Gerald A. Meehl

National Center for Atmospheric Research · Climate & Global Dynamics Laboratory

Consequences from a slowdown or collapse of the Atlantic Meridional Overturning Circulation (AMOC) could include modulations to El Niño-Southern Oscillation (ENSO) and development of the Pacific Meridional Overturning Circulation (PMOC). Despite potential ramifications to the global climate, our understanding of the influence of various AMOC and PM...

The Atlantic meridional overturning circulation (AMOC) is an important global-scale circulation and changes in AMOC can induce significant regional and global climate impacts. Here we study the stability of AMOC and its influence on global ocean circulation and the surface climate though analyzing a set of sensitivity experiments using the Communit...

Decadal climate prediction presumes there are decadal-timescale processes and mechanisms that, if initialized properly in models, potentially provide predictive skill more than one or two years into the future. Candidate mechanisms involve Pacific decadal variability and Atlantic multidecadal variability, elements of which involve slow fluctuations...

The Paris Agreement calls for efforts to limit anthropogenic global warming to less than 1.5oC above pre-industrial levels. However, natural internal variability may exacerbate anthropogenic warming to produce temporary excursions above 1.5oC. Such excursions would not necessarily exceed the Paris Agreement, but would provide a warning that the thr...

Historical observations show that one in two La Niña events last for two consecutive years. Despite their outsized impacts on drought, these 2-year La Niña are not predicted on a routine basis. Here, we assess the predictability of 2-year La Niña using retrospective forecasts performed with a climate model that simulates realistic multi-year events...

Previous studies have investigated the centennial and multidecadal trends of the Pacific and Indian Ocean Walker cells (WCs) during the past century, but have obtained no consensus owing to data uncertainties and weak signals of the long-term trends. This paper focuses on decadal variability (periods of one to few decades) by first documenting the...

The semiannual oscillation (SAO) is a twice-yearly northward movement (in May-June-July (MJJ) and November-December-January (NDJ)) of the circumpolar trough of sea level pressure (SLP) in the Southern Hemisphere with effects throughout the troposphere. During MJJ the second harmonic of SLP, describing the SAO, has low values of SLP north of 50°S in...

Robust evidence exists that certain extreme weather and climate events, especially daily temperature and precipitation extremes, have changed in regard to intensity and frequency over recent decades. These changes have been linked to human-induced climate change, while the degree to which climate change impacts an individual extreme climate event (...

In the early twenty-first century, satellite-derived tropospheric warming trends were generally smaller than trends estimated from a large multi-model ensemble. Because observations and coupled model simulations do not have the same phasing of natural internal variability, such decadal differences in simulated and observed warming rates invariably...

Multiple lines of observational evidence indicate that the global climate has been getting warmer since the early 20th century. This warmer climate has led to a global mean sea level rise of about 18 cm during the 20th century, and over 6 cm for the first 15 years of the 21st century. Regionally the sea level rise is not uniform due in large part t...

Sea level rise (SLR) can exert significant stress on highly populated coastal societies and low-lying island countries around the world. Because of this, there is huge societal demand for improved decadal predictions and future projections of SLR, particularly on a local scale along coastlines. Regionally, sea level variations can deviate considera...

Key Points • Systematic evaluation of temporal and spatial features of the IPO in CMIP5 using TPI • Decadal variance of the IPO is commonly underestimated in models relative to observations • Models that simulate the IPO spatial pattern well also tend to simulate low frequency temporal characteristics Accelerated warming and hiatus periods in...

Sea level rise (SLR) can exert significant stress on highly populated coastal societies and low-lying island countries around the world. Because of this, there is huge societal demand for improved decadal predictions and future projections of SLR, particularly on a local scale along coastlines. Regionally, sea level variations can deviate considera...

Significance Future changes in temperature extremes over the continental United States are represented by the ratio of daily record high maximum temperatures to daily record low minimum temperatures and computed as a function of mean temperature increase that does not depend on scenario. A nonlinear empirical fit of the relation between this ratio...

The Coupled Model Intercomparison Project (CMIP) has successfully provided the climate community with a rich collection of simulation output from Earth system models (ESMs) that can be used to understand past climate changes and make projections and uncertainty estimates of the future. Confidence in ESMs can be gained because the models are based o...

The Decadal Climate Prediction Project (DCPP) is a coordinated multi-model investigation into decadal climate prediction, predictability, and variability. The DCPP makes use of past experience in simulating and predicting decadal variability and forced climate change gained from the fifth Coupled Model Intercomparison Project (CMIP5) and elsewhere....

Projections of future climate change play a fundamental role in improving understanding of the climate system as well as characterizing societal risks and response options. The Scenario Model Intercomparison Project (ScenarioMIP) is the primary activity within Phase 6 of the Coupled Model Intercomparison Project (CMIP6) that will provide multi-mode...

