Program for Arctic Regional Climate Assessment (PARCA)

The Program for Arctic Regional Climate Assessment (PARCA) is a NASA-sponsored initiative with the prime objective of understanding the mass balance of the Greenland ice sheet. In October 1998, PARCA investigators met to review activities of the previous year, assess the program's progress, and plan future investigations directed at accomplishing that objective. Some exciting results were presented and discussed, including evidence of dramatic thinning of the ice sheet near the southeastern coast. Details of the investigations and many of the accomplishments are given in this report, but major highlights are given in the Executive Summary of the report

Comparison of sea-ice freeboard distributions from aircraft data and cryosat-2

The only remote sensing technique capable of obtain- ing sea-ice thickness on basin-scale are satellite altime- ter missions, such as the 2010 launched CryoSat-2. It is equipped with a Ku-Band radar altimeter, which mea- sures the height of the ice surface above the sea level. This method requires highly accurate range measure- ments. During the CryoSat Validation Experiment (Cry- oVEx) 2011 in the Lincoln Sea, Cryosat-2 underpasses were accomplished with two aircraft, which carried an airborne laser-scanner, a radar altimeter and an electro- magnetic induction device for direct sea-ice thickness re- trieval. Both aircraft flew in close formation at the same time of a CryoSat-2 overpass. This is a study about the comparison of the sea-ice freeboard and thickness dis- tribution of airborne validation and CryoSat-2 measure- ments within the multi-year sea-ice region of the Lincoln Sea in spring, with respect to the penetration of the Ku- Band signal into the snow

Determination and Characterization of 20th Century Global Sea Level Rise

The Report describes the PhD dissertation research completed by Chung-Yen Kuo on November, 2005, supervised by C.K. Shum and other PhD Committee Members including Douglas Alsdorf, Michael Bevis, Laury Miller, and Yuchan Yi.This research is supported by grants from NOAA under NA16RG2252, NA86RG0053 (R/CE-5) and NA030AR4170060 (R/CE-8), and NASA's Ocean and Ice, Cryosphere, Physical Oceanography and Interdisciplinary Science Programs (NNG04GA53G, NAG5-9335, NAG5-12585, JPL- 1265252).Sea level rise has been widely recognized as a measurable signal and as one of the consequences of a possible anthropogenic (human-induced) effect on global climate change. The small rate of sea level rise, 1–2 mm/yr during the last century [Church et al., 2001, Chapter 11, Changes in Sea Level, in the Third Assessment Report (TAR) of the Intergovernmental Panel for Climate Change (IPCC), Working Group I, Houghton et al., 2001], could only be partially explained by a number of competing geophysical processes, each of which is a complex process within the Earth-atmosphere-ocean-cryosphere-hydrosphere system. In particular, the observed 20th Century sea level rise rate of 1.84±0.35 mm/yr [Douglas, 2001; Peltier, 2001] could not explain up to one half of the predicted 20th Century global sea level rise based on the IPCC TAR estimate of 1.1 mm/yr (0.6 mm/yr of melted water from ice sheets and glaciers, and 0.5 mm/yr from the steric effect in the ocean) [Church et al., 2001] and remains an enigma [Munk, 2002]. The quest to resolve the controversy [Meier and Wahr, 2002] and to further understand sea level change [Chao et al., 2002] is well underway including efforts being conducted during the current IPCC Fourth Assessment Report (FAR), 2003–2007. In this study, we provide a determination of the 20th Century (1900–2002) global sea level rise, the associated error budgets, and the quantifications or characterization of various geophysical sources of the observed sea level rise, using data and geophysical models. We analyzed significant geographical variations of global sea level change including those caused by the steric component (heat and salinity) in the ocean, and sea level redistribution resulting from ice sheets and glacier melting in consequence of self gravitation, and the effects of glacial isostatic adjustment (GIA) since the Pleistocene affecting sea level signals in the observations. In particular, relative sea level data from up to 651 global long-term (longest