Julien Emile-Geay

Julien Emile-Geay
University of Southern California | USC · Department of Earth Sciences

PhD
Bringing 21st century methods to the study of past climates

About

110
Publications
39,604
Reads
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3,675
Citations
Introduction
I work on bridging the gap between paleoclimate data and models, with the goal of better constraining the dynamics of climate on human-relevant timescales.
Additional affiliations
January 2009 - April 2016
University of Southern California
Position
  • Professor (Assistant)
December 2006 - December 2008
Georgia Institute of Technology
Position
  • PostDoc Position
September 2001 - October 2006
Columbia University
Position
  • PhD Student
Education
August 2001 - October 2006
Columbia University
Field of study
  • Climate Dynamics

Publications

Publications (110)
Article
Full-text available
It has recently been proposed, within the framework of the linear shallow-water equations, that tropical Pacific decadal variability (PDV) can be accounted for by basin modes with eigenperiods of 10 to 20 yr, amplifying a midlatitude wind forcing with an essentially white spectrum. Here the authors use a different formalism of linear equatorial wav...
Article
Full-text available
Climate field reconstructions (CFRs) enable spatially-resolved estimates of past climates, providing important insights about climate variability over the Common Era. In particular, a reconstructed “La Niña-like” pattern during the transition from the Medieval Climate Anomaly (MCA) to the Little Ice Age (LIA) has been widely tied to medieval drough...
Article
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Paleoclimate observations constitute the only constraint on climate behavior prior to the instrumental era. However, such observations only provide indirect (proxy) constraints on physical variables. Proxy system models aim to improve the interpretation of such observations and better quantify their inherent uncertainties. However, existing models...
Article
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AbstractThis study formulates the design of optimal observing networks for past surface climate conditions as the solution to a data assimilation problem, given a realistic proxy system model (PSM), paleoclimate observational uncertainties, and a network of current and proposed observing sites. We illustrate the method with the design of optimal ne...
Article
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The potential for explosive volcanism to affect the El Niño-Southern Oscillation (ENSO) has been debated since the 1980s. Several observational studies, based largely on tree-ring proxies, have since found support for a positive ENSO phase in the year following large eruptions. In contrast, recent coral data from the heart of the tropical Pacific s...
Article
Full-text available
Studying past climate variability is fundamental to our understanding of current changes. In the era of Big Data, the value of paleoclimate information critically depends on our ability to analyze large volume of data, which itself hinges on standardization. Standardization also ensures that these datasets are more Findable, Accessible, Interoperab...
Article
A decades-long affair Decadal climate variability and change affects nearly every aspect of our world, including weather, agriculture, ecosystems, and the economy. Predicting its expression is thus of critical importance on multiple fronts. Power et al . review what is known about tropical Pacific decadal climate variability and change, the degree...
Article
We use theNorthern Hemisphere Tree-RingNetwork Development (NTREND) tree-ring database to examine the effects of using a small, highly-sensitive proxy network for paleotemperature data assimilation over the last millennium. We first evaluate our methods using pseudo-proxy experiments. These indicate that spatial assimilations using this network are...
Article
Full-text available
Chronological uncertainty is a hallmark of the paleoenvironmental sciences and geosciences. While many tools have been made available to researchers to quantify age uncertainties suitable for various settings and assumptions, disparate tools and output formats often discourage integrative approaches. In addition, associated tasks like propagating a...
Article
Surface temperature is a vital metric of Earth’s climate state, but is incompletely observed in both space and time: over half of monthly values are missing from the widely used HadCRUT4.6 global surface temperature dataset. Here we apply GraphEM, a recently developed imputation method, to construct a spatially complete estimate of HadCRUT4.6 tempe...
Article
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In the past 40 years, the global annual mean surface temperature has experienced a non-uniform warming, differing from the spatially uniform warming simulated by the forced responses of large multi-model ensembles to anthropogenic forcing. Rather, it exhibits significant asymmetry between the Arctic and Antarctic, intermittent and spatially varying...
Preprint
Full-text available
The potential for explosive volcanism to affect the state of the El Niño-Southern Oscillation (ENSO) has been debated since the1980s. Several observational studies, largely based on tree rings, have since found support for a positive ENSO phase in the year following large eruptions. Models of different complexities also simulate such a response, de...
Chapter
This chapter investigates ENSO variability before the instrumental era. Though generally indirect, paleoclimate observations provide information that no other source can, sampling ENSO behavior across different base states, subject to many types and intensities of external forcing, and providing a much longer statistical sample than afforded by the...
Article
Full-text available
