Gerald Meehl

Introduction

Gerald A. Meehl is a Senior Scientist at the National Center for Atmospheric Research (NCAR) and heads the Climate Change Research Section. He is also Chief Scientist of the Cooperative Agreement To Analyze variabiLity, change and predictabilitY in the earth SysTem (CATALYST), a project that involves NCAR and the U.S. Department of Energy (DOE). His research interests include studying the interactions between El Niño/Southern Oscillation (ENSO) and the monsoons of Asia; identifying possible effects on global climate of changing anthropogenic forcings, such as carbon dioxide and aerosols, as well as natural forcings, such as solar variability; quantifying possible future changes of weather and climate extremes in a warmer climate; and understanding the interplay between internally generated climate variability and the response to external forcings, particularly in the context of understanding and predicting decadal climate variability. He was a member of the Intergovernmental Panel on Climate Change (IPCC) science team that was awarded the Nobel Peace Prize in 2007. As part of that effort, he was an author on the first five IPCC climate change assessment reports, serving as contributing author (1990), lead author (1995), coordinating lead author (2001, 2007), and lead author on the near-term climate change chapter for the IPCC AR5 that was completed in 2013. He received his Ph.D. in climate dynamics from the University of Colorado, and was a recipient of the Jule G. Charney Award of the American Meteorological Society in 2009, and the Sverdrup Gold Medal of the American Meteorological Society in 2023. Dr. Meehl is a Fellow of both the American Meteorological Society and the American Geophysical Union, and a Visiting Senior Fellow at the University of Hawaii Joint Institute for Marine and Atmospheric Research. He serves as a member of the Department of Energy Biological and Environmental Research Advisory Committee (BERAC). Additionally, he has been a member and chair of the National Academy of Sciences Climate Research Committee (CRC), co-chair of the National Academy of Sciences Board on Atmospheric Sciences and Climate (BASC), co-chair of the Community Earth System Model (CESM) Climate Variability and Change Working Group (CVCWG), member and co-chair of the World Climate Research Programme (WCRP) Working Group on Coupled Models (WGCM), the group that coordinates the international global climate model experiments addressing anthropogenic climate change through the Coupled Model Intercomparison Project (CMIP) where he was member and chair of the CMIP Panel, and is currently a member of the WCRP Decadal Climate Prediction Panel (DCPP).

Notable Research Highlights

Discovered, defined, and named the Tropospheric Biennial Oscillation (TBO) that connects interannual variability of the Asian-Australian monsoon system with tropical Pacific sea surface temperature on biennial timescales, and subsequently showed how processes involved with decadal climate variability of the Interdecadal Pacific Oscillation (IPO) in the Indo-Pacific region could modulate the TBO so that in some decades the system is more biennial than others (Meehl, 1987, Mon. Wea. Rev.; Meehl, 1993, J. Climate; Meehl, 1994, Science; Meehl, 1997, J. Climate; Meehl and Arblaster, 2001, Geophys. Res. Lett.; Meehl and Arblaster, 2002a,b, J. Climate; Meehl et al., 2003, J. Climate; Meehl and Arblaster, 2011, J. Climate; Meehl and Arblaster, 2012, Geophys. Res. Lett.)

Led the first effort to address science questions involved with possible changes in future weather and climate extremes in the physical climate system (Meehl et al., 2000a,b, Bulletin Amer. Meteorol. Soc.)

Published the first study documenting future changes of heat waves in a warmer climate and the associated physical processes that are responsible (Meehl and Tebaldi, 2004, Science)

First to quantify processes and changes associated with weather and climate extremes in a warmer climate due to increasing greenhouse gases for frost days (Meehl et al., 2004, Clim. Dyn.) and precipitation extremes (Meehl et al., 2005, Geophys. Res. Lett.); connected patterns of extremes over North America to El Niño anomalies; and showed how those patterns could change in the future with changes in the tropical Pacific base state (Meehl et al., 2007a,b, Geophys. Res. Lett.)

