Frank PATTYN Laboratoire de Glaciologie Université Libre de Bruxelles Belgium

Transcription

1 Frank PATTYN Laboratoire de Glaciologie Université Libre de Bruxelles Belgium

2 200 x 10 6 people living in coastal floodplains IPCC, x 10 6 km 2 and US$ worth of assets lying <1 m a.s.l. (Stern, 2007) 27/10/2010 Controverses Climatiques F. Pattyn (ULB) 2

3 Relative SL stability >100 m 27/10/2010 Controverses Climatiques F. Pattyn (ULB) 3

4 Annual averages of the global mean sea level (mm) Sea level rise of mm/year IPCC, /10/2010 Controverses Climatiques F. Pattyn (ULB) 4

5 Sea level rise estimates from satellite altimetry are 3.1 ± 0.4 mm/yr for Leuliette et al, /10/2010 Controverses Climatiques F. Pattyn (ULB) 5

6 Sea level 27/10/2010 Controverses Climatiques F. Pattyn (ULB) 6

7 IPCC, 2007 Sea level rise of cm by 2100: probably underestimation Time series of global mean sea level (deviation from the mean) in the past and as projected for the future. For the period before 1870, global measurements of sea level are not available. The green line shows global mean sea level observed from satellite altimetry. The blue shading represents the range of model projections for the SRES A1B scenario for the 21st century, relative to the 1980 to 1999 mean, and has been calculated independently from the observations. 27/10/2010 Controverses Climatiques F. Pattyn (ULB) 7

8 IPCC, 2007 Present day sea level rise Projections and uncertainties (5 to 95% ranges) of global average sea level rise and its components in 2090 to 2099 (relative to 1980 to 1999) for the six SRES marker scenarios. The projected sea level rise assumes that the part of the present day ice sheet mass imbalance that is due to recent ice flow acceleration will persist unchanged. It does not include the contribution shown from scaled up ice sheet discharge, which is an alternative possibility. It is also possible that the present imbalance might be transient, in which case the projected sea level rise is reduced by 0.02 m. It must be emphasized that we cannot assess the likelihood of any of these three alternatives, which are presented as illustrative. The state of understanding prevents a best estimate from being made 27/10/2010 Controverses Climatiques F. Pattyn (ULB) 8

9 Models used to date do not include the full effects of changes in ice sheet flow, because a basis in published literature is lacking. Ice flow from Greenland and Antarctica could increase or decrease in the future. Understanding of these effects is too limited to assess their likelihood or to provide a best estimate or an upper bound for sea level rise. Dynamical processes related to ice flow not included in current models but suggested by recent observations could increase the vulnerability of the ice sheets to warming, increasing future sea level rise. Understanding of these processes is limited and there is no consensus on their magnitude. 27/10/2010 Controverses Climatiques F. Pattyn (ULB) 9

10 From thermal expansion to ice sheet dynamics 27/10/2010 Controverses Climatiques F. Pattyn (ULB) 10

11 Milne et al, /10/2010 Controverses Climatiques F. Pattyn (ULB) 11

12 Thermal expansion (57% of total sea level rise) IPCC, 2007 Global sea level change due to thermal expansion for 1955 to 2003 (Levitus et al. (2005a; black line), Ishii et al. (2006; red line) for the 0 to 700 m layer, Willis et al. (2004; green line) for the upper 750 m. 27/10/2010 Controverses Climatiques F. Pattyn (ULB) 12

13 Mean rates of sea level change over the period estimated from observations of ocean temperature Milne et al, 2009 Mean rate of sea surface height change during October 1992 to May 2007, determined from satellite altimetry measurements 27/10/2010 Controverses Climatiques F. Pattyn (ULB) 13

14 Sea level change due to Greenland mass loss of 1 mm/yr Sea level change due to West Antarctic mass loss of 1 mm/yr Milne et al, /10/2010 Controverses Climatiques F. Pattyn (ULB) 14

15 % IPCC, /10/2010 Controverses Climatiques F. Pattyn (ULB) 15

16 GIC contribution to sea level rise (Kaser, 2010) 27/10/2010 Controverses Climatiques F. Pattyn (ULB) 16

17 Greenland ice mass Antarctic ice mass 30±11 Gt/yr 2 26±14 Gt/yr 2 A quadratic function of time describes the observations well, but to make projections we need physically based models Velicogna, /10/2010 Controverses Climatiques F. Pattyn (ULB) 17