The mid-Holocene (6000 years ago) is a standard time period for the evaluation of the simulated response of global climate models using palaeoclimate reconstructions. The latest mid-Holocene simulations are a palaeoclimate entry card for the Palaeoclimate Model Intercomparison Project (PMIP4) component of the current phase of the Coupled Model Inte...
Article
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Robock claims that our analysis fails to acknowledge that pan-tropical surface cooling caused by large volcanic eruptions may mask El Niño warming at our central Pacific site, potentially obscuring a volcano-El Niño connection suggested in previous studies. Although observational support for a dynamical response linking volcanic cooling to El Niño...
Preprint
Full-text available
Chronological uncertainty is a hallmark of the paleosciences. While many tools have been made available to researchers to quantify age uncertainties suitable for various settings and assumptions, disparate tools and output formats often discourage integrative approaches. In addition, associated tasks like propagating age model uncertainties to subs...
Article
Full-text available
The American West exemplifies drought-sensitive regions with growing populations. Paleoclimate investigations have documented severe droughts in this region before European settling, with major implications for water management and planning. Here, we leverage paleoclimate data assimilation to reconstruct past climate states, enabling a large-scale...
Article
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Explosive volcanism imposes impulse-like radiative forcing on the climate system, providing a natural experiment to study the climate response to perturbation. Previous studies have identified disagreements between paleoclimate reconstructions and climate model simulations with respect to the magnitude and recovery from volcanic cooling, questionin...
Article
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A comprehensive database of paleoclimate records is needed to place recent warming into the longer-term context of natural climate variability. We present a global compilation of quality-controlled, published, temperature-sensitive proxy records extending back 12,000 years through the Holocene. Data were compiled from 679 sites where time series co...
Article
Not a big deal after all Do volcanic eruptions affect El Niño–Southern Oscillation (ENSO) variability? Models indicate that sulfate aerosols resulting from large eruptions can initiate an El Niño–like response in the tropical Pacific, but observations have not shown evidence of such behavior. Dee et al . present an oxygen-isotope time series of fos...
Preprint
Full-text available
Abstract. The mid-Holocene (6000 years ago) is a standard experiment for the evaluation of the simulated response of global climate models using paleoclimate reconstructions. The latest mid-Holocene simulations are a contribution by the Palaeoclimate Model Intercomparison Project (PMIP4) to the current phase of the Coupled Model Intercomparison Pro...
Article
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Speleothem δ18O is widely used to reconstruct past hydroclimate variability, particularly over Asia. However, the interpretation of this proxy is still in debate. While this proxy is originally interpreted as regional rainfall amount of the Asian monsoon, other studies have interpreted it as upstream monsoon rainfall or atmospheric circulation chan...
Article
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The progress of science is tied to the standardization of measurements, instruments, and data. This is especially true in the Big Data age, where analyzing large data volumes critically hinges on the data being standardized. Accordingly, the lack of community-sanctioned data standards in paleoclimatology has largely precluded the benefits of Big Da...
Article
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Multidecadal surface temperature changes may be forced by natural as well as anthropogenic factors, or arise unforced from the climate system. Distinguishing these factors is essential for estimating sensitivity to multiple climatic forcings and the amplitude of the unforced variability. Here we present 2,000-year-long global mean temperature recon...
Article
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Paleoclimate reconstruction on the Common Era (1–2000 AD) provides critical context for recent warming trends. This work leverages integrated nested Laplace approximations (INLA) to conduct inference under a Bayesian hierarchical model using data from three sources: a state-of-the-art proxy database (PAGES 2k), surface temperature observations (Had...
Article
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The Last Millennium Reanalysis (LMR) utilizes an ensemble methodology to assimilate paleoclimate data for the production of annually resolved climate field reconstructions of the Common Era. Two key elements are the focus of this work: the set of assimilated proxy records and the forward models that map climate variables to proxy measurements. Resu...
Article
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Climate records exhibit scaling behavior with large exponents, resulting in larger fluctuations at longer timescales. It is unclear whether climate models are capable of simulating these fluctuations, which draws into question their ability to simulate such variability in the coming decades and centuries. Using the latest simulations and data synth...
Article
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The δ¹⁸O signal preserved in paleoarchives is widely used to reconstruct past climate conditions. In many speleothems, this signal is classically interpreted via the amount effect. However, recent work has shown that precipitation δ¹⁸O (δ¹⁸OP) is greatly influenced by convective processes distinct from precipitation amount, and new observations ind...
