Alexander Gershunov

Scripps Institution of Oceanography

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Co-Authors

Daniel R. Cayan

Scripps Institution of Oceanography

Kristen Guirguis

Scripps Institution of Oceanography

Tarik Benmarhnia

Scripps Institution of Oceanography

F. Martin Ralph

Scripps Institution of Oceanography

Rachel E. S. Clemesha

University of California, San Diego

Tamara Shulgina

Scripps Institution of Oceanography

Rosana Aguilera

Scripps Institution of Oceanography

Michael J. DeFlorio

Scripps Institution of Oceanography

Rupa Basu

California Environmental Protection Agency

Michael D. Dettinger

Scripps Institution of Oceanography

Cooperative Institutions

University of California, San Diego

130

Scripps Institution of Oceanography

124

University of Colorado Boulder

University of California, Los Angeles

NSF National Center for Atmospheric Research

National Oceanic and Atmospheric Administration

Cooperative Institute for Research in Environmental Sciences

University of California, Berkeley

University of California, Davis

University of Arizona

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Observed Impacts of Anthropogenic Climate Change on Wildfire in California

Earth s Future (2019)

Park Williams John T. Abatzoglou Alexander Gershunov Janin Guzman‐Morales Daniel A. Bishop

Abstract Recent fire seasons have fueled intense speculation regarding the effect of anthropogenic climate change on wildfire in western North America and especially in California. During 1972–2018, California experienced a fivefold increase in annual burned area, mainly due to more than an eightfold increase in summer forest‐fire extent. Increased summer forest‐fire area very likely occurred due to increased atmospheric aridity caused by warming. Since the early 1970s, warm‐season days warmed by approximately 1.4 °C as part of a centennial warming trend, significantly increasing the atmospheric vapor pressure deficit (VPD). These trends are consistent with anthropogenic trends simulated by climate models. The response of summer forest‐fire area to VPD is exponential, meaning that warming has grown increasingly impactful. Robust interannual relationships between VPD and summer forest‐fire area strongly suggest that nearly all of the increase in summer forest‐fire area during 1972–2018 was driven by increased VPD. Climate change effects on summer wildfire were less evident in nonforested lands. In fall, wind events and delayed onset of winter precipitation are the dominant promoters of wildfire. While these variables did not change much over the past century, background warming and consequent fuel drying is increasingly enhancing the potential for large fall wildfires. Among the many processes important to California's diverse fire regimes, warming‐driven fuel drying is the clearest link between anthropogenic climate change and increased California wildfire activity to date.

Fire regime

10.1029/2019ef001210

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Citations (825)

North Pacific Decadal Climate Variability since 1661

Journal of Climate (2001)

Franco Biondi Alexander Gershunov Daniel R. Cayan

Climate in the North Pacific and North American sectors has experienced interdecadal shifts during the twentieth century. A network of recently developed tree-ring chronologies for Southern and Baja California extends the instrumental record and reveals decadal-scale variability back to 1661. The Pacific decadal oscillation (PDO) is closely matched by the dominant mode of tree-ring variability that provides a preliminary view of multiannual climate fluctuations spanning the past four centuries. The reconstructed PDO index features a prominent bidecadal oscillation, whose amplitude weakened in the late l700s to mid-1800s. A comparison with proxy records of ENSO suggests that the greatest decadal-scale oscillations in Pacific climate between 1706 and 1977 occurred around 1750, 1905, and 1947.