A strengthening of the Amundsen Sea low from 1979 to 2013 has been shown to largely explain the observed increase in Antarctic sea ice concentration in the eastern Ross Sea and decrease in the Bellingshausen Sea. Here it is shown that while these changes are not generally seen in freely running coupled climate model simulations, they are reproduced...

Longer-term externally forced trends in global mean surface temperatures (GMSTs) are embedded in the background noise of internally generated multidecadal variability. A key mode of internal variability is the Interdecadal Pacific Oscillation (IPO), which contributed to a reduced GMST trend during the early 2000s. We use a novel, physical phenomeno...

Antarctic sea-ice extent has been slowly increasing in the satellite record that began in 1979. Since the late 1990s, the increase has accelerated, but the average of all climate models shows a decline. Meanwhile, the Interdecadal Pacific Oscillation, an internally generated mode of climate variability, transitioned from positive to negative, with...

The Coupled Model Intercomparison Project (CMIP) is an ongoing coordinated international activity of numerical experimentation of unprecedented scope and impact on climate science. Its most recent phase, the fifth phase (CMIP5), has created nearly 2 PB of output from dozens of experiments performed by dozens of comprehensive climate models availabl...

The Coupled Model Intercomparison Project (CMIP) has successfully provided the climate community with a rich collection of simulation output from Earth system models (ESMs) that can be used to understand past climate changes and make projections and uncertainty estimates of the future. Confidence in ESMs can be gained because the models are based o...

The negative phase of the Interdecadal Pacific Oscillation (IPO), a dominant mode of multi-decadal variability of sea surface temperatures (SSTs) in the Pacific, contributed to the reduced rate of global surface temperature warming in the early 2000s. A proposed mechanism for IPO multidecadal variability indicates that the presence of decadal times...

By coordinating the design and distribution of global climate model simulations of the past, current, and future climate, the Coupled Model Intercomparison Project (CMIP) has become one of the foundational elements of climate science. However, the need to address an ever-expanding range of scientific questions arising from more and more research co...

The global fully coupled half-degree Community Climate System Model Version 4 (CCSM4) was integrated for a suite of climate change ensemble simulations including five historical runs, five Representative Concentration Pathway 8.5 [RCP8.5) runs, and a long Pre-Industrial control run. This study focuses on precipitation at regional scales and its sen...

The Decadal Climate Prediction Project (DCPP) is a coordinated multi-model investigation into decadal climate prediction, predictability, and variability. The DCPP makes use of past experience in simulating and predicting decadal variability and forced climate change gained from CMIP5 and elsewhere. It builds on recent improvements in models, in th...

Global Global environmental changes, such as climate change, result from the interaction of human and natural systems. Understanding these changes and options for addressing them requires research in the physical, environmental, and social sciences, as well as engineering and other applied fields. In this essay, the authors provide their personal p...

It has been claimed that the early-2000s global warming slowdown or hiatus, characterized by a reduced rate of global surface warming, has been overstated, lacks sound scientific basis, or is unsupported by observations. The evidence presented here contradicts these claims.

Compared to changes in the climatological mean temperature, we have less confidence in how much and by what mechanisms temperature variability may be affected by anthropogenic climate change. Here based on a 30-member climate change projection from an earth system model, we find summertime subseasonal temperature variability in the US Great Plains...

Multi-model simulations show a post-Pinatubo eruption sequence of Pacific sea surface temperatures (SSTs) that includes a La Niña-like pattern the third northern winter after an eruption, opposite in sign to what was observed after Pinatubo. This leads to the loss of hindcast skill for years in the 1990s affected by the Pinatubo eruption because th...

Aspen Global Change Institute Workshop on Decadal Climate Predictions: Improving Our Understanding of Processes and Mechanisms to Make Better Predictions; Aspen, Colorado, 7–12 June 2015

By coordinating the design and distribution of global climate model simulations of the past, current and future climate, the Coupled Model Intercomparison Project (CMIP) has become one of the foundational elements of climate science. However, the need to address an ever-expanding range of scientific questions arising from more and more research com...

Regardless of the harmful effects of burning fossil fuels on global climate, other energy sources will become more important in the future because fossil fuels could run out by the early twenty-second century given the present rate of consumption. This implies that sooner or later humanity will rely heavily on renewable energy sources. Here we mode...

Observed surface air temperatures over the contiguous U.S. for the second half of the twentieth century showed a slight cooling over the southeastern part of the country, the so-called “warming hole,” while temperatures over the rest of the country warmed. This pattern reversed after 2000. Climate model simulations show that the disappearance of th...

The atmospheric teleconnections associated with the Eastern Pacific El Niño and El Niño Modoki events onto the tropical Atlantic Ocean are investigated. The Eastern Pacific El Niños drive significant warming of the tropical North Atlantic basin during boreal spring after its peak via the atmospheric bridge and tropospheric temperature mechanisms. H...