record is 150 years) tide gauges from Permanent Service for Mean Sea Level (PSMSL) and other sources, and geocentric sea level data from multiple satellite radar altimetry (1985–2005) have been used to determine and characterize the 20th Century global sea level rise. Altimeter and selected tide gauge data have been used for sea level determination, accounting for relative biases between different altimeters and offsets between the tide gauges, effects of thermosteric sea level variations, vertical motions affecting tide gauge sea level measurements, sea level redistribution due to ice melt resulting from self gravitation, and barotropic ocean response due to atmospheric forcing. This study is also characterized by the role of the polar ocean in the global sea level study and addressing the question whether there is a detectable acceleration of sea level rise during the last decade. Vertical motions have been estimated by combining geocentric sea level measurements from satellite altimetry (TOPEX/POSEIDON, T/P) and long-term relative (crustfixed) sea level records from global tide gauges using the Gauss-Markov model with stochastic constraints. The study provided a demonstration of improved vertical motion solutions in semienclosed seas and lakes, including Fennoscandia and the Great Lakes region, showing excellent agreement with independent GPS observed radial velocities, or with predictions from GIA models. In general, the estimated uncertainty of the observed vertical motion is <0.5 mm/yr, significantly better than other studies. Finally, improved algorithms to account for nonlinear vertical motions caused by other geodynamic processes than GIA including post-seismic deformations, have been developed and applied to tectonically active regions such as Alaska and compared well with GPS velocities and other studies. This novel technique could potentially provide improved vertical motion globally where long-term tide gauge records exist. The thermosteric sea level trend of the upper (0–500 m) layers of the ocean accounts for about 70% of the variations when deeper ocean (0–3000 m) is considered using the World Ocean Altas 2001 (WOA01). The estimated global thermosteric sea level trend of 0.33 mm/yr (0–500 m) and 0.43 mm/yr (0–3000 m) agrees well with other studies [Levitus et al., 2005; Ishii et al., 2005]. A detailed analysis using in situ temperature and salinity data from the Ocean Station Data (OSD), in the Eastern Pacific (where the in situ data are more abundant) is conducted to assess the contributions of respective roles of thermal and salinity effects on sea level changes. The analysis indicates that the estimated thermosteric sea level trends integrated from 0–500 m and from 0–1000 m depths using OSD are almost identical at ~0.64 mm/yr in the Eastern Pacific ocean. In this region, the halosteric (salinity) sea level trend is small, at the 0.04 mm/yr level for both the 0–500 m and 0–1000 m cases. The result is consistent with Miller and Douglas [2004], which analyzed data from selected oceanic regions including the Eastern Pacific, Eastern Atlantic, and Caribbean. In general, observed sea level (from tide gauges or altimetry) and thermosteric sea level are highly correlated except in regions of high mesoscale variability. A detailed comparison of the estimated thermosteric sea level trends with the observed sea level trend near proximities of global tide gauges (~50 years, 1950–2004, up to 597 sites) indicates that the thermosteric sea level trends account for 36% and 68% of the observed global sea level trend, for the 0–500 m and 0–3000 m cases, respectively, indicating the importance of deeper ocean thermal effects. Comparison of global thermosteric sea level trend with altimetry observed sea level trends indicates that the results are dependant on data spans used: thermosteric sea level trend (0–500 m) accounts for over 81% of total observed sea level using T/P altimetry during 1993–2003, while accounts for only 26% when data span extends to 1985–2005 using multiple altimetry. The primary reason is the presence of long-period oceanic variability (interannual, decadal or longer) in trend estimates causing inconsistent conclusions. Comparison of thermosteric sea level trend with sea level observed by selected tide gauges reaches different conclusions