Preprint
Full-text available
A Paleoclimate Reconstruction on the Common Era (1-2000AD) was performed using a Hierarchical Bayesian Model from three types of data: proxy data from PAGES2k project dataset, HadCRUT4 temperature data from the Climatic Research Unit at the University of East Anglia, and external forcing data from several sources. Five data reduction techniques wer...
Article
Full-text available
The Last Millennium Reanalysis utilizes an ensemble methodology to assimilate paleoclimate data for the production of annually resolved climate field reconstructions of the Common Era. Two key elements are the focus of this work: the set of assimilated proxy records, and the forward models that map climate variables to proxy measurements. Results b...
Article
Full-text available
In the version of this Review Article originally published, ref. 10 was mistakenly cited instead of ref. 107 at the end of the sentence: “This complexity of residual ice cover makes it likely that HTM warming was regional, rather than global, and its peak warmth thus had different timing in different locations.” In addition, for ref. 108, Scientifi...
Article
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Over the past 3.5 million years, there have been several intervals when climate conditions were warmer than during the pre-industrial Holocene. Although past intervals of warming were forced differently than future anthropogenic change, such periods can provide insights into potential future climate impacts and ecosystem feedbacks, especially over...
Article
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The Last Millennium Reanalysis (LMR) employs a data assimilation approach to reconstruct climate fields from annually resolved proxy data over years 0–2000 CE. We use the LMR to examine Atlantic multidecadal variability (AMV) over the last 2 millennia and find several robust thermodynamic features associated with a positive Atlantic Multidecadal Os...
Conference Paper
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Traditional approaches to ontology development have a large lapse between the time when a user using the ontology has found a need to extend it and the time when it does get extended. For scientists, this delay can be weeks or months and can be a significant barrier for adoption. We present a new approach to ontology development and data annotation...
Article
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Reproducible climate reconstructions of the Common Era (1 CE to present) are key to placing industrial-era warming into the context of natural climatic variability. Here we present a community-sourced database of temperature-sensitive proxy records from the PAGES2k initiative. The database gathers 692 records from 648 locations, including all conti...
Article
Full-text available
The Last Millennial Reanalysis (LMR) employs a data assimilation approach to reconstruct climate fields from annually-resolved proxy data over years 0–2000CE. We use the LMR to examine Atlantic Multidecadal Variability (AMV) over the last two millennia, and find several robust thermodynamic features associated with a positive Atlantic Multidecadal...
Article
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A concerted effort has begun to gather and preserve archives of marine samples and descriptive data, giving scientists ready access to insights on ancient environments.
Article
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Paleoclimate data assimilation has recently emerged as a promising technique to estimate past climate states. Here we test two of the underlying assumptions of paleoclimate data assimilation as applied so far: (1) climate proxies can be modeled as linear, univariate recorders of temperature; and (2) structural errors in GCMs can be neglected. To in...
Article
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An “offline” approach to DA is used, where static ensemble samples are drawn from existing CMIP climate-model simulations to serve as the prior estimate of climate variables. We use linear, univariate forward models (“proxy system models”; PSMs) that map climate variables to proxy measurements by fitting proxy data to 2m air temperature from gridde...
Article
The El Niño-Southern Oscillation (ENSO) is the primary driver of interannual climate variability in the tropics and subtropics. Despite substantial progress in understanding ocean–atmosphere feedbacks that drive ENSO today, relatively little is known about its behavior on centennial and longer timescales. Paleoclimate records from lakes, corals, mo...
Article
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Paleoclimatology is a highly collaborative scientific endeavor, increasingly reliant on online databases for data sharing. Yet there is currently no universal way to describe, store and share paleoclimate data: in other words, no standard. Data standards are often regarded by scientists as mere technicalities, though they underlie much scientific a...
Article
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The El Niño/Southern Oscillation (ENSO) is the leading mode of interannual climate variability. However, it is unclear how ENSO has responded to external forcing, particularly orbitally induced changes in the amplitude of the seasonal cycle during the Holocene. Here we present a reconstruction of seasonal and interannual surface conditions in the t...
Article
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The use of Mg/Ca in marine carbonates as a paleothermometer has been challenged by observations that implicate salinity as a contributing influence on Mg incorporation into biotic calcite and that dissolution at the sea-floor alters the original Mg/Ca. Yet, these factors have not yet been incorporated into a single calibration model. We introduce a...