryosphere (including sea level) and its modelling Ralf GREVE Institute of Low Temperature Science Hokkaido University Sapporo, 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 mountainous regions. Sea ice frozen ocean water. River ice, lake ice frozen river/lake water. Ground ice frozen ground, permafrost. Snow frozen precipitation.

2 3 Cryosphere (2) Time-scales Snow cover: Sea ice: Glaciers: Ice shelves: Ice sheets: Days, weeks. Months 10 years years years years. 4 Climate and cryosphere (1) Global warming Mean global surface temperature: Increase of 0.74 ± 0.18 C during Source: IPCC WG1 AR4 Main cause: Emission of greenhouse gases (CO 2, CH 4, N 2 O, HC = halogenated carbohydrates, e.g. CFC) anthropogenic greenhouse effect.

3 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% (!), CO 2 ~ 22%, O 3 ~ 7%, N 2 O ~ 4%, CH 4 ~ 2.5%. Anthropogenic greenhouse effect: - Cause: Anthropogenic emissions of greenhouse gases. - Contributions: CO 2 ~ 60%, CH 4 ~ 20%, N 2 O ~ 6%, HC ~ 14%. 6 Climate and cryosphere (3) Atmospheric CO 2 Ice sheets as climate archive Pre-industrial concentration (1750): [CO 2 ] ~ 280 ppm. Today (2005): [CO 2 ] ~ 379 ppm. Present emissions: ca. 26 Gt/a (due to usage of fossil fuels). Of this amount ~ 50% release to the atmosphere, ~ 50% absorption by oceans and vegetation (forests). Data: Source: IPCC WG1 TAR Resulting increase at present: d[co 2 ]/dt ~ 1.9 ppm/a.

4 7 Climate and cryosphere (4) Natural variability during the last 420 ka (Vostok measurements): Source: IPCC WG1 TAR Ice sheets as climate archive Correlation with atmospheric temperature: - Interglacial maxima: [CO 2 ] ~ ppm. - Glacial minima: [CO 2 ] ~ ppm. Present value and rate of increase never occured. 8 Climate and cryosphere (5) Projections of the IPCC WG1 AR4 (2007) CO 2 concentration in the year 2100 (SRES scenarios): ~ ppm. Mean global surface temperature ( relative to ): Increase by C. Mean global sea level ( relative to ): Increase by cm. Increase of extreme weather events: Ice-sheet + glacier melt Heat waves, heavy rainfall, tropical cyclons. Increase in areas affected by droughts.

5 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 CO 2 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.

6 11 Ice sheets in the climate system (2) Antarctic ice sheet Larsen WAIS Ice volume: km 3 (2% ice shelves). Sea-level equivalent: c. 61 m. Ice-covered area: km 2 (8.5% ice shelves). Tertiary origin, 30 Ma old. Present mass loss: 1% melting, 99% calving. 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: km 3 (no ice shelves). Sea-level equivalent: c. 7.2 m. Ice-covered area: km 2. Quaternary origin, 2-3 Ma old. Present mass loss: 50% melting, 50% calving susceptible to temperature rise of ~ 5 C.

7 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 ago... 0 (present). Atmospheric forcing: - Glacial index g(t) from ice-core records (GRIP, Vostok). colder climate warmer climate - Surface temperature, precipitation: Interpolation between present and LGM conditions, weighed by g(t). - Surface melting: Degree-day parameterization. Grid spacing (resolution): Δx = 20 km.

8 15 Greenland: Paleoclimatic simulation (3) Results: Topography 127 ka ago (Eem): 21 ka ago (LGM): Present: Present (data): 16 Greenland: Paleoclimatic simulation (4) Results: Present-day surface velocity Main drainage systems Jacobshavn ice stream

9 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 CO 2 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): Δx = 20 km. 18 Greenland: Greenhouse simulations (2) Results: Temporal evolution Volume change (sea-level equivalent): Freshwater discharge (1 Sv = 10 6 m 3 /s):

10 19 Greenland: Greenhouse simulations (3) Results: Topography Present (year 1990): Year 2350, WRE 1000: 20 Sea-level change (1) Observation: Increase of 1.8 ± 0.5 mm/a ( ), 3.1 ± 0.7 mm/a ( ). Source: IPCC WG1 AR4 Main contributions: Thermal expansion of ocean water, melting of glaciers and ice sheets.

11 21 Sea-level change (2) Prediction of the Greenland simulations driven by WRE for the 21 st 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). - Ice-dynamical effects may lead to a significantly larger positive contribution of Antarctica and Greenland to sea-level rise. 22 Sea-level change (3) Thus, IPCC WG1 AR4 prediction for the 21 st century quite uncertain: relative to : Rise of cm, or mm/a. Some consequences of a sea-level rise of about 1 m: 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 Pacific Ocean. Necessity to reconstruct seaports (expensive!).

12 23 References Bamber, J. L., and A. J. Payne (eds.) Mass Balance of the Cryosphere: Observations and Modelling of Contemporary and Future Changes. Cambridge University Press, Cambridge, UK, and New York, NY, USA, 662 pp. Fisher, H., M. Whalen, J. Smith, D. Mastroianni and B. Deck Ice core records of atmospheric CO 2 around the last three glacial terminations. Science 283, Greve, R Evolution and dynamics of the Greenland ice sheet over past glacial-interglacial cycles and in future climate-warming scenarios. Proceedings of the 5th International Workshop on Global Change: Connection to the Arctic (GCCA5), University of Tsukuba, Japan, Greve, R Relation of measured basal temperatures and the spatial distribution of the geothermal heat flux for the Greenland ice sheet. Annals of Glaciology 42, IPCC, Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change [Houghton, J. T., Y. Ding, D. J. Griggs, M. Noguer, P. J. van der Linden, X. Dai, K. Maskell and C. A. Johnson (eds.)]. Cambridge University Press, Cambridge, UK, and New York, NY, USA, 881 pp. IPCC, Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K. B. Averyt, M. Tignor and H. L. Miller (eds.)]. Cambridge University Press, Cambridge, UK, and New York, NY, USA, 996 pp. Petit, J. R., and 18 others Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica. Nature 399 (6735), End of Chapter Cryosphere (including sea level) and its modelling