Persistently active El Niño-Southern Oscillation since the Mesozoic.

The El Niño-Southern Oscillation (ENSO), originating in the central and eastern equatorial Pacific, is a defining mode of interannual climate variability with profound impact on global climate and ecosystems. However, an understanding of how the ENSO might have evolved over geological timescales is still lacking, despite a well-accepted recognition that such an understanding has direct implications for constraining human-induced future ENSO changes. Here, using climate simulations, we show that ENSO has been a leading mode of tropical sea surface temperature (SST) variability in the past 250 My but with substantial variations in amplitude across geological periods. We show this result by performing and analyzing a series of coupled time-slice climate simulations forced by paleogeography, atmospheric CO2 concentrations, and solar radiation for the past 250 My, in 10-My intervals. The variations in ENSO amplitude across geological periods are little related to mean equatorial zonal SST gradient or global mean surface temperature of the respective periods but are primarily determined by interperiod difference in the background thermocline depth, according to a linear stability analysis. In addition, variations in atmospheric noise serve as an independent contributing factor to ENSO variations across intergeological periods. The two factors together explain about 76% of the interperiod variations in ENSO amplitude over the past 250 My. Our findings support the importance of changing ocean vertical thermal structure and atmospheric noise in influencing projected future ENSO change and its uncertainty.

Elevated atmospheric CO2 drove an increase in tropical cyclone intensity during the early Toarcian hyperthermal.

The occurrence of sedimentary storm deposits around the Tethys Ocean during the early Toarcian hyperthermal (~183 Ma) suggests that intensified tropical cyclone (TC) activity occurred in response to CO2 rise and marked warming. However, this hypothesized linkage between extreme warmth and storm activity remains untested, and the spatial pattern of any changes in TCs is unclear. Here, model results show that there were two potential storm genesis centers over Tethys during the early Toarcian hyperthermal located around the northwestern and southeastern Tethys. The empirically determined doubling of CO2 concentration that accompanied the early Toarcian hyperthermal (~500 to ~1,000 ppmv) leads to increased probability of stronger storms over Tethys, in tandem with more favorable conditions for coastal erosion. These results match well with the geological occurrence of storm deposits during the early Toarcian hyperthermal and confirm that increased TC intensity would have accompanied global warming.

Coupled atmospheric chemistry, radiation, and dynamics of an exoplanet generate self-sustained oscillations.

Nonlinearity in photochemical systems is known to allow self-sustained oscillations, but they have received little attention in studies of planetary atmospheres. Here, we present a unique, self-oscillatory solution for ozone chemistry of an exoplanet from a numerical simulation using a fully coupled, three-dimensional (3D) atmospheric chemistry-radiation-dynamics model. Forced with nonvarying stellar insolation and emission flux of nitric oxide (NO), atmospheric ozone abundance oscillates by a factor of thirty over a multidecadal timescale. As such self-oscillations can only occur with biological nitrogen fixation contributing to NO emission, we propose that they are a unique class of biosignature. The resulting temporal variability in the atmospheric spectrum is potentially observable. Our results underscore the importance of revisiting the spectra of exoplanets over multidecadal timescales to characterizing the atmospheric chemistry of exoplanets and searching for exoplanet biosignatures. There are also profound implications for comparative planetology and the evolution of the atmospheres of terrestrial planets in the solar system and beyond. Fully coupled, 3D atmospheric chemistry-radiation-dynamics models can reveal new phenomena that may not exist in one-dimensional models, and hence, they are powerful tools for future planetary atmospheric research.

Role of ocean heat transport in climates of tidally locked exoplanets around M dwarf stars.

The distinctive feature of tidally locked exoplanets is the very uneven heating by stellar radiation between the dayside and nightside. Previous work has focused on the role of atmospheric heat transport in preventing atmospheric collapse on the nightside for terrestrial exoplanets in the habitable zone around M dwarfs. In the present paper, we carry out simulations with a fully coupled atmosphere-ocean general circulation model to investigate the role of ocean heat transport in climate states of tidally locked habitable exoplanets around M dwarfs. Our simulation results demonstrate that ocean heat transport substantially extends the area of open water along the equator, showing a lobster-like spatial pattern of open water, instead of an "eyeball." For sufficiently high-level greenhouse gases or strong stellar radiation, ocean heat transport can even lead to complete deglaciation of the nightside. Our simulations also suggest that ocean heat transport likely narrows the width of M dwarfs' habitable zone. This study provides a demonstration of the importance of exooceanography in determining climate states and habitability of exoplanets.

