The Record of Sea Level Change From Satellite Measurements: What Have We Learned?

R. S. Nerem Colorado Center for Astrodynamics Research Cooperative Institute for Research in Environmental Sciences University of Colorado at Boulder The Record of Sea Level Change from Satellite Measurements: What Have We Learned? Bowie Lecture Fall AGU 2005

Many Thanks! “ Unselfish Cooperation in Research”

Global Surface Temperature Anomalies GISS [ Hansen et al ., 2005]

Global Climate Model Predictions [ Suzuki et al ., 2005]

Earth’s Heat Balance (1955-1998) Estimates of Earth's heat balance components (10 22 J) for the 1955 - 1 998 period. [ Levitus et al ., 2005]

Effects of Sea Level Rise 1 meter 2 meters 4 meters 8 meters GFDL

The Bathtub Sea Level Model + -

Causes of Sea Level Change Inflow Bathtub Outflow

Causes of Sea Level Change Thermal Expansion (~ 1 meter potential) Water Exchange with Continents (potential) Greenland Ice (7 meters) Antarctic Ice (60 meters) Mountain Glaciers (0.5 meter) Terrestrial Water Storage Variations (< 0.5 meter) Other (halosteric, etc.)

Satellite Measurements of Sea Level Change G RAVITY R EBOUND

Glacial Isostatic Adjustment [ James et al ., 2005]

Glacial Isostatic Adjustment -7.0 -2.0 -1.5 -1.0 -0.5 0 0.5 1.0 1.5 2.0 25.0 Present-Day Radial Deformation (mm/year) [ Milne , 2005]

Sea Level Change Budget Total Sea Level Last Decade (mm/year) 20th Century (mm/year)

Past Sea Level Changes Alley et al. [2005] Avg 10 mm/yr Peak 50 mm/yr

PSMSL Sea Level Records > 10 years

PSMSL Sea Level Records > 50 years

Tide Gauge Sea Level Measurements Rate ~ 1.8 mm/year

Tide Gauge Sea Level Measurements The rate of sea level change over the 20th century is generally taken as 1.8 mm/year [e.g. Douglas , 1991]. Suggestions of a tide gauge geographic sampling bias [ Cabanes et al ., 2001] appear to have been resolved [ Miller and Douglas , 2004], though spatial sampling is still a concern.

Tide Gauges versus Satellite Altimetry

TOPEX/Poseidon and Jason 10-day Groundtrack

TOPEX/Poseidon: In the beginning…… Nerem, R. S., “Global Mean Sea Level Variations from TOPEX/POSEIDON Altimeter Data,” Science , Vol. 268, pp. 708-710, May 5, 1995. Nerem, R. S., Global Mean Sea Level Change: Correction, Science , Letters to the Editor, Vol. 275, p. 1053, February 21, 1997.

Tide Gauges Available for Altimeter Calibration

TOPEX and Jason Tide Gauge Calibrations TOPEX Jason

Platform Harvest Calibration Site GPS ANTENNA (JPL) WVR (JPL) BUBBLER (NOAA) ACOUSTIC (NOAA) EQUIP SHED MET SENSORS Point Arguello, CA + 54 m + 27 m + 6 m LASER SENSOR (CU)

Global Mean Sea Level Variations

Global Mean Sea Level Variations Seasonal variations removed

Global Mean Sea Level Variations Seasonal variations removed GIA correction applied (0.3 mm/year) Rate = 3.2 ± 0.4 mm/year http://sealevel.colorado.edu

Sea Level Change Budget Thermal Expansion 3.2 1.8 Last Decade (mm/year) 20th Century (mm/year)

Thermosteric Sea Level Change [ Levitus et al ., 2005; Antonov et al ., 2005] Rate = 0.4 mm/year (1955-2004) Rate = 1.2 - 1.6 mm/year (1993-2004)

Decadal Variability Due to Volcanic Forcing? [ Church et al ., 2005]