depending on the choice and number of tide gauges (1955–1996) used: thermosteric sea level trend (0–3000 m) accounts for over 88% of the observed sea level from 27 tide gauges, while it accounts for only 38% if 515 tide gauges are used. A study using temporal gravity field measurements observed by the Gravity Recovery and Climate Experiment (GRACE) in terms of oceanic mass variations at long-wavelength (>800 km) and monthly sampling demonstrates its potential use, when combined with satellite altimetry, to improve steric sea level trend estimates over the Southern Ocean, where the OSD is extremely sparse or non-existent. The global (covering within ±81°.5 latitude) sea level trend (corrected for IB) observed by multiple satellite altimetry (GEOSAT, ERS-1, T/P, ERS-2, GFO, JASON and ENVISAT, data covering 1985–2002 except for 1988–1991), and by ~530 tide gauges are 2.8±0.5 mm/yr (corrected for GIA geoid effect) and 2.7±0.4 mm/yr, respectively, indicating excellent agreement. The observed sea level trend using multiple satellite altimeters covering 1985–2004 is 2.9±0.5 mm/yr, although the data span is still too short to yield reliable trend estimates. The multiple altimeter data sets allow coverage in the polar oceans, as opposed to only using data from T/P (coverage within ±66°), and have a longer data span (~19 years versus 10 year for T/P). The data have been calibrated against each other and with tide gauge data for robust determination of their relative biases with respect to T/P. The 20th Century (1900–2002) global sea level trend determined from 651 tide gauge stations is 1.6±0.4 mm/yr, while the trend estimate for the last 50 years (1948–2002) is similar, but used 620 tide gauges. The atmospheric effect via the barotropic IB correction (globally averaged at ~0.11 mm/yr and over 50 years at global tide gauge locations) to sea level (tide gauges or altimetry) observations is warranted as it reduces variances of the data and causes improved agreements between altimetry and tide gauge sea level data. The multiple altimeter data allows sea level studies in the Arctic and Sub-Arctic Oceans, which have been less studied. The sea level trend in the Sub-Arctic Ocean, bounded by latitude 55°N–82°N and longitude 315°E–60°E, is estimated at 1.8 mm/yr using primarily ERS-1 and ERS-2 altimetry, which have been calibrated by TOPEX/POSEIDON in the lower latitude (±66° latitude) oceans and remove constant offsets and a latitude-dependent bias. The sea level trend can mostly be explained by atmospheric forcing and thermosteric effects. In the Arctic Ocean, sea level trend during 1948–2002 is estimated at 1.9–2.0 mm/yr using tide gauges after correction of land motion using different GIA models. After applying the inverted barometric (IB) correction, the sea level rate reduces to 1.5–1.6 mm/yr, indicating that the barotropic response of the ocean contributes significantly more to the sea level trend in the Arctic Ocean than for the global ocean. In an attempt to estimate the global sea level trend and to quantify some of the known contributions in the 20th Century using data from sparsely distributed long-term tide gauges and ~20 years of satellite altimetry, we assume that the spatial patterns of sea level trends from melt water sources (Antarctica, Greenland and mountain glaciers), the thermosteric effects, and geoid change and land uplift due to GIA are known, while their magnitudes are unknown. We ignored contributions from fresh water imbalances due to continental hydrological processes [Milly et al., 2003; Ngo-Duc et al., 2005], human-impoundment of water in reservoirs or lakes [Chao et al., 1994; Sahagian et al., 1994] and other effects, including permafrost melt [Zhang et al., 2005]. Using two different estimators, the Weighted Least Squares (WLS) and the Elementwise- Weighted Total Least Squares (EW-TLS), and using long-term tide gauges and satellite altimetry, we estimate the respective contributions of each of the sources to the global sea level rise. The EW-TLS technique is found to be more stable while the WLS technique produces solutions with larger error of ~0.3 mm/yr in a simulated study. The EW-TLS solution yields the estimated 20th Century (1900–2002) sea level trend contributions by melt water from the Antarctic and Greenland ice sheets and mountain