Proxy (statistics)

Multivariate ENSO index

Climate system

10.1175/1520-0442(2001)014<0005:npdcvs>2.0.co;2

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Citations (547)

Climate Change and Microbiological Water Quality at California Beaches

EcoHealth (2012)

Jan C. Semenza Joshua S. Caplan Guido Buescher Tapash Das Mitchell V. Brinks

Animal ecology

10.1007/s10393-012-0779-1

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Citations (10)

Megaflooding on Earth and Mars by Devon M. Burr, Paul A. Carling & Victor R. Baker

Hydrological Sciences Journal (2010)

Alexander Gershunov

10.1080/02626667.2010.498564

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Citations (0)

Climatic‐scale space‐time variability of tropical precipitation

Journal of Geophysical Research Atmospheres (1996)

Alexander Gershunov Joel Michaelsen

More than 15 years of monthly microwave sounding unit rainfall data over the tropical oceans are analyzed to illustrate rainfall variability on various timescales and delineate its spatial patterns. The annual and semiannual components of the seasonal cycle are modeled with first and second annual harmonics at every 2.5° × 2.5° grid square. Regions of highest rainfall variability tend to be characterized by a powerful annual cycle. The semiannual cycle is generally a trivial component of the seasonal cycle, except in some regions where either the mean climatological precipitation is low or where the total seasonal cycle is weak. An interesting exception, in this respect, is a band of the southeastern tropical Pacific extending immediately to the south of the eastern equatorial Pacific cold tongue. Regions of highest climatological mean rainfall are characterized by weak seasonality but strong nonseasonal variability. After seasonality is described and removed from the data, nonseasonal variability is considered via principal component analysis in the time domain. The two dominant modes together describe precipitation variability associated with the El Niño‐Southern Oscillation: they outline the evolution of warm‐ and cold‐event precipitation anomalies and contrast the intense 1982–1983 warm event with the moderate events of 1986–1987 and 1992–1993. These two modes display oscillations with predominantly quasi‐biennial and ∼5‐year periods. Another coherent mode summarizes intraseasonal variability which, although inadequately resolved by the monthly average rainfall data, displays typical signs of the 40‐ to 50‐day oscillation. All coherent modes, despite having much of their energy concentrated around rather different frequencies, show signs of interaction.

Annual cycle

Special sensor microwave/imager

Madden–Julian oscillation

Seasonality

Water cycle

10.1029/96jd01382

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Citations (8)

ARTMIP-early start comparison of atmospheric river detection tools: how many atmospheric rivers hit northern California’s Russian River watershed?

Climate Dynamics (2018)

F. Martin Ralph Anna M. Wilson Tamara Shulgina Brian Kawzenuk Scott Sellars

10.1007/s00382-018-4427-5

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Citations (76)

How agricultural droughts are contributing to child undernutrition in sub-Saharan Africa

Anna Dimitrova Alexander Gershunov Tarik Benmarhnia

Countries in sub-Saharan Africa (SSA) have some of the highest levels of child malnutrition, with more than one-third of children under five in the region characterized as chronically undernourished. High reliance on subsistence farming, poor adoption of irrigation technologies, and variable climate conditions make populations in SSA highly vulnerable to malnutrition during droughts. We use anthropometric data for 520,734 children under the age of five from 34 countries in SSA collected between 1990 and 2022 in combination with high-resolution agricultural and climate data to estimate the association between agricultural droughts and child undernutrition in the region. We use global gridded data on the geographical distribution of crop areas for 15 major crops. Data on crop planting and harvesting dates are also collected for each crop. The Standardized Precipitation Evapotranspiration Index (SPEI), a multi-scalar drought index, is used to measure the intensity and spatial distribution of droughts during key periods of agricultural production (planting, growth, and harvesting) and of different duration (seasonal and long-lasting droughts). Our analysis shows that droughts during the crop-growing seasons are associated with an increased risk of child undernutrition in SSA. The findings presented in this study call for urgent action to improve drought monitoring and response in SSA where the risks to child health posed by global warming are considerable. Under climate change, the severity and frequency of extreme weather and climate events, including droughts, are projected to increase, which will place millions of children at risk of hunger unless timely action plans are taken to improve food security in the region.

