observed solid earth contributions to sea level variation...

2013-09-23 GeoSci Seminar @ UNR 1

Observed Solid Earth Contributions toSea Level Variation in Western US

Geoff Blewitt

Nevada Bureau of Mines & GeologyUniversity of Nevada, Reno

[email protected]

with contributions from: Bill Hammond (NBMG)Steve Nerem (U. Colorado)


A Non-Grant-Funded Study: Why?

Requested by the National Research Council

− Service to the nation

− Broader question of planning for sea-level rise in western US

• initiative of CA Governor, Arnold Schwartzenegger (2008)

• NRC Committee set up, and publishes its findings in 2012

• One finding is that more hard data are needed for firm conclusions

− Question posed to me (2013): What is contribution of vertical land motion to sea level rise in CA?

Will impact decision making of:

− Army Corps of Engineers

• planning for coastal protection

− California Dept of Water Resources

• Levies of Sacramento-S. Joachim Delta are longer than CA coastline!


Sea Level Context: Last 20,000 Years

Melting Ice of the last Ice Age

− Starting ~22,000 years ago

− Ending ~7,000 years ago

− Caused 125 meters of sea level rise

− ~8 mm/yr (Milne et al., 2005)

− with some rapid pulses

Stable since last 7,000 years

− on 1,000 year time scale:< 1 mm/yr

− falling with time

− recently << 1 mm/yr

Sources: Milne et al., 2005; Flemming et al., 1998Figure constructed by R.R. Rohde


Sea Level Context: Last 2,000 years

Sea level rose 0.2 m over last century: ~ 2 mm/yr


Global Warming Context: Last 2,000 years

Temperature rose 0.6 C over last century (National Academies, 2006)

Composite of 10 reconstructions by various authors and proxies:-- corals, stalagmites, sediments, tree rings, boreholes, ice cores, glacier retreat, ...


“Hockey Stick” Curves


So What is Happening to Sea Level Now?

Is sea level continuing to rise?

− tide gauges (right) indicate ~2 mm/yr

− too noisy to assess acceleration

“Jason”: satellite altimetry

− precise way to sample all the ocean

− first we take a look at how this works

− then we look at the data

− 3 NASA missions over last 20 years:

• Topex (1992)

• Jason 1 (2001)

• Jason 2 (2008) ...

(Tide gauge data from NOAA)


Jason’s name derives from Greek Mythologyand has nothing to do with hockey sticks

Jason’s never ending sequels

− Jason 3 launches in 2015


Jason and Sea Level

How Jason works

− Radar measures range to sea surface below satellite

− GPS is used to position the radar altimeter satellite

• relative to Earth’s center of mass

• NASA’s GIPSY software

− Radar bias is calibrated

• compare with GPS buoys

− Hence we can measure geocentric sea level (GSL) relative to Earth’s center


Sea Level Rise: Last 20 years

Global mean sea level is now rising 3.2 mm/yr

− Over 1 mm/yr faster than what tide gauges tell us over last century

− Evidence for recent “acceleration” in sea level


Sea Level Rise and Climate Change

Why would global mean sea level be rising?

− primary cause can only be global warming

− density: thermal expansion and salinity of the ocean

− mass: ice sheets on land (e.g., Greenland) melt into the ocean

• “Melting land ice is now the largest contributor to global sea-level rise”

Why would global mean sea level be accelerating?

− Independent evidence of non-linear response to global warming

• ice sheets self-lubricate as they melt and advance to the ocean

• rising sea level lifts ice sheet at the coast, reducing basal friction

− Unlike thermal expansion, the mass effect could be catastrophic

• Mountain Glaciers (0.5 meters)

• Greenland (7 meters)

• Antarctica (70 meters)

Source: USGS (2000)


Sea Level Rise and Coastal Impact

Coastal Impact

− Map shows loss of coastline as sea level rises

− Problem is compounded, e.g., Louisana, by subsidence of land as sea level rises

− Need to consider vertical land motion (VLM)

− Impact of storm surges amplifies as sea level rises

• Hurricane Sandy...

− Economic impact accelerates if no action taken


Coastal Impact: Local Sea Level (LSL)

Geocentric Sea Level (GSL)

− measured by Jasonrelative to Earth center

− in direction normal to ellipsoid GRS80 (fits mean sea level)

Local Sea Level (LSL)

− measured by tide gaugesrelative to the land

− therefore LSL is relevant to coastal & societal impacts

Tide gauge moves with the landLSL ≠ GSL Vertical Land Motion

VLM


Coastal Impact: Vertical Land Motion (VLM)

Vertical Land Motion (VLM)

− Coastal subsidence (-VLM)maps 1:1 into LSL

− VLM must be measuredin same reference frameas Jason

− International Terrestrial Reference Frame, ITRF

• Global Geodetic Observing System(GPS, VLBI, SLR,...)

• Earth center of mass to ±0.5 mm/yr

Source: P. Whitehousehttp://www.antarcticglaciers.org/recovering-from-an-ice-age

Local Sea Level Rise = Geocentric Sea Level Rise + Coastal Subsidence

LSL = GSL − VLM


Q: Contribution of VLM to LSL at US West Coast?

Investigative Approach

− Consider possible contributing physical effects on

• Sea Level

• Vertical Land Motion (VLM)

− GPS data over last decade to characterize VLM as vertical rates

• same method as for Jason: NASA’s GIPSY software in ITRF

− Attempt to understand spatial pattern of VLM along the coast


Effects on LSL: Glacial Isostatic Rebound (GIA)

(1) GIA affects VLM hence LSL

− during Ice Age

• ice sheets squeezed mantle outward to form a “forebulge”

• forebulge peaks in region of US-Canada border

• US west coast tilts up

− after Ice Age (now!)