Of the rise in global atmospheric temperature over the past century, nearly 30% occurred between 1910 and 1940 when anthropogenic forcings were relatively weak(1). This early warming has been attributed to internal factors, such as natural climate variability in the Atlantic region, and external factors, such as solar variability and greenhouse gas...

An interdisciplinary approach to global change research is required for scientific advances that are both fundamental and relevant to real-world problems. The Aspen Global Change Institute (AGCI), under the leadership of director John Katzenberger, has provided global leadership for such interdisciplinary science over the past 25 years. From its fi...

It has been demonstrated that climate models initialized with observations produce better simulations of Pacific sea surface temperature (SST) patterns than uninitialized simulations for the two major climate regime changes of the last 40 years, the mid-1970s climate shift and early-2000s hiatus. A fundamental feature of these hindcasts is the simu...

The slowdown in the rate of global warming in the early 2000s is not evident in the multi-model ensemble average of traditional climate change projection simulations1. However, a number of individual ensemble members from that set of models successfully simulate the early-2000s hiatus when naturally-occurring climate variability involving the Inter...

Using the climate feedback response analysis method, the authors examine the individual contributions of the CO2 radiative forcing and climate feedbacks to the magnitude, spatial pattern, and seasonality of the transient surface warming response in a 1% yr−1 CO2 increase simulation of the NCAR Community Climate System Model, version 4 (CCSM4). The...

to uninitialized climate change projections, a multi-model ensemble from the CMIP5 10 year decadal prediction experiments produces more warming during the mid-1970s climate shift and less warming in the early 2000s hiatus in both the tropical Indo-Pacific region and globally averaged surface air temperature (TAS) in closer agreement with observatio...

Since 1995, the Coupled Model Intercomparison Project (CMIP) has coordinated climate model experiments involving multiple international modeling teams. Through CMIP, climate modelers and scientists from around the world have analyzed and compared state-of-the-art climate model simulations to gain insights into the processes, mechanisms, and consequ...

Previous studies have suggested that the status of the Bering Strait may have a significant influence on global climate variability on centennial, millennial, and even longer time scales. Here we use multiple versions of the National Center for Atmospheric Research (NCAR) Community Climate System Model (CCSM, versions 2 and 3) to investigate the in...

Despite ongoing increases in atmospheric greenhouse gases, the Earth’s global average surface air temperature has remained more or less steady since 2001. A variety of mechanisms have been proposed to account for this slowdown in surface warming. A key component of the global hiatus that has been identified is cool eastern Pacific sea surface tempe...

This paper provides an update on research in the relatively new and fast moving field of decadal climate prediction, and addresses the use of decadal climate predictions not only for potential users of such information but also for improving our understanding of processes in the climate system. External forcing influences the predictions throughout...

[1] When the PDO is in phase with the 11 year sunspot cycle there are positive SLP anomalies in the Gulf of Alaska, nearly no anomalous zonal SLP gradient across the equatorial Pacific, and a mix of small positive and negative SST anomalies there. When the two indices are out of phase, positive SLP anomalies extend farther south in the Gulf of Alas...

[1] Under a strong global warming scenario, the global mean temperature could rise up to 10°C, causing the global ocean conveyor belt to collapse and the summer sea ice to disappear. This will lead to profound changes in our climate system and to impact drastically the living conditions of the globe. Here we study how the global heat redistribution...

Adaptation requires science that analyzes decisions, identifies vulnerabilities, improves foresight, and develops options.

Heat waves are thought to result from subseasonal atmospheric variability. Atmospheric phenomena driven by tropical convection, such as the Asian monsoon, have been considered potential sources of predictability on subseasonal timescales. Mid-latitude atmospheric dynamics have been considered too chaotic to allow significant prediction skill of lea...

Previous studies have linked the rapid sea level rise (SLR) in the western tropical Pacific (WTP) since the early 1990s to the Pacific decadal climate modes, notably the Pacific Decadal Oscillation in the north Pacific or Interdecadal Pacific Oscillation (IPO) considering its basin wide signature. Here, the authors investigate the changing patterns...

Globally averaged surface air temperatures in some decades show rapid increases (accelerated warming decades), and in other decades there is no warming trend (hiatus decades). A previous study showed that the net energy imbalance at the top of the atmosphere of about 1Wm22 is associated with greater increases of deep ocean heat content below 750m d...

[1] A future Maunder Minimum type grand solar minimum, with total solar irradiance reduced by 0.25% over a 50 year period from 2020 to 2070, is imposed in a future climate change scenario experiment (RCP4.5) using, for the first time, a global coupled climate model that includes ozone chemistry and resolved stratospheric dynamics (Whole Atmosphere...