Identified for the first time that the ratio of daily record high temperatures to record low minimum temperatures over the U.S. has been increasing to values greater than one in the presence of mean surface warming, and will continue to increase in the future (Meehl et al., 2009, Geophys. Res. Lett.); and generalized the future changes in the ratio over the U.S. based on mean temperature increase (Meehl and Tebaldi, 2016, Proc. Nat. Acad. Sci.)

Published the first study using analyses of two global coupled climate models to quantify climate change commitment due to human-caused warming (Meehl et al., 2005, Science), and confirmed that result with additional analyses in a single model (Meehl et al., 2006, J. Climate)

Discovered and defined a new mechanism connecting solar variability to tropical Pacific climate whereby small solar forcing changes are amplified by dynamically coupled interactions and cloud feedbacks in the Pacific to produce a La Niña-like response coincident with peaks in solar forcing followed a couple years later by an El Niño-like response (referred to as the “bottom-up” mechanism) (Meehl et al., 2003, J. Climate; Meehl et al., 2008, J. Climate; Meehl and Arblaster, 2009, J. Climate; Meehl et al., 2009, Science; Meehl et al., 2013, Geophys. Res. Lett.)

Documented observed and modeled processes for, and defined future changes to, the Asian-Australian monsoon system, El Niño, and El Niño teleconnections (Meehl, 1990, J. Climate; Meehl et al., 1993, J. Climate; Meehl and Washington, 1993, Science; Meehl et al., 2006, Clim. Dyn.; Meehl and Teng, 2007, Clim. Dyn.)

Discovered and defined a new mechanism that produces decadal timescale variability associated with the Interdecadal Pacific Oscillation (IPO) (Meehl and Hu, 2006, J. Climate) and identified its importance for decadal climate prediction (Meehl et al., 2010, J. Climate)

Led the effort to address the science questions involved with the new field of decadal climate prediction (Meehl et al., 2009, 2014, Bull. Amer. Meteorol. Soc.)

First to address the early-2000s slowdown of global warming (sometimes called the “hiatus”) by identifying where heat goes during hiatus decades (defined as decades with little global mean surface temperature trend, when heat goes into the subsurface ocean), defined mechanisms for producing hiatus and accelerated warming decades, and put these naturally-occurring processes in the context of the response to increasing GHGs and global temperature trends (Meehl et al., 2011, Nature Clim. Change; Meehl et al., 2013, J. Climate; Meehl et al., 2016, Nature Climate Change)

First to quantify the role of internal decadal variability associated with the negative phase of the IPO to expansions of Antarctic sea ice from 2000 to 2014 (Meehl et al., 2016, Nature Geoscience)

First to publish an initialized decadal climate prediction based on a physical process--connecting interannual ENSO variability with decadal timescale transitions of the Interdecadal Pacific Oscillation--to predict a transition of the IPO to positive in the 2015-2019 time frame, with larger rates of global warming from 2013 to 2022 (Meehl et al., 2016, Nature Communications)

Documented, for the first time, processes and mechanisms associated with the seasonal cycle and interannual variability in the Southern Hemisphere (Meehl, 1987, Mon. Wea. Rev.; Meehl, 1991, J. Climate; Meehl et al., 1998, Tellus; Meehl, van Loon, and Arblaster, 2017, Geophys. Res. Lett.)

Documented processes and mechanisms associated with decadal climate variability globally (Meehl et al., 1998, Geophys. Res. Lett.; Meehl et al., 2000, J. Climate), in the Pacific (Meehl et al., 2009, J. Climate), and over North America (Meehl et al., 2012, J. Climate; Meehl et al., 2015, Geophys. Res. Lett.)