glaciers to be 0.80±0.14 mm/yr, 0.56±0.12 mm/yr and 0.31±0.08 mm/yr, respectively; the estimated contribution from the thermosteric effect is 0.06±0.04 mm/yr, and the estimated scale of the GIA (ICE-4G) model to correct tide gauge sea level data is 1.27±0.08, indicating that the correction should be higher by 27%. The resulting 20th Century (1900–2002) globally averaged sea level trend is estimated to be 1.73±0.42 mm/yr (95% confidence or 2s) after summing the above forcing factors. The estimate of the resulting last 50 year (1948–2002) global sea level trend is 1.74±0.48 mm/yr (95% confidence or 2s). Finally, we address the issue of whether the sea level trend acceleration is detectable. An analysis indicates that the minimum data span to obtain a stable rate of sea level trend from 27 selected tide gauges [Douglas, 2001] is 20 years or more, while one should use a 30-year or longer data span to derive a stable thermosteric sea level trend from WOA01. It is concluded that, with 95% confidence, there is no statistically significant evidence of sea level acceleration during 1900–2000 from tide gauge data, and during 1950-2000 from thermosteric data. The estimated 20th Century global sea level rise is compared with the more recently estimated total geophysical effects contributing to global sea level rise of 1.41–1.53 mm/yr, which include the sum of the steric effect (~0.4 mm/yr) [e.g., Levitus et al., 2005; Antonov et al., 2005], mountain glacier melting (~0.51 mm/yr) [e.g., Arendt et al., 2002; Dyurgerov and Meier, 2005; Raper and Braithwaite, 2005; and others], ice sheet mass imbalance (~0.45 mm/yr) [e.g., Krabill et al., 2004, Rignot et al., 2005, Thomas et al., 2005], hydrological imbalance (~0.0–0.12 mm/yr) [Milly et al., 2003, Ngo-Duc et al., 2005], and anthropogenic effect (~0.05 mm/yr) [Dork Sahagian, pers. comm.]. This study (1.74±0.48 mm/yr) reduces the existing discrepancy [Chruch et al., 2001] to 0.21–0.33 mm/yr between the total observed and predicted contributions to 20th Century global sea level rise. However, the estimated individual contributions of the ice sheet imbalance and the oceanic steric effect remain significantly different from the current results from observations, and not much progress has been made since the last IPCC study on the effect of hydrologic imbalance and anthropogenic causes. Much future work remains to improve our understanding of the complex processes governing global sea level changes

Determination and Characterization of 20th Century Global Sea Level Rise

The Report describes the PhD dissertation research completed by Chung-Yen Kuo on November, 2005, supervised by C.K. Shum and other PhD Committee Members including Douglas Alsdorf, Michael Bevis, Laury Miller, and Yuchan Yi.This research is supported by grants from NOAA under NA16RG2252, NA86RG0053 (R/CE-5) and NA030AR4170060 (R/CE-8), and NASA's Ocean and Ice, Cryosphere, Physical Oceanography and Interdisciplinary Science Programs (NNG04GA53G, NAG5-9335, NAG5-12585, JPL- 1265252).Sea level rise has been widely recognized as a measurable signal and as one of the consequences of a possible anthropogenic (human-induced) effect on global climate change. The small rate of sea level rise, 1–2 mm/yr during the last century [Church et al., 2001, Chapter 11, Changes in Sea Level, in the Third Assessment Report (TAR) of the Intergovernmental Panel for Climate Change (IPCC), Working Group I, Houghton et al., 2001], could only be partially explained by a number of competing geophysical processes, each of which is a complex process within the Earth-atmosphere-ocean-cryosphere-hydrosphere system. In particular, the observed 20th Century sea level rise rate of 1.84±0.35 mm/yr [Douglas, 2001; Peltier, 2001] could not explain up to one half of the predicted 20th Century global sea level rise based on the IPCC TAR estimate of 1.1 mm/yr (0.6 mm/yr of melted water from ice sheets and glaciers, and 0.5 mm/yr from the steric effect in the ocean) [Church et al., 2001] and remains an enigma [Munk, 2002]. The quest to resolve the controversy [Meier and Wahr, 2002] and to further understand sea level change [Chao et al., 2002] is well underway including efforts being conducted during the current IPCC Fourth Assessment Report (FAR), 2003–2007. In this study, we provide a determination of