• mantle flows back

• forebulge collapses

• US west coast tilts down

− more down to the North

WestCoast

←N

James, T.S., et al. (2000). Quat. Sci. Rev. 19, 1527-1541

WestCoast

←N



For West Coast, GIA contributes to LSL rising ~ +0.5 mm/yr

− LSL increasing to the north

− Coast tilting downward to the north

Tamisiea, M.E., and J.X. Mitrovica (2011). Oceanography 24(2), 24–39

CA

N

LSL = GSL-VLM



(2) “Continental levering”

− after ice-age, sea level rises about 125 meters

− mantle squeezes from under ocean to under continents

• offshore crust subsides

• onshore crust uplifts

• tilts normal to coastline

• minimal VLM at coastline

− global mean sea level drops

• -0.3 mm/yr mean GSL

− gravity field changes shape

• ±1 mm/yr in GSL

Near former ice sheet

Far from former ice sheet


Effects on GSL and VLM: GIA

For West Coast, GIA predicts GSL lowering: GSL ~ -0.5 mm/yr

− VLM = (GSL - LSL) = (-0.5 - 0.5) = -1.0 mm/yr

− Predicted LSL rise results from coast sinking faster than sea surface

Tamisiea, M.E., and J.X. Mitrovica (2011). Oceanography 24(2), 24–39

GSL


Effects on LSL and VLM: Current Land Ice Melt

Land ice melting causes global mass redistribution

− ccean does not fill up like a bathtub

− changes Earth’s shape, rotation, and gravity field

− causes local departure from “bathtub” rise at US West Coast:

• -10% (Greenland ice), and +30% (Antarctic ice)

− at US West Coast, negligible VLM

• Can approximate GSL = LSL

Land ice melting causes “thermosteric” density changes

− ocean temperature and salinity control density

− ice is cold, fresh water

− affects ocean topography and currents

− affects regional GSL (hence LSL) rates

− negligible effect on VLM


Effects on LSL and VLM: Regional Tectonics

Regional Tectonics

− megathrust fault north of Cape Mendicino can give upward VLM

− lower mantle viscosity significantly reduces current GIA VLM

− mainly strike-slip faulting in CA can give small local VLM

• San Andreas-Hayward Fault system near coastline in Bay Area

− small local post-seismic VLM from large earthquakes (1 mm/yr?)


GPS Measurement of VLM:>11,000 stations monitored by UNRhttp://geodesy.unr.edu


GPS Measurement of VLM:US stations monitored by UNRhttp://geodesy.unr.edu


GPS Measurement of VLM:Western US stations monitored by UNRhttp://geodesy.unr.edu


GPS Measurement of Horizontal Land Motion:Western US stations monitored by UNRhttp://geodesy.unr.edu

Kreemer et al., (2012). NBMG Map 178

Horizontal Strain from GPS


GPS Measurement of VLM:Western US stations monitored by UNRhttp://geodesy.unr.edu

“Contemporary uplift of the Sierra Nevada, western United States, from GPS and InSAR measurements”

Hammond et al. (2012) Geology, v. 40; no. 7; p. 667–670


GPS Measurements of VLM: 2 MethodsSierra Nevada Uplift ~1 mm/yr

Two Methods Demonstrated in a Western US Transect(1) red triangles: vertical (GPS point positions)

(2) gray cloud: vertical (InSAR line-of-sight − GPS horizontal strain)

For this study we have only used method (1)

Hammond, W., et al. (2010), Geology vol. 40


GPS Measurement of VLM:West Coast US stations monitored by UNRhttp://geodesy.unr.edu

Select GPS stations

− within 15 km of coast

− at least 4.5 years of datato get an accurate rate

Procedure for each station

− generate time series of daily vertical positions

− reject time series that are poorly fit by a constant vertical rate

− removes stations subject to very local motion that do not represent the crust

− final selection of 89 stations


Results: Vertical Land Motion from GPS Alone<15 km of coast

MTJ →

Mendocino Triple Junctionintersects coast at 40.3 N

From NRC Report on Sea Level (2012), Figure 4.14

CA/OR →

OR/WA →

VLM (mm/yr)


Coastal VLM from GPS:Oregon and Washington

Why does subsidence peak at the Mendicino Triple Junction (MTJ)?

− GIA predicts northward increase in subsidence

− tectonic boundary at MTJ

• strike-slip to the south with little tectonic VLM

• thrust to the north with tectonic uplift predicted

− GIA reduced

• lower mantle viscocity to the north (James et al.)

• GIA may have already stopped

MTJ →

Mendocino Triple Junctionintersects coast at 40.3 N

From NRC Report on Sea Level (2012), Figure 4.14

CA/OR →

OR/WA →

VLM (mm/yr)


Coastal VLM from GPS:San Francisco Bay Area

General Pattern:

− northern California has largest coastal subsidence western US: ~ 1 mm/yr

− subsidence decreases going inland

Physical Processes:

− GIA predicts northward increase in subsidence

− continental levering predicts decreasing subsidence going inland

− San Andreas-Hayward faults

− differential loading of delta


Conclusions

Science

− Sea level impacts relate to:

• global sea level rise

• regional sea level variations

• vertical land motion: VLM

− Methodology demonstrates:

• Regional VLM can be accurately quantified

• Regional VLM can be qualitatively understood in terms of physical processes

− VLM of the same magnitude as current sea level rise

Outlook

− Global network of GPS can be used to extend this method around the globe in a seamless way

− GPS-InSAR looks promising

− Complimentary to Jason studies of sea level rise

− Need to maintain International Terrestrial Reference Frame

• connect Jason missions

• connect to VLM studies

• connect to tide gauges

observed solid earth contributions to sea level variation...

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