Quantified Objective: Determining changes in ice sheet mass balance within 15 Gt yr−1 per decade or 1.5 Gt yr−2.
Ice sheets are losing mass at an accelerating rate of 300 Gt yr−1 per decade, or 30 Gt yr−2. Detecting changes at the 5 percent level is essential for understanding the interactions of ice sheets and climate at the regional level and for improving projections from numerical models. Details of ice flow dynamics must be resolved at the glacier scale.
The melting of glaciers and ice sheets into the ocean is a dominant part of the total contribution to sea level rise (Vaughan et al., 2013) and by far the largest uncertainty in projecting sea level rise in the coming centuries (Church et al., 2013).
The contribution of ice sheets to sea level was 0.3±0.1 mm yr−1 in 1997-2001 and increasing to 1.2 ± 0.3 mm yr−1 in 2007-2011 (1 mm = 360 Gt). For small glaciers and ice caps, the contribution was 0.76 mm yr−1 in 1993-2009 and 0.83 mm yr−1 in 2005-2009 (Vaughan et al., 2013). Projections of sea level rise by 2100 range from 0.21 to 0.83 m, however with low confidence in the ability of ice sheet numerical models to project rapid dynamical changes in Antarctica and Greenland, which results in a systematic underestimation of ice sheet contributions (Church et al., 2013).
Detecting the rate of change in ice mass balance per decade is essential, along with the observation of rapid changes and the detection of dynamic instabilities. In 1997-2011, ice sheet loss accelerated at 300 Gt yr−1 per decade or 0.9 mm yr−1 per decade (Vaughan et al., 2013).
A glacier and ice sheet observing system has demonstrated its capability and value to provide modern, consistent, comprehensive estimates of ice sheet mass balance over the last decades, observe rapid glacier changes in Greenland and West Antarctica, and detect instabilities in ice dynamics. This system consists of satellite altimetry (Ice, Cloud, and land Elevation Satellite [ICESat] series), airborne altimetry and radar sounding (Operation
IceBridge—OIB), satellite radar interferometry (international synthetic aperture radar (SARs) with NASA participation), satellite time-variable gravity (Gravity Recovery and Climate Experiment [GRACE] series), and in situ automated weather stations (AWS). Employing this multiple-instrument approach provides complementary and essential information about net changes in mass and the underlying physical components that drive these changes such as surface mass balance processes and ice dynamics. Taken together, the geophysical variables measured by this observing system are sufficient to achieve the quantified objective.
Gravity measurements provide the most accurate synoptic-scale information about net mass change, and help evaluate surface mass balance fields at the large scale. Ice sheet velocity measurements provide detailed information about ice flow dynamics, the largest uncertainty in sea level rise. Ice volume measurements complement gravity and velocity measurements. Given the complementarity of the three geophysical variables for achieving the quantified objective, each variable is given the same utility score in this example.
Satellite radar interferometry shall measure ice motion with a precision of 100 mm at 100 m spatial scale, balanced pole to pole, with a near daily frequency re-visit to observe rapid change in ice dynamics and measure long term trends and spatial patterns in ice mass change. This enables measurements of ice sheet discharge into the ocean with a precision of 3 percent, combined with estimates of surface mass balance from regional atmospheric climate models at the 7 percent level, which in turns helps detect changes in ice mass loss with a precision of 1 Gt yr−2 in Greenland and 2 Gt yr−2 in Antarctica (Rignot et al., 2011).
Satellite laser altimetry shall measure ice sheet surface elevation to detect ice-sheet elevation change rates to accuracies better than 1 cm yr−1 on an annual basis and 25 cm yr−1 on fast moving glaciers at spatial scales of 10 km. This will enable detection of changes of 80 Gt yr1 in Antarctica and 25 Gt yr−1 in Greenland if the density at which volume changes are taking place is well known. Detection of acceleration in mass loss not well documented for laser altimetry (Shepherd et al., 2012). Available data showing non-linear, dynamically-controlled glacier response (Csatho et al, 2014) suggest a high degree of gap intolerance in maintaining the required quality of continuity measurements,
Satellite time-variable gravity shall measure change in Earth’s geoid with a precision better than 1 mm to degree 55 (363 km). This will enable measurements of acceleration in mass loss of 1 Gt yr−2 in Greenland and 4 Gt yr−2 in Antarctica (Velicogna et al., 2014).
Based on the above assessment of measurement precisions and accuracies and using the subjective quality rating scale given in Table 4.5, the committee attaches quality ratings of 0.9 to expected space-borne gravity and ice velocity measurements and 0.6 for laser altimetry from a combination of satellite and airborne platforms.