the 20th Century (1900–2002) global sea level rise, the associated error budgets, and the quantifications or characterization of various geophysical sources of the observed sea level rise, using data and geophysical models. We analyzed significant geographical variations of global sea level change including those caused by the steric component (heat and salinity) in the ocean, and sea level redistribution resulting from ice sheets and glacier melting in consequence of self gravitation, and the effects of glacial isostatic adjustment (GIA) since the Pleistocene affecting sea level signals in the observations. In particular, relative sea level data from up to 651 global long-term (longest record is 150 years) tide gauges from Permanent Service for Mean Sea Level (PSMSL) and other sources, and geocentric sea level data from multiple satellite radar altimetry (1985–2005) have been used to determine and characterize the 20th Century global sea level rise. Altimeter and selected tide gauge data have been used for sea level determination, accounting for relative biases between different altimeters and offsets between the tide gauges, effects of thermosteric sea level variations, vertical motions affecting tide gauge sea level measurements, sea level redistribution due to ice melt resulting from self gravitation, and barotropic ocean response due to atmospheric forcing. This study is also characterized by the role of the polar ocean in the global sea level study and addressing the question whether there is a detectable acceleration of sea level rise during the last decade. Vertical motions have been estimated by combining geocentric sea level measurements from satellite altimetry (TOPEX/POSEIDON, T/P) and long-term relative (crustfixed) sea level records from global tide gauges using the Gauss-Markov model with stochastic constraints. The study provided a demonstration of improved vertical motion solutions in semienclosed seas and lakes, including Fennoscandia and the Great Lakes region, showing excellent agreement with independent GPS observed radial velocities, or with predictions from GIA models. In general, the estimated uncertainty of the observed vertical motion is <0.5 mm/yr, significantly better than other studies. Finally, improved algorithms to account for nonlinear vertical motions caused by other geodynamic processes than GIA including post-seismic deformations, have been developed and applied to tectonically active regions such as Alaska and compared well with GPS velocities and other studies. This novel technique could potentially provide improved vertical motion globally where long-term tide gauge records exist. The thermosteric sea level trend of the upper (0–500 m) layers of the ocean accounts for about 70% of the variations when deeper ocean (0–3000 m) is considered using the World Ocean Altas 2001 (WOA01). The estimated global thermosteric sea level trend of 0.33 mm/yr (0–500 m) and 0.43 mm/yr (0–3000 m) agrees well with other studies [Levitus et al., 2005; Ishii et al., 2005]. A detailed analysis using in situ temperature and salinity data from the Ocean Station Data (OSD), in the Eastern Pacific (where the in situ data are more abundant) is conducted to assess the contributions of respective roles of thermal and salinity effects on sea level changes. The analysis indicates that the estimated thermosteric sea level trends integrated from 0–500 m and from 0–1000 m depths using OSD are almost identical at ~0.64 mm/yr in the Eastern Pacific ocean. In this region, the halosteric (salinity) sea level trend is small, at the 0.04 mm/yr level for both the 0–500 m and 0–1000 m cases. The result is consistent with Miller and Douglas [2004], which analyzed data from selected oceanic regions including the Eastern Pacific, Eastern Atlantic, and Caribbean. In general, observed sea level (from tide gauges or altimetry) and thermosteric sea level are highly correlated except in regions of high mesoscale variability. A detailed comparison of the estimated thermosteric sea level trends with the observed sea level trend near proximities of global tide gauges (~50 years, 1950–2004, up to 597 sites) indicates that the thermosteric sea level trends account for 36% and 68% of the observed global sea level trend, for the 0–500 m and 0–3000 m cases, respectively, indicating