Total versus Thermosteric Sea Level Change [ Willis et al ., 2005]

Global Sea Level Trends: 1993-2005

Global Sea Level Trends Altimetry Climate Model (MIROC)

Sea Level Variations versus Latitude

Sea Level Change versus Latitude

Sea Level Change Budget Greenland and Antarctica Ice Melt 3.2 1.8 1.2 0.4 Last Decade (mm/year) 20th Century (mm/year) Mountain Glaciers and Water Storage Water mass redistribution changes the Earth’s gravity field

GRACE Gravity Recovery and Climate Experiment

GRACE Surface Water Mass Variations Swenson, Leuliette, Nerem, 2005

Ocean/Continents Mass Change from GRACE

GRACE Continental Water Mass Estimates

GRACE Ocean Water Mass Estimates

GRACE Surface Water - South America [ Lemoine et al ., 2005]

Contribution of Amazon to Sea Level Change

GRACE Geoid Change Trend: 2002-2005

Sea Level Change Budget Greenland and Antarctica Ice Melt 3.2 1.8 1.2 0.4 Last Decade (mm/year) 20th Century (mm/year)

Polar Ice and Sea Level Change Polar ice gravitationally attracts the ocean water Melting of ice causes regional fall of sea level in the vicinity of the ice sheet Patterns fit to tide gauge observations: [ Mitrovica et al ., 2001] 1.20 1.10 1.00 0.90 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.0 Antarctica Greenland

The Sea Level “Enigma” Observed sea level rise over the 20th century is ~1.8 mm/year. Approximately 0.4 mm/year is attributed to thermal expansion, leaving ~1.4 mm/year to be attributed to other sources (most likely melting ice) The Earth’s rotation is very sensitive to melting large ice sheets and glaciers, but also to Glacial Isostatic Adjustment. Observations of changes in the Earth’s rotation appear to be well described by models of Glacial Isostatic Adjustment (GIA), and thus little room is left for a contribution from melting ice. Munk, W., Twentieth century sea level: An engima, Proc. Nat. Acad. Sci ., 99, 6550-6555, 2002.

Sea Level “Enigma” Ancient Eclipse Data Drift of Rotation Pole Oblateness Variations Satellite Laser Ranging

Earth’s Rotational Stability [ Mitrovica et al ., 2005]

Sea Level Enigma: Recent Developments Improved theory for Earth’s rotational stability significantly changes the GIA modeling predictions. Revised estimates of the Earth’s oblateness variations are now available, which include uncertainties in the knowledge of the 18.6 year tide. Better estimates of the rotation pole drift are now available. These developments now require a 20th century contribution from melting ice of ~1 mm/year in order to explain the Earth rotation observations. [ Mitrovica et al ., 2005]

Salinity Changes - Ocean Freshening [ Munk , 2003]

Greenland Ice Contributions [ Johannessen et al ., 2005] [ Rignot and Kanagaratnam , 2005] InSAR: 0.23-0.55 mm/yr [ Krabill et al. , 2004] Laser Alt: 0.25 mm/yr Radar Alt: Accumulation

Antarctic Ice Mass Flux from InSAR Rapid coastal thinning in Bellinghausen and Amundsen sectors of West Antarctica -177±30 km 3 /yr Interior thickening and peripheral thinning in East Antarctica +9±30 km 3 /yr SLR 0.4 to 0.6 mm/yr -114 km 3 /yr - 37 ±20 km 3 /yr +33 km 3 /yr -38 km 3 /yr +48 km 3 /yr -2 km 3 /yr +5 km 3 /yr -22 km 3 /yr -3 km 3 /yr +5 km 3 /yr -2 km 3 /yr -56 km 3 /yr -2 km 3 /yr +21 km 3 /yr -4 km 3 /yr -33 km 3 /yr -4 km 3 /yr - 49 ±20 km 3 /yr Peninsula: loss in Graham land, gain in Palmer land. [ Rignot , 2005]