PROBABILITY OF SUCCESS
ICESat-1 had a lifetime of 7 years or twice its designed lifetime of 3 years, yet did not operate in continuous mode. ICESat-2 will employ a new photon counting technology for its sole instrument, the Advanced Topographic Laser Altimeter System (ATLAS). ATLAS represents a new approach to spaceborne determination of surface elevation in order to improve elevation estimates in sloped areas, as well as rough land surfaces such as crevasses. Specifically, ATLAS is a micropulse, multibeam, photon-counting laser altimeter with lower energy, a shorter pulse width, and a higher repetition rate relative to the instrument that was onboard ICESat. The probability of success is 0.8 (see Section 4.4 for scoring rationale).
Operation Ice Bridge has been operating in a precursor mode since 1993 as a suborbital program at NASA.1 The technology and implementation is mature to the level of a mission series. The probability of success is rated the highest with a score of 1.0.
1 The Arctic Ice Mapping group (Project AIM) at the NASA Goddard Space Flight Center Wallops Flight Facility has been conducting systematic topographic surveys of the Greenland Ice Sheet since 1993, using scanning airborne laser altimeters (NASA ATM) combined with Global Positioning System (GPS) technology. Operation IceBridge campaigns began in 2009; see http://icebridge.gsfc.nasa.gov/.
TABLE F.1 Final Continuity Scoring for the Quantified Objective Ice Sheet Mass Balance
|ICESat-2 + OIB (laser altimetry series)||5||1||0.6||0.8||2.4|
|GRACE FO (gravity series)||5||1||0.9||1.0||4.5|
|NISARa (ice velocity series)||5||1||0.9||0.8||3.6|
a Interferometric synthetic aperture radar (InSAR) is a technique that uses two or more synthetic aperture radar (SAR) images over the same region to derive surface topography or surface motion. NISAR refers to the NASA-ISRO (Indian Space Research Organization) [Interferometric] Synthetic Aperture Radar (see http://nisar.jpl.nasa.gov).
Satellite imaging radars have flown since 1991, without the purpose to provide interferometry data over ice sheets, but providing hands-on experience with ice sheet observations. NASA flew Seasat in 1978 and several shuttle-borne radar missions in the 1990s, with plans to return with a dedicated interferometric SAR free flyer mission in 2020. The technology and implementation both benefit from decades of experience with SAR. The probability of success is 0.8.
The GRACE mission was designed to last 5 years and has lasted more than 12 years. The probability of success of GRACE follow-on mission using a combination of existing and new technology is high and has a score of 1.
NASA is extending the gravity, ice volume, and ice velocity measurements by operating OIB and developing GRACE Follow-on, ICESat-2, and NISAR (NASA-ISRO synthetic aperture radar). Relative to this example quantified Earth science objective, previous experience indicates that laser altimetry, through a combination of satellite and airborne capabilities, will obtain a high-quality measurement that partially meets the quantified objective, whereas the quality of the gravity and ice velocity measurements will largely meet the quantified objective. Accordingly, the final continuity scoring for the quantified objective is given in Table F.1 using the benefit (B) formula from Chapter 3 of B = I × U × Q × S, where I ranges from 1 to 5, U ranges from 0 to 1.0, Q ranges from 0 to 1.0, and S ranges from 0 to 1.0 (see Section 4.4 for scoring rationale).
Church, J.A., P.U. Clark, A. Cazenave, J.M. Gregory, S. Jevrejeva, A. Levermann, M.A. Merrifield, et al. 2013. Sea Level Change. Chapter in Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex, and P.M. Midgley eds.). Cambridge University Press, Cambridge, U.K., and New York, N.Y.
Csatho, B.M., A.F. Schenk, C.J. van der Veen, G. Babonis, K. Duncan, S. Rezvanbehbahani, M.R. van den Broeke, S.B. Simon-sen, S. Nagarajan, and J.H. van Angelen. 2014. Laser altimetry reveals complex patter of Greenland ice sheet dynamics. Proceedings of the National Academies of Sciences 111(52):18478-18483.
Rignot, E., I. Velicogna, M.R. van den Broeke, A. Monaghan, and J.T.M. Lenaerts. 2011. Acceleration of the contribution of the Greenland and Antarctic ice sheets to sea level rise. Geophysical Research Letters 38:L05503.
Shepherd A., E. Ivins, A. Geruo, V.R. Barletta, M. Bentley, S. Bettadpur, K.H. Briggs, et al. 2012. A reconciled estimate of ice sheet mass balance. Science 338(6111):1183-1189.
Vaughan, D.G., J.C. Comiso, I. Allison, J. Carrasco, G. Kaser, R. Kwok, P. Mote, et al. 2013. Observations: Cryosphere. Chapter 4 in Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex, and P.M. Midgley eds.). Cambridge University Press, Cambridge, U.K., and New York, N.Y.
Velicogna, I., T.C. Sutterley, and M.R. van den Broeke. 2014. Regional acceleration in ice mass loss from Greenland and Antarctica using GRACE time-variable gravity. Journal of Geophysical Research: Space Physics 119:8130-8137.