the importance of deeper ocean thermal effects. Comparison of global thermosteric sea level trend with altimetry observed sea level trends indicates that the results are dependant on data spans used: thermosteric sea level trend (0–500 m) accounts for over 81% of total observed sea level using T/P altimetry during 1993–2003, while accounts for only 26% when data span extends to 1985–2005 using multiple altimetry. The primary reason is the presence of long-period oceanic variability (interannual, decadal or longer) in trend estimates causing inconsistent conclusions. Comparison of thermosteric sea level trend with sea level observed by selected tide gauges reaches different conclusions depending on the choice and number of tide gauges (1955–1996) used: thermosteric sea level trend (0–3000 m) accounts for over 88% of the observed sea level from 27 tide gauges, while it accounts for only 38% if 515 tide gauges are used. A study using temporal gravity field measurements observed by the Gravity Recovery and Climate Experiment (GRACE) in terms of oceanic mass variations at long-wavelength (>800 km) and monthly sampling demonstrates its potential use, when combined with satellite altimetry, to improve steric sea level trend estimates over the Southern Ocean, where the OSD is extremely sparse or non-existent. The global (covering within ±81°.5 latitude) sea level trend (corrected for IB) observed by multiple satellite altimetry (GEOSAT, ERS-1, T/P, ERS-2, GFO, JASON and ENVISAT, data covering 1985–2002 except for 1988–1991), and by ~530 tide gauges are 2.8±0.5 mm/yr (corrected for GIA geoid effect) and 2.7±0.4 mm/yr, respectively, indicating excellent agreement. The observed sea level trend using multiple satellite altimeters covering 1985–2004 is 2.9±0.5 mm/yr, although the data span is still too short to yield reliable trend estimates. The multiple altimeter data sets allow coverage in the polar oceans, as opposed to only using data from T/P (coverage within ±66°), and have a longer data span (~19 years versus 10 year for T/P). The data have been calibrated against each other and with tide gauge data for robust determination of their relative biases with respect to T/P. The 20th Century (1900–2002) global sea level trend determined from 651 tide gauge stations is 1.6±0.4 mm/yr, while the trend estimate for the last 50 years (1948–2002) is similar, but used 620 tide gauges. The atmospheric effect via the barotropic IB correction (globally averaged at ~0.11 mm/yr and over 50 years at global tide gauge locations) to sea level (tide gauges or altimetry) observations is warranted as it reduces variances of the data and causes improved agreements between altimetry and tide gauge sea level data. The multiple altimeter data allows sea level studies in the Arctic and Sub-Arctic Oceans, which have been less studied. The sea level trend in the Sub-Arctic Ocean, bounded by latitude 55°N–82°N and longitude 315°E–60°E, is estimated at 1.8 mm/yr using primarily ERS-1 and ERS-2 altimetry, which have been calibrated by TOPEX/POSEIDON in the lower latitude (±66° latitude) oceans and remove constant offsets and a latitude-dependent bias. The sea level trend can mostly be explained by atmospheric forcing and thermosteric effects. In the Arctic Ocean, sea level trend during 1948–2002 is estimated at 1.9–2.0 mm/yr using tide gauges after correction of land motion using different GIA models. After applying the inverted barometric (IB) correction, the sea level rate reduces to 1.5–1.6 mm/yr, indicating that the barotropic response of the ocean contributes significantly more to the sea level trend in the Arctic Ocean than for the global ocean. In an attempt to estimate the global sea level trend and to quantify some of the known contributions in the 20th Century using data from sparsely distributed long-term tide gauges and ~20 years of satellite altimetry, we assume that the spatial patterns of sea level trends from melt water sources (Antarctica, Greenland and mountain glaciers), the thermosteric effects, and geoid change and land uplift due to GIA are known, while their magnitudes are unknown. We ignored contributions from fresh water imbalances due to continental hydrological processes [Milly et al., 2003; Ngo-Duc et al., 2005], human-impoundment of water in reservoirs or lakes [Chao