Sources and Processes Affecting Fine Particulate Matter Pollution over North China: An Adjoint Analysis of the Beijing APEC Period.

The stringent emission controls during the APEC 2014 (the Asia-Pacific Economic Cooperation Summit; November 5-11, 2014) offer a unique opportunity to quantify factors affecting fine particulate matter (PM2.5) pollution over North China. Here we apply a four-dimensional variational data assimilation system using the adjoint model of GEOS-Chem to address this issue. Hourly surface measurements of PM2.5 and SO2 for October 15-November 14, 2014 are assimilated into the model to optimize daily aerosol primary and precursor emissions over North China. Measured PM2.5 concentrations in Beijing average 50.3 µg m(-3) during APEC, 43% lower than the mean concentration (88.2 µg m(-3)) for the whole period including APEC. Model results attribute about half of the reduction to meteorology due to active cold surge occurrences during APEC. Assimilation of surface measurements largely reduces the model biases and estimates 6%-30% lower aerosol emissions in the Beijing-Tianjin-Hebei region during APEC than in late October. We further demonstrate that high PM2.5 events in Beijing during this period can be occasionally contributed by natural mineral dust, but more events show large sensitivities to inorganic aerosol sources, particularly emissions of ammonia (NH3) and nitrogen oxides (NOx) reflecting strong formation of aerosol nitrate in the fall season.

Reducing spread in climate model projections of a September ice-free Arctic.

This paper addresses the specter of a September ice-free Arctic in the 21st century using newly available simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5). We find that large spread in the projected timing of the September ice-free Arctic in 30 CMIP5 models is associated at least as much with different atmospheric model components as with initial conditions. Here we reduce the spread in the timing of an ice-free state using two different approaches for the 30 CMIP5 models: (i) model selection based on the ability to reproduce the observed sea ice climatology and variability since 1979 and (ii) constrained estimation based on the strong and persistent relationship between present and future sea ice conditions. Results from the two approaches show good agreement. Under a high-emission scenario both approaches project that September ice extent will drop to ∼1.7 million km(2) in the mid 2040s and reach the ice-free state (defined as 1 million km(2)) in 2054-2058. Under a medium-mitigation scenario, both approaches project a decrease to ∼1.7 million km(2) in the early 2060s, followed by a leveling off in the ice extent.

The super-cycle of the global land-monsoon system.

The global land-monsoon system demonstrates super-cycles in both area and precipitation intensity in the Phanerozoic. The cyclicity of the global land-monsoon closely follows the supercontinent cycle, indicating the governance of continental configurations on the global land-monsoon over tectonic timescales.

Competing climate feedbacks of ice sheet freshwater discharge in a warming world.

Freshwater discharge from ice sheets induces surface atmospheric cooling and subsurface ocean warming, which are associated with negative and positive feedbacks respectively. However, uncertainties persist regarding these feedbacks' relative strength and combined effect. Here we assess associated feedbacks in a coupled ice sheet-climate model, and show that for the Antarctic Ice Sheet the positive feedback dominates in moderate future warming scenarios and in the early stage of ice sheet retreat, but is overwhelmed by the negative feedback in intensive warming scenarios when the West Antarctic Ice Sheet undergoes catastrophic collapse. The Atlantic Meridional Overturning Circulation is affected by freshwater discharge from both the Greenland and the Antarctic ice sheets and, as an interhemispheric teleconnection bridge, exacerbates the opposing ice sheet's retreat via the Bipolar Seesaw. These results highlight the crucial role of ice sheet-climate interactions via freshwater flux in future ice sheet retreat and associated sea-level rise.

Band-by-band spectral radiative kernels based on the ERA5 reanalysis.

