Spring 2017

Spring 2017

Tuesday Afternoons at 3:30 PM
Natural Science Annex 101

April 4, 2017

Speaker: Alyssa Atwood, UC Berkeley

Title: The tropical Pacific response to abrupt climate change: A case study of the 8,200 BP event

Abstract: The ability of the Earth’s climate system to undergo dramatic and rapid reorganization on decadal timescales is exemplified by the abrupt climate transitions that occurred over the last glacial cycle. Such millennial variability, associated with variations in the strength of the Atlantic Meridional Overturning Circulation (AMOC), stretch our understanding of the dynamical principles that govern the climate system, given the lack of such events in the modern record and the inability of climate models to reproduce such variability. The most recent abrupt climate event occurred ~ 8,200 yr BP when an inferred weakening of the AMOC drove large and abrupt climate changes across the globe, including 2-6 °C cooling in Greenland over less than 20 years. While the tropical Pacific plays a fundamental role in global climate variability on interannual to orbital timescales, very few proxy records resolve tropical Pacific climate across the 8,200 BP event with sufficient resolution. In this talk I will present new evidence based on paired biomarker records from a freshwater lake in the Galápagos Islands suggesting that during the 8,200 BP event: (1) the Intertropical Convergence Zone (ITCZ) shifted south in the eastern equatorial Pacific and (2) the frequency and/or intensity of Eastern Pacific El Niño events decreased. While abundant climate theory and modeling studies support a southward shift of the ITCZ in response to a weakened AMOC, the dynamical drivers for the observed change in ENSO variability are less well developed. To explore these linkages, a set of simulations with the CESM coupled General Circulation Model with a prescribed freshwater forcing was performed in tandem with an intermediate complexity model of the tropical Pacific. The results provide evidence for a mechanistic link between abrupt North Atlantic cooling, a southward shift of the Pacific ITCZ, and weakened ENSO variability in the simulations that is consistent with the proxy reconstructions. These results demonstrate the utility of coupling paleoclimate data with dynamical theory and model simulations to more deeply probe the mechanisms of global climate change.

Hosts: Stephanie Bosch

April 11, 2017

Speaker: Maureen Walton, USGS

Title: Assessing marine geohazards along transform margins of western North America

Hosts: William Rush

April 18, 2017

Speaker: Matteo Spagnolo, UC Berkeley

Title: The patterned topography of ice stream beds

Hosts: Andrea Rhode

April 25, 2017

Speaker: Kimberly Blisnuik, San Jose State University

Title: New geomorphic and sedimentologic insights on fault activity, slip rates, and seismic hazard along the San Andreas fault system in the San Gorgonio Pass and the Santa Cruz Mountains: What we know and what we think we know

Hosts: Gavin Piccione

May 2, 2017

Speaker: Tali Babila, UCSC

Title: Capturing the global signature of ocean acidification during the PETM

Hosts: Megan Kelley

May 9, 2017

Speaker: Laurel Larson, UC Berkeley

Title: Unraveling complexity from two directions: Hydroecological systems seen through the lenses of modeling and data

Abstract: One consequence of earth systems moving out of a regime of stationarity is that statistical models based on past behavior may no longer be useful for predicting the future. Rather, an understanding of the mechanisms driving dynamic earth systems is needed. The mechanisms responsible for nonlinear—even surprising—behavior often involve feedbacks between biotic and abiotic processes. Examples of these feedback processes abound in aquatic sciences, where flow, vegetation, sediment, topographic, and biogeochemical processes tend to exhibit strong coupling. Effects of biotic-abiotic feedback processes may be examined using numerical models (a bottom-up approach), but because multiple feedback processes may produce the same outcome, these studies may not be conclusive. Likewise, although correlative field studies may be useful for generating hypotheses about system drivers, they are often not sufficient to resolve causallyimportant processes in complex hydrologic systems with multiple limiting factors or nonlinearities.

An emerging alternative to the bottom-up method is a top-down approach in which causal interactions and their critical spatiotemporal scales are delineated directly from data using emerging frameworks of inference. Challenges of not knowing the functional form of the relationship between drivers or which of several potential drivers is limiting a response at any time are dealt with by performing analyses in the framework of uncertainty reduction rather than prediction. If a variable (such as discharge) uniquely and independently reduces the uncertainty of another variable (such as ecosystem respiration) over particular time lags, we conclude that it exerts a causal influence. In this manner, and using a stream gaging station data record, we show that fine sediment deposition by intermediate-size storms boosts stream metabolism over timescales of 100+ days in an urban stream that is limited by particulate carbon. Conversely, large storms depress stream respiration over timescales of one month in streams with close coupling between gross primary productivity and respiration but over 1-2 day timescales in streams where these processes remain uncoupled. Application of these methods over larger spatial scales can reveal critical spatial scales and pathways of rainfall recycling in transitional forests of Brazil, or the most prominent drivers of global temperature changes during historic transitions from glaciated to unglaciated states. These case studies highlight the potential for application of these emerging causal inference techniques to the vast library of hydroecological data already collected, provide insight into optimal strategies for data collection in new sensor networks, and showcase how data-driven research may improve mechanistic model generation.

Hosts: Graham Edwards

May 16, 2017

Speaker: Luca Malatesta, UCSC

Title: Interpretation of terraces and preservation of environmental signals across alluvial piedmonts

Hosts: Basia Marcks

May 23, 2017

Speaker: Andy Ridgwell, USGS

Title: Carbon isotopes and the evolution of life and the Planet

Hosts: Edward Ballaron

May 30, 2017

Speaker: Chris Edwards, UCSC

Title: Ocean Modeling Studies of the California Current System

Hosts: Rachel Maxwell

June 6, 2017

Speaker: Jon Perkins, USGS

Title: Feedbacks between glacial stratigraphy, landslide hazards, and landscape evolution in NW Washington State

Hosts: Heather Shaddox