Antarctic Ice Changes from Radar Altimetry Interior thickening in East Antarctica - coastal thinning or near balance of coastal East Antarctica (-0.12 mm/yr SLR). Rapid thinning in Amundsen/ Belliingshausen sea sectors of West Antarctica. [ Davis et al ., 2005]

Ice Height from Radar Altimeter Measurements [ Zwally et al ., 2005] Greenland Margins ~ -0.12 mm/year Greenland Interior ~ +0.14 mm/year WAIS ~ -0.12 mm/year EAIS ~ +0.04 mm/year

Greenland Mass Variability from GRACE [ Velicogna and Wahr , 2005] 0.45 ± .06 mm/year

Antarctica Ice Mass Change from GRACE [ Velicogna and Wahr , 2005] 0.44 ± .23 mm/year

Ice Mass Variability from GRACE [ Luthcke et al ., 2005]

Preliminary ICESat Results dh/dt Laser 3a (10/7 - 11/7, 2004) - Laser 2a (9/25 - 11/18, 2003) [ Luthcke et al ., 2005]

Sea Level Change Budget Melting of Mountain Glaciers 3.2 1.8 Last Decade (mm/year) 20th Century (mm/year) 1.2 0.4 0.4 0.2 0.5 0.3

GPS Crustal Uplift in Alaska [ Larsen et al ., 2005]

Alaska Glacier Mass Changes from GRACE [ Tamisiea et al ., 2005] Sea Level Contribution of 0.3 mm/year over 2002-2004

Ice Volume Changes in Patagonia 1975 - 1995 - 2000  MSL = 0.04 mm/year (1975 - 1990s)  MSL = 0.1 mm/year (1995-2000) [ Rignot et al ., 2003]

Mountain Glaciers: Contribution to Sea Level [ Dyurgerov and Meier , 2005] 1961-2003: 0.5 mm/year 1994-2003: 0.9 mm/year

Sea Level Change Budget Land Water Storage 3.2 1.8 Last Decade (mm/year) 20th Century (mm/year) 1.2 0.4 0.4 0.2 0.5 0.3 0.9 0.4

Sea Level Change Budget 3.0 = 1.2 0.4 0.5 0.9 ? ? 1.4 = 0.4 0.2 0.3 0.5 ? ? 20th Century: 1.8 mm/year (observed) Last Decade: 3.2 mm/year (observed) Last Decade (mm/year) 20th Century (mm/year)

What Have We Learned? The “enigma” of 20th century sea level change has been largely resolved and a larger contribution from melting ice is now allowed (~1 mm/year). Satellite altimetry has observed an average increase in sea level of 3.2 mm/year over 1993-2005, which probably reflects an acceleration versus the 20th century rate (~1.8 mm/year). Roughly half the current rise may be attributed to thermal expansion and half to the addition of water to the oceans (likely from melting of mountain glaciers and polar ice). Although there are uncertainties in the sea level contribution from present-day melting of the ice sheets and mountain glaciers, there is a growing body of evidence that the contribution is large (~1.5-2 mm/year) and accelerating.

Satellite Missions Studying Sea Level Change Planned/Pending approval In orbit Approved JASON-2/OSTM TOPEX/POSEIDON JASON-1 Envisat RA-2 Medium accuracy RA (SSH) from high-inclination orbit High accuracy RA (SSH) from mid-inclination orbit CRYOSAT-2/LRM OceanSat-3 AltiKa ERS-2 RA GMES S-3 ICESAT Geosat Follow-On NPOESS GRACE GRACE follow-on Laser Altimetry (ice) Temporal Gravity (ocean mass) ICESAT-2 ? 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14

Future Greenland Ice Changes Alley et al. [2005]

The Record of Sea Level Change From Satellite Measurements: What Have We Learned?

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The Record of Sea Level Change From Satellite Measurements: What Have We Learned?