Century-scale continent-to-ocean ice mass transport and measurement of lithospheric thickness and mantle viscosity using GPS Erik R. Ivins (JPL/Caltech) Focus: Little Ice Age & Modern “Anthropogene” Glacier Demise Climate and Tectonics Slab-window environs of the Eastern Pacific Rim Patagonian and Antarctic Peninsula Glacier Change

Little Ice Age and Constraints on Upper Mantle Viscosity of Slab Windows Who cares about slab window mantle viscosity? What do we know about the Little Ice Age above slab windows? UNAVCO - 30th Anniversary2 How ubiquitous are slab windows? Are there independent estimates of the viscosity? We use GPS uplift data – robust (non-elastic) correction for GRACE mass balance – to derive new estimates for two active slab windows.

Central America: example of ongoing slab window tectonics From Johnston and Thorkelson (EPSL 146, , 1997).

The physics of determining the mantle viscosity o Little Ice Age spatial scales: 100’s km o Little Ice Age time scales: 100’s years GIA is measured with GPS geodesy

Southernmost South America UNAVCO - 30th Anniversary5 Breitsprecher and Thorkelson (2009)

Active Slab Windows: How ubiquitous? How Important? UNAVCO - 30th Anniversary Thorkelson et al. Geology 2012

What do geodesists have to say? Model by Pollitz and Nyst (2005) o Strain-rate and velocity using PBO GPS data o  x   Pa sec Zandt and Carrigan (1993) (seismic tomographic flow model) o  x   Pa sec Studies Pertinent to Window south of Mendicino Triple Junction UNAVCO - 30th Anniversary7

What do the climate scientists have to say? Importance of trend in the Atlantic Multi-decadal Oscillation UNAVCO - 30th Anniversary8 Two competing hypotheses 1.Rapid rates of land ice loss begin only after ~ Loss initiated at times coincident with LIA terminating conditions – at substantial mass rates during

Little Ice Age to modern-day Patagonia How much mass at LIA max? mass loss rate via GRACE (26 ± 8 Gt/yr) History of the rate of land- ocean mass transport. Do we know it? UNAVCO - 30th Anniversary9

Little Ice Age to modern-day Antarctic Peninsula Observations: i.Outlet glacier height changes since 1948 ii.9 Ice shelf catastrophic break- ups since 1989 iii.Surface atmospheric temperature rising since 1900 iv.GRACE & ICESat ice loss rate -37 ± 10 Gt/yr Question: When does land ice mass loss begin? UNAVCO - 30th Anniversary10

What do geodesists have to say? Observe GPS uplift Southern Patagonian network: Heiner Lange PhD thesis Antarctic Peninsula: IGS stations – Reinhard Dietrich (TU Dresden) network has legacy to early 1990s – NSF & BAS effort (LARISA) UNAVCO - 30th Anniversary11 SPI 31 stations – max uplift rate at 39 ± 2.5 mm/yr AP 11 stations – max uplift rate at 14 ± 3 mm/yr

Patagonian data and interpretation UNAVCO - 30th Anniversary12 Input for elastic model from Willis, Pritchard et al., GRL 2012

Patagonia Ice History (loss rates in Gt/yr) UNAVCO - 30th Anniversary13 Rate of loss in Gt/yr

   fit Model A versus Model B UNAVCO - 30th Anniversary14  8 x Pa sec  1.6 x Pa sec 

Antarctic Peninsula UNAVCO - 30th Anniversary15

Ice loss history UNAVCO - 30th Anniversary16

Why is solving for GIA important to climate change studies? UNAVCO - 30th Anniversary17

Antarctic Peninsula GRACE corrected mass loss UNAVCO - 30th Anniversary18 Conclusions: GPS station uplift data Determine 2 of 5 slab-window upper mantle viscosity values: 0.6 – 3 x Pa sec --- consistent with Pollitz and Nyst (2005). Sensitivity to both LIA ice history & to space era loss is demonstrated – But independent constraints are unlikely Data + model derive a Gt/yr level GIA correction for GRACE mass balance. Conclusions: GPS station uplift data Determine 2 of 5 slab-window upper mantle viscosity values: 0.6 – 3 x Pa sec --- consistent with Pollitz and Nyst (2005). Sensitivity to both LIA ice history & to space era loss is demonstrated – But independent constraints are unlikely Data + model derive a Gt/yr level GIA correction for GRACE mass balance.

What do climate scientists have to say? Antarctic Peninsula & Southern Patagonia o Sea Surface & Atmospheric surface temperatures rise (SST & SAT) – trend since AD 1900 o Sea Ice Concentration (SIC) around Antarctica during o Atlantic Multi-decadal Oscillation (AMO) serves as a dominant teleconnection mechanism for LIA – to ‘modern’ glacier demise in Antarctic Peninsula & S. Patagonia (Li, Holland, Gerber & Yoo, Science 2014) (CAM4 & GFDL) Two hypotheses: o Global warming and ice mass loss are post-1960’s phenomenon – e.g. Greenland, etc., are ‘stable’ until about late 1980’s o Mass transport (ice-to-ocean) is better characterized by the tide-gauge record – sustained ice losses traced back to the late 1880’s UNAVCO - 30th Anniversary19

Ice Losses and Slab Windows – Overview of this talk Slab windows What parts of the climate are responsible for Little Ice Age demise? Patagonian GPS Network Antarctic Peninsula GPS Networks GIA model Results – GRACE JPL Mascon Solutions Speculations – Cryosphere Speculations – Mantle - Lithosphere UNAVCO - 30th Anniversary20