10.5194/egusphere-egu24-20931

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Citations (0)

Climate and Wildfire in the Western United States

Bulletin of the American Meteorological Society (2003)

A. L. Westerling Alexander Gershunov Tim Brown Daniel R. Cayan Michael D. Dettinger

Abstract A 21-yr gridded monthly fire-starts and acres-burned dataset from U.S. Forest Service, Bureau of Land Management, National Park Service, and Bureau of Indian Affairs fire reports recreates the seasonality and interannual variability of wildfire in the western United States. Despite pervasive human influence in western fire regimes, it is striking how strongly these data reveal a fire season responding to variations in climate. Correlating anomalous wildfire frequency and extent with the Palmer Drought Severity Index illustrates the importance of prior and accumulated precipitation anomalies for future wildfire season severity. This link to antecedent seasons' moisture conditions varies widely with differences in predominant fuel type. Furthermore, these data demonstrate that the relationship between wildfire season severity and observed moisture anomalies from antecedent seasons is strong enough to forecast fire season severity at lead times of one season to a year in advance.

10.1175/bams-84-5-595

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Citations (519)

Future dryness in the southwest US and the hydrology of the early 21st century drought

Proceedings of the National Academy of Sciences (2010)

Daniel R. Cayan Tapash Das David W. Pierce T. P. Barnett Mary Tyree

Recently the Southwest has experienced a spate of dryness, which presents a challenge to the sustainability of current water use by human and natural systems in the region. In the Colorado River Basin, the early 21st century drought has been the most extreme in over a century of Colorado River flows, and might occur in any given century with probability of only 60%. However, hydrological model runs from downscaled Intergovernmental Panel on Climate Change Fourth Assessment climate change simulations suggest that the region is likely to become drier and experience more severe droughts than this. In the latter half of the 21st century the models produced considerably greater drought activity, particularly in the Colorado River Basin, as judged from soil moisture anomalies and other hydrological measures. As in the historical record, most of the simulated extreme droughts build up and persist over many years. Durations of depleted soil moisture over the historical record ranged from 4 to 10 years, but in the 21st century simulations, some of the dry events persisted for 12 years or more. Summers during the observed early 21st century drought were remarkably warm, a feature also evident in many simulated droughts of the 21st century. These severe future droughts are aggravated by enhanced, globally warmed temperatures that reduce spring snowpack and late spring and summer soil moisture. As the climate continues to warm and soil moisture deficits accumulate beyond historical levels, the model simulations suggest that sustaining water supplies in parts of the Southwest will be a challenge.

Dryness

Snowpack

10.1073/pnas.0912391107

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Citations (677)

Atmospheric River Tracking Method Intercomparison Project (ARTMIP): project goals and experimental design

Geoscientific model development (2018)

Christine A. Shields Jonathan J. Rutz L. Ruby Leung F. Martin Ralph Michael Wehner

Abstract. The Atmospheric River Tracking Method Intercomparison Project (ARTMIP) is an international collaborative effort to understand and quantify the uncertainties in atmospheric river (AR) science based on detection algorithm alone. Currently, there are many AR identification and tracking algorithms in the literature with a wide range of techniques and conclusions. ARTMIP strives to provide the community with information on different methodologies and provide guidance on the most appropriate algorithm for a given science question or region of interest. All ARTMIP participants will implement their detection algorithms on a specified common dataset for a defined period of time. The project is divided into two phases: Tier 1 will utilize the Modern-Era Retrospective analysis for Research and Applications, version 2 (MERRA-2) reanalysis from January 1980 to June 2017 and will be used as a baseline for all subsequent comparisons. Participation in Tier 1 is required. Tier 2 will be optional and include sensitivity studies designed around specific science questions, such as reanalysis uncertainty and climate change. High-resolution reanalysis and/or model output will be used wherever possible. Proposed metrics include AR frequency, duration, intensity, and precipitation attributable to ARs. Here, we present the ARTMIP experimental design, timeline, project requirements, and a brief description of the variety of methodologies in the current literature. We also present results from our 1-month “proof-of-concept” trial run designed to illustrate the utility and feasibility of the ARTMIP project.

Time line

Tracking (education)

Baseline (sea)

10.5194/gmd-11-2455-2018

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Citations (296)