et al., 1994; Sahagian et al., 1994] and other effects, including permafrost melt [Zhang et al., 2005]. Using two different estimators, the Weighted Least Squares (WLS) and the Elementwise- Weighted Total Least Squares (EW-TLS), and using long-term tide gauges and satellite altimetry, we estimate the respective contributions of each of the sources to the global sea level rise. The EW-TLS technique is found to be more stable while the WLS technique produces solutions with larger error of ~0.3 mm/yr in a simulated study. The EW-TLS solution yields the estimated 20th Century (1900–2002) sea level trend contributions by melt water from the Antarctic and Greenland ice sheets and mountain glaciers to be 0.80±0.14 mm/yr, 0.56±0.12 mm/yr and 0.31±0.08 mm/yr, respectively; the estimated contribution from the thermosteric effect is 0.06±0.04 mm/yr, and the estimated scale of the GIA (ICE-4G) model to correct tide gauge sea level data is 1.27±0.08, indicating that the correction should be higher by 27%. The resulting 20th Century (1900–2002) globally averaged sea level trend is estimated to be 1.73±0.42 mm/yr (95% confidence or 2s) after summing the above forcing factors. The estimate of the resulting last 50 year (1948–2002) global sea level trend is 1.74±0.48 mm/yr (95% confidence or 2s). Finally, we address the issue of whether the sea level trend acceleration is detectable. An analysis indicates that the minimum data span to obtain a stable rate of sea level trend from 27 selected tide gauges [Douglas, 2001] is 20 years or more, while one should use a 30-year or longer data span to derive a stable thermosteric sea level trend from WOA01. It is concluded that, with 95% confidence, there is no statistically significant evidence of sea level acceleration during 1900–2000 from tide gauge data, and during 1950-2000 from thermosteric data. The estimated 20th Century global sea level rise is compared with the more recently estimated total geophysical effects contributing to global sea level rise of 1.41–1.53 mm/yr, which include the sum of the steric effect (~0.4 mm/yr) [e.g., Levitus et al., 2005; Antonov et al., 2005], mountain glacier melting (~0.51 mm/yr) [e.g., Arendt et al., 2002; Dyurgerov and Meier, 2005; Raper and Braithwaite, 2005; and others], ice sheet mass imbalance (~0.45 mm/yr) [e.g., Krabill et al., 2004, Rignot et al., 2005, Thomas et al., 2005], hydrological imbalance (~0.0–0.12 mm/yr) [Milly et al., 2003, Ngo-Duc et al., 2005], and anthropogenic effect (~0.05 mm/yr) [Dork Sahagian, pers. comm.]. This study (1.74±0.48 mm/yr) reduces the existing discrepancy [Chruch et al., 2001] to 0.21–0.33 mm/yr between the total observed and predicted contributions to 20th Century global sea level rise. However, the estimated individual contributions of the ice sheet imbalance and the oceanic steric effect remain significantly different from the current results from observations, and not much progress has been made since the last IPCC study on the effect of hydrologic imbalance and anthropogenic causes. Much future work remains to improve our understanding of the complex processes governing global sea level changes

Satellite altimeter remote sensing of ice caps

This thesis investigates the use of satellite altimetry techniques for measuring surface elevation changes of ice caps. Two satellite altimeters, Radar Altimeter 2 (RA-2) and Geoscience Laser Altimeter System (GLAS) are used to assess the surface elevation changes of three Arctic ice caps. This is the first time the RA-2 has been used to assess the elevation changes of ice caps - targets much smaller than the ice sheets which are the instrument’s primary land ice targets. Algorithms for the retrieval of elevation change rates over ice caps using data acquired by RA-2 and GLAS are presented. These algorithms form a part of a European Space Agency (ESA) glacier monitoring system GlobGlacier. A comparison of GLAS elevation data to those acquired by the RA-2 shows agreement between the two instruments. Surface elevation change rate estimates based on RA-2 are given for three ice caps: Devon Ice Cap in Arctic Canada (−0.09 ± 0.29 m/a), Flade Isblink in Greenland (0.03 ± 0.03 m/a) and Austfonna on Svalbard (0.33 ± 0.08 m/a). Based on RA-2 and GLAS measurements it is shown that the areas of Flade Isblink below the