18 Greenland dynamic change is not caused by surface meltwater lubricating the bed 27/10/2010 Controverses Climatiques F. Pattyn (ULB) 18

19 Water temperature, m depth averaged Holland et al, /10/2010 Controverses Climatiques F. Pattyn (ULB) 19

20 Holland et al, 2008 Elevation Velocity 27/10/2010 Controverses Climatiques F. Pattyn (ULB) 20

21 interior thickening related to changes in snowfall coastal thinning in WAIS (Pine Island, Smith and Thwaites Glaciers) and EAIS (Cook and Totten Glaciers) close correspondence to ice velocity (ice streams) Observed elevation change ( ) 27/10/2010 Controverses Climatiques F. Pattyn (ULB) 21

22 interior thickening related to changes in snowfall Pritchard et al, 2009 coastal thinning in WAIS (Pine Island, Smith and Thwaites Glaciers) and EAIS (Cook and Totten Glaciers) close correspondence to ice velocity (ice streams) Shepherd and Wingham, /10/2010 Controverses Climatiques F. Pattyn (ULB) 22

23 slow flowing (~10 m/yr) interior flow dominated by vertical shear floating ice shelves dominated by longitudinal stretching fast flowing (~km/yr) areas (ice streams or outlet glaciers) lateral shear but potentially fully combination 27/10/2010 Controverses Climatiques F. Pattyn (ULB) 23

24 collapse of ice shelf A in 1995 and B in 2002 meltwaterdriven fracture understood natural experiment testing link between floating and grounded ice MacAyeal et al, 2003 Hulbe et al, /10/2010 Controverses Climatiques F. Pattyn (ULB) 24

25 Scambos et al, /10/2010 Controverses Climatiques F. Pattyn (ULB) 25

26 Wingham et al, /10/2010 Controverses Climatiques F. Pattyn (ULB) 26

27 Jenkins et al. (2010) identify a bedrock ridge under the ice shelf ~40 km from the current grounding line Jenkins et al, 2010 observed GL retreat rates consistent with GL occupying ridge in mid 1990s Why is thinning accelerating? 27/10/2010 Controverses Climatiques F. Pattyn (ULB) 27

28 likely cause changes in water masses occupying Amundsen Sea Circumpolar Deep Water (CDW) is dominant water mass ~1 C or ~4 C above melt point at grounding line 27/10/2010 Controverses Climatiques F. Pattyn (ULB) 28

29 Thoma and others (2008) use regional ocean model MICOM Isopycnal layer model shows thickening of CDW layers in mid 1990s forced by NCEP sea surface data Thoma et al., /10/2010 Controverses Climatiques F. Pattyn (ULB) 29

30 Evidence of thermohaline circulation underneath small ice shelves and in shallow bays Marine ice formation Ice sheet bedrock Sub ice melting Ice shelf Pattyn et al, in prep 27/10/2010 Controverses Climatiques F. Pattyn (ULB) 30

31 Mechanisms of grounding line behavior and ocean interaction understood Unstable GL retreat across overdeepenings Efforts underway to include complex mechanisms of marine ice sheet behavior in large scale ice sheet models (ice2sea) Docquier et al, submitted Nick et al, /10/2010 Controverses Climatiques F. Pattyn (ULB) 31

32 MISMIP = Marine Ice Sheet Model Intercomparison Project Tools for verifying marine ice sheet models under idealized geometry Tests for marine ice sheet instability Pattyn et al, 2006 Grounding line position (km) Experiment 1a Mode M Log(ice viscosity) SCHO AVI1 AVI2 CSC1 CSC3 DNG1 DPO1 DPO2 DPO3 DPO4 EBU1 EBU2 FNI1 FPA4 FSA1 FSA2 ORY1 RHI1 RGL5 RGL6 Schoof et al, 2007; Pattyn et al, in prep. 27/10/2010 Controverses Climatiques F. Pattyn (ULB) 32

33 Need to understand the 20th century sea level budget, particularly to know whether there is an observational gap or other factors (deep ocean heat uptake, constant pre industrial term) Recent rapid ice sheet dynamical changes are driven by ice melting caused by ocean warming. To project such effects, couple high resolution models of ice stream dynamics including grounding line migration to high resolution regional ocean models. Such improvements are on their way. 27/10/2010 Controverses Climatiques F. Pattyn (ULB) 33