Radiative kernel is a widely adopted method for diagnosing radiation variability and climate feedback. However, most of the existing radiative kernels are broadband flux kernels and lack the spectral information. Motivated by the growing interest in the spectral changes of the Earth radiation budget, we generate a new set of band-by-band radiative kernels based on the fifth generation European Center for Medium-Range Weather Forecasts (ERA5) reanalysis, which can be used for analyzing the spectrally decomposed changes in the top of atmosphere, surface and atmospheric radiation. The radiative sensitivity quantified by the ERA5 band-by-band kernel is compared to another spectral kernel and validated in a spectral radiation closure test. The use and benefits of the new ERA5 kernels are then demonstrated in an analysis of spectral feedbacks of an ensemble of global climate models (GCMs).

Hydrologic cycle weakening in hothouse climates.

The hydrologic cycle has wide impacts on the ocean salinity and circulation, carbon and nitrogen cycles, and the ecosystem. Under anthropogenic global warming, previous studies showed that the intensification of the hydrologic cycle is a robust feature. Whether this trend persists in hothouse climates, however, is unknown. Here, we show in climate models that mean precipitation first increases with rising surface temperature, but the precipitation trend reverses when the surface is hotter than ~320 to 330 kelvin. This nonmonotonic phenomenon is robust to the cause of warming, convection scheme, ocean dynamics, atmospheric mass, planetary rotation, gravity, and stellar spectrum. The weakening occurs because of the existence of an upper limitation of outgoing longwave emission and the continuously increasing shortwave absorption by H2O and is consistent with atmospheric dynamics featuring the strong increase of atmospheric stratification and marked reduction of convective mass flux. These results have wide implications for the climate evolutions of Earth, Venus, and potentially habitable exoplanets.

Spatial continuous modeling of early Cenozoic carbon cycle and climate.

A real spatial continuous modeling of climate and carbon cycle is developed, and tested for early Cenozoic from 60 Ma to 40 Ma.

Controlling factors for the global meridional overturning circulation: A lesson from the Paleozoic.

The global meridional overturning circulation (GMOC) is important for redistributing heat and, thus, determining global climate, but what determines its strength over Earth's history remains unclear. On the basis of two sets of climate simulations for the Paleozoic characterized by a stable GMOC direction, our research reveals that GMOC strength primarily depends on continental configuration while climate variations have a minor impact. In the mid- to high latitudes, the volume of continents largely dictates the speed of westerly winds, which in turn controls upwelling and the strength of the GMOC. At low latitudes, open seaways also play an important role in the strength of the GMOC. An open seaway in one hemisphere allows stronger westward ocean currents, which support higher sea surface heights (SSH) in this hemisphere than that in the other. The meridional SSH gradient drives a stronger cross-equatorial flow in the upper ocean, resulting in a stronger GMOC. This latter finding enriches the current theory for GMOC.

The Hadley circulation in the Pangea era.

The Pangea era is an exceptional phase in Earth's history. It is characterized by its hothouse climate state and the latest supercontinent. Thus, it is expected that atmospheric circulation in the Pangea era was largely different from that of the modern world. Here, we study the Hadley circulation in the Pangea era and compare it with that of the present, by performing climate simulations. Our results show that the annual mean Hadley cells are about 20% and 45% weaker than that in the pre-industrial (PI) climate, and their poleward edges are about 2° wider in latitude. The austral winter cell is weakened by 27% and expanded by 2.6°, while the changes of the boreal winter cell are not significant. One distinctive feature is that the ascending branches of the boreal and austral winter cells shift to 23°S and 18°N, respectively, which are much more poleward than their present locations. Our analyses demonstrate that the weakening and widening of the Hadley circulation is due to increasing tropical and subtropical static stability, and that the poleward shifts of the ascending branches of the winter cells are associated with the geographic configuration of the supercontinent Pangea.

Continental drift shifts tropical rainfall by altering radiation and ocean heat transport.

Shifts in the position of the intertropical convergence zone (ITCZ) have great importance for weather, climate, and society. The ITCZ shifts have been extensively studied in current and future warmer climate; however, little is known for its migration in the past on geological time scales. Using an ensemble of climate simulations over the past 540 million years, we show that ITCZ migrations are controlled primarily