Evidence from observations indicates a net loss of global land-based ice and a rise of global sea level. Other than sea level rise, it is not clear how this loss of land-based ice could affect other aspects of global climate in the future. Here, the authors use the Community Climate System Model version 3 (CCSM3) to evaluate the potential influence...

Future climate change projections for phase 5 of the Coupled Model Intercomparison Project (CMIP5) are presented for the Community Earth System Model version 1 that includes the Community Atmospheric Model version 5 [CESM1(CAM5)]. These results are compared to the Community Climate System Model, version 4 (CCSM4) and include simulations using the r...

Decadal predictions have a high profile in the climate science community and beyond, yet very little is known about their skill. Nor is there any agreed protocol for estimating their skill. This paper proposes a sound and coordinated framework for verification of decadal hindcast experiments. The framework is illustrated for decadal hindcasts tailo...

Given the increases in spatial resolution and other improvements in climate modeling capabilities over the last decade since the CMIP3 simulations were completed, CMIP5 provides a unique opportunity to assess scientific understanding of climate variability and change over a range of historical and future conditions. With participation from over 20...

Case studies involving notable past decadal climate variability are analyzed for the mid-1970s climate shift, when the tropical Pacific warmed over a decade and globally averaged temperature rapidly increased, and the early 2000s hiatus when the tropical Pacific cooled over a decade and global temperatures warmed little. Ten year hindcasts followin...

The phase of the Interdecadal Pacific Oscillation (IPO) can influence the Tropospheric Biennial Oscillation (TBO) such that a shift from a negative to positive phase of the IPO was associated with a weakening of the TBO after the mid-1970s. Here it is shown that the most recent transition in the late 1990s of the IPO from positive to negative was a...

A linear trend calculated for observed annual mean surface air temperatures over the United States for the second-half of the twentieth century shows a slight cooling over the southeastern part of the country, the so-called warming hole, while temperatures over the rest of the country rose significantly. This east west gradient of average temperatu...

There is a common perception that, if human societies make the significant adjustments necessary to substantively cut emissions of greenhouse gases, global temperature increases could be stabilized, and the most dangerous consequences of climate change could be avoided. Here we show results from global coupled climate model simulations with the new...

An analysis of sea-level pressure anomalies at 14 sunspot peaks in the 11 year solar cycle in the Indian region in summer shows that the mean sea level pressure anomalies consist of relatively high pressure over land surrounded by low pressure anomalies over the sea. This signal is robust enough to appear when the data are divided into two segments...

This study uses an atmosphere-ocean fully coupled climate model to investigate possible remote impacts of Asian carbonaceous aerosols on US climate change. We took a 21st century mitigation scenario as a reference, and carried out three sets of sensitivity experiments in which the prescribed carbonaceous aerosol concentrations over a selected Asian...

To assess the climate impacts of historical and projected land cover change in the Community Climate System Model, version 4 (CCSM4), new time series of transient Community Land Model, version 4 (CLM4) plant functional type (PFT) and wood harvest parameters have been developed. The new parameters capture the dynamics of the Coupled Model Intercompa...

Abrupt climate transitions, known as Dansgaard-Oeschger and Heinrich events, occurred frequently during the last glacial period, specifically from 80–11 thousand years before present, but were nearly absent during interglacial periods and the early stages of glacial periods, when major ice-sheets were still forming. Here we show, with a fully coupl...

The simulation characteristics of theAsian-Australianmonsoon are documented for the Community Climate System Model, version 4 (CCSM4). This is the first part of a two part series examining monsoon regimes in the global tropics in theCCSM4.Comparisons aremade to anAtmosphericModel Intercomparison Project (AMIP) simulation of the atmospheric componen...

This is the second part of a two part series studying simulation characteristics of the Community Climate System Model, version 4 (CCSM4) for various monsoon regimes around the global tropics. Here, the West African, East African, North American, and South American monsoons are documented in CCSM4. Comparisons are made to an Atmospheric Model Inter...

Initial-value predictability measures the degree to which the initial state can influence predictions. In this paper, the initial-value predictability of six atmosphere-ocean general circulation models in the North Pacific and North Atlantic is quantified and contrasted by analyzing long control integrations with time invariant external conditions....

The El Nino/Southern Oscillation (ENSO) response to anthropogenic climate change is assessed in the following 1� nominal resolution CCSM4 CMIP5 simulations: 20th century ensemble, pre-industrial control, 21st century projections and stabilized 2100-2300 `extension runs'. ENSO variability weakens slightly with CO2; however, various significance test...

Paleo proxy data and previous modeling studies both indicate that the massive discharge of icebergs into the North Atlantic may have led to a (nearly) collapsed Atlantic meridional overturning circulation (AMOC), resulting in a seesaw-like climate change between the North Pacific and North Atlantic, with a warming in the former and a cooling in the...

Abstract Available from http://www.agu.org