ICESat dH/dt Thinning Thickening ICESat key findings

Slope Fraction of area Estimated Mass Loss Total Ice Sheet Loss Fraction of Total >0.5°33%175 GT145 GT>100% * >1°18%132 GT145 GT83% >2°8.5%68 GT145 GT43% >3°5.1%40 GT145 GT26% ICESat-derived ice loss as a function of slope * there is a net gain of ice in areas with slopes <0.5° Most Ice Loss at Rough Steep Margins Slope < 3° Total GT/yr Slope > 3° Total GT/yr Zwally, Saba, GSFC

Summer sea ice extent is decreasing faster than predicted by IPCC models. From ICESat: Sea ice thickness has decreased by about 0.70 m; Area of thick, multiyear ice has decreased by 42%. Arctic Sea Ice Observations Stroeve et al., 2007 Kwok, 2009

ICESat-2 Mission Concept In contrast to ICESat design, ICESat-2 will use micro-pulse multi-beam photon counting approach Provides: Dense cross-track sampling to resolve surface slope on an orbit basis (different sampling geometries are currently discussed with the SDT). High repetition rate (10 kHz) generates dense along-track sampling (~70 cm). Advantages: Improved elevation estimates over high slope areas and very rough (e.g. crevassed) areas Improved lead detection

Current ICESat-2 design:

Flight direction

3km 100 m Footprint size: 10 m PRF: 10 kHz (0.7 m)

Planned ICESat coverage over Jakobshavn glacier. Actual was 1/3 of this. Planned ICESat-2 Spacing ICESat vs ICESat-2

Bridging the gap between ICESat and ICESat-2 The big picture: Ideally, there is temporal overlap of sequential satellite missions (ideally one year). IceBridge goal: selective monitoring of ICESat tracks, over both ice sheets and sea ice. Monitoring key areas that are undergoing rapid and potentially long-term changes, such as the multiyear sea ice cover in the Arctic. ICESat established time series of ice sheet and sea ice thickness changes IceBridge’s objective is to continue this time series by flying over key regions every year until the launch of ICESat-2 CryoSat2 is a radar altimeter but will yield better spatial and temporal coverage; X- calibration with IceBridge ICESat-2 will continue monitoring with improved measurement concept.

Arctic IceBridge Flights

(1) How can Ice Bridge instruments and flight lines benefit the scientific analysis of existing ICEsat-1 data? For grounded ice, we have already collected all the ICESat – airborne coincident data that can be collected. Ice Bridge can contribute by continuing to support the analysis of these existing data sets. Continued monitoring of the cal/val targets used by ICESat (e.g. Track 412 or other reference tracks) will contribute by placing the ICESat observation period into wider context, and better characterizing ice sheet change on a variety of time and length scales. Monitoring tracks that span a range of surface slope, roughness, and reflectivity would be ideal. For sea ice, a major point of interest is the study of snow on sea ice and how this impacts ice thickness retrievals from laser altimetry data. There are currently three data sets used to determine snow thickness: passive microwave data, model data, and climatological data. However, the error sources and biases in each of these methods are still largely unknown. IceBridge data will provide a valuable data set to determine errors and biases in these data sets. This will allow us to reanalyze previous ice thickness estimates from ICESat data and better place the results into the climate record.

(2) How can Ice Bridge data collections enhance the science transition from ICESat-1 to ICESat-2? By continuing the time series along selected tracks, we can place the changes observed by ICESat into the context of the longer term changes in both the ice sheets, as well as the sea ice. Ideally, an instrument with a given set of characteristics used to cross-calibrate with ICESat would also be used to cross-calibrate with ICESat-2. In practice, instruments evolve over time. Consequently, it is imperative that altimetry instruments used in Ice Bridge continue to be well-calibrated and well- chatacterized to maximize ICESat and ICESat-2 cross calibration capability. ICESat-2 will be at lower orbit than ICESat. Once the ICESat-2 orbit is fixed, line up some flights with ICESat-2 orbit. Flight coordination with ICESat-2 calibration instrument MABEL. This will allow comparison between ICESat-2 like instrument and other airborne laser altimeters. (MABEL is ICESat-2 simulator; first flights (primarily engineering in late October; first science flights April 2011 out of Goose Bay and coordination with IceBridge (focus on ice sheets); April 2012 (out of Alaska?) will have focus on sea ice).

MABEL April 2011 Goose Bay

(3) How do you think Ice Bridge data could help tie together ICEsat 1/2 and Cryosat science? Continue to fly laser altimeters with radar altimeter measurements, ideally at similar frequency as used on CryoSat. Underflights of CryoSat tracks with both laser and radar altimeters X-calibration of radar and laser altimeter measurements may allow us to extrapolate IceBridge data to larger scales. Coordination with flights of the ASIRAS instrument under CryoSat when possible.

(4) How can Ice Bridge data best aid planning for ICESat-2? MABEL data will be instrumental for ICESat-2 algorithm development, i.e. extraction of geopysical information. Coordination with IceBridge and its vast array of data sets will enable us to estimate algorithm performance, errors sources, and uncertainties…..and improve algorithms….

(5) Incorporate ICESat 1/2 project needs into a decision matrix for IceBridge flight planning ICESat / ICESat-2 needs largely fall into repeat flights along selected ground tracks, and Cross-calibration flights of opportunity between ASIRAS and MABEL. Use previously collected ICESat data to identify areas of rapid change and determine where IceBridge data collections will have the greatest impact. Could be formalized through a decision matrix developed by the OIB science team.