First to document skill of initialized decadal hindcasts in simulating large climate shifts associated with the IPO (Meehl and Teng, 2012, 2014a,b, Geophys. Res. Lett.; Meehl et al., 2014, Nature Climate Change; Meehl et al., 2015, Geophys. Res. Lett.; Meehl et al., 2016, Nature Communications)

Performed multiple pioneering studies to quantify possible future climate changes due to increases of greenhouse gases, with a particular focus on the Asia-Pacific and North American regions (Meehl and Washington, 1986, Mon. Wea. Rev.; Meehl and Washington, 1990, Climatic Change; Meehl and Washington, 1996, Nature; Meehl et al., 2000, J. Climate; Meehl and Arblaster, 2003, Clim. Dyn.; Meehl et al., 2004, J. Climate; Meehl et al., 2012, J. Climate; Meehl et al., 2013, J. Climate, and more)

Published the first initialized decadal climate prediction for a near-term Pacific climate state involving the IPO that was based on an understanding of a physical process that could provide credibility for the prediction (Meehl et al., 2016, Nature Communications)

Discovered that decadal variability of SSTs in the tropical Pacific associated with the IPO was primarily responsible for the rapid rate of expansion of Antarctic sea ice from 2000-2014 (Meehl et al., 2016, Nature Geo.), and that a seasonally-dependent combination of tropical Pacific (negative IPO) and tropical Atlantic (positive AMO) SST decadal variability primarily drove a comparable rapid decrease of Arctic sea ice during that period (Meehl et al., 2018, Geophys. Res. Lett.)

Discovered that the sudden and precipitous decline of Antarctic sea ice starting in late 2016 was driven by a combination of interannual forcing from the tropical eastern Indian/western Pacific, with consequent teleconnections involving surface winds around Antarctica, and decadal forcing from the tropical Pacific (negative IPO); the intensified mid- and high-latitude westerlies around Antarctica from about 2000-2014 gradually warmed the upper 600m of the Southern Ocean to sustain the initial large decreases of Antarctic sea ice that began in late 2016 (Meehl et al., 2019, Nature Communications)

Discovered and defined a new paradigm for trans-basin interaction whereby the tropical Atlantic and Pacific are mutually and sequentially interactive on decadal timescales, thus providing insights into mechanisms that could provide skill in multi-year climate predictions (Meehl et al., 2020, Nature Geoscience)

Returning to an earlier theme of timescale interactions in the climate system (Meehl et al., 2001, Clim. Dyn.), discovered a set of processes whereby a buildup of off-equatorial western Pacific ocean heat content can be triggered by an interannual El Niño event to produce a decadal timescale transition of the Interdecadal Pacific Oscillation (IPO) that could be a key for skill in decadal climate predictions (Meehl et al., 2021, Clim. Dyn.) where model error that produces drift in predictions is identified as the biggest challenge in making skillful initialized multi-year climate predictions (Meehl et al., 2022, Clim. Dyn.)

Notable Community Service Highlights

Starting in 1995, under the auspices of the World Climate Research Programme, initiated and led the Coupled Model Intercomparison Project (CMIP) as part of the WCRP/CLIVAR Working Group on Coupled Models; this produced the first coordinated comparison of a set of climate change experiments performed by modeling groups from around the world for CMIP3 and assessed in the IPCC AR4, subsequently leading to CMIP5 simulations assessed in the IPCC AR5, and CMIP6 simulations assessed in the IPCC AR6. CMIP has transformed the field of climate science by making state-of-the-art climate change experiments openly available to researchers and students around the world (Meehl et al., 1997, EOS; Meehl et al., 2000, Bull. Amer. Meteorol. Soc.; Meehl et al., 2005, Bull. Amer. Meteorol. Soc.; Meehl et al., 2007, Bull. Amer. Meteorol. Soc.; Meehl et al., 2014, EOS)

Participated as an author in the first five Intergovernmental Panel on Climate Change (IPCC) assessments (contributing author in the First, lead author in the Second, coordinating lead author in the Third and Fourth, all for the climate change projections chapters; and lead author in the Fifth for the near-term climate change chapter)