late summer snow line have been thinning whereas the areas above the late summer snow line have been thickening. Also GLAS observed dynamic thickening rates of more than 3 m/a are presented. On Flade Isblink and Austfonna RA-2 measurements are compared to surface mass balance (SMB) estimates from a regional atmospheric climate model RACMO2. The comparison shows that SMB is the driver of interannual surface elevation changes at Austfonna. In contrast the comparison reveals areas on Flade Isblink where ice dynamics have an important effect on the surface elevation. Furthermore, RACMO2 estimates of surface mass budget at Austfonna before the satellite altimeter era are presented. This thesis shows that both traditional radar and laser satellite altimetry can be used to quantify the response of ice caps to the changing climate. Direct altimeter measurements of surface elevation and, in consequence volume change of ice caps, can be used to improve their mass budget estimates

Variations de hauteur de la calotte antarctique par altimétrie radar par satellite : amincissement dynamique, vidanges de lacs sous-glaciaires et autres curiosités

La calotte polaire Antarctique est une région immense et peu accessible, mais partie intégrante du système climatique planétaire. Pour mieux comprendre son fonctionnement et prévoir ses réactions face à un climat qui évolue, les mesures satellites sont des outils précieux. Nous exploitons l'un de ces capteurs satellites : l'altimètre radar d'Envisat. Cet instrument permet de mesurer la hauteur de la surface de la calotte et, par des mesures répétées dans le temps, son évolution temporelle. Nous explorons les changements de volume de la calotte sur une période entre 2002 et 2010. Cette période est très courte en regard de certains phénomènes agissant sur la calotte mais permet néanmoins de détecter d'importants changements, dus à des excès de précipitations ou à une accélération de l'écoulement de la glace. Par ailleurs, la densité spatiale et temporelle de l'échantillonnage d'Envisat permet d'observer des événements rapides (quelques mois) et localisés (quelques kilomètres) tels que des vidanges de lacs sous-glaciaires. Ces phénomènes sont encore mal connus et l'altimétrie est un des principaux outils aptes à les observer. La manière dont l'onde radar est réfléchie et rétrodiffusée par la surface de la calotte est un problème complexe, principalement parce que le manteau neigeux est lui-même changeant et complexe. Nous évoquons l'état de l'art de la compréhension des phénomènes impliqués. Nous terminons ces travaux par une ouverture sur les techniques qui permettront d'avancer dans la compréhension des calottes polaires : nouveaux altimètres, séries de données plus longues, fusion de jeux de données provenant de capteurs différents et complémentarité avec les données in situ.The Antarctic Ice Sheet is a vast and remote hostile land. It is nonetheless an important part of the planetary climate system. Space-borne instruments are among the best tools to study the evolution of the ice sheet. In this work, we use data from one of these space sensors: the Envisat radar altimeter. This instrument provided us repeated measurements of the ice sheet surface elevation every 35 days during 8 years. From this dataset, we investigated volume change of the ice sheet between 2002 and 2010. This period is relatively short compared to the typical duration of ice sheet response (thousands of years after an ice age) but the data show some evolution, either extreme precipitation events or accelerated flow and associated thinning. The high space and time resolution also allowed us to observe rapid and local events such as subglacial lake drainages. These were only recently discovered in Antarctica and altimetry is one of the best suited tools to study them. The reflection and backscatter of the radar wave by the snowpack is still a complex problem that has to be further investigated. The own behavior of the snowpack must be better understood. We present the state of the art of the understanding of the radar/snowpack interaction. We conclude with an outlook on future techniques that will enhance our understanding of the ice sheet process and ice sheet evolution: new altimeters, longer time series, multi-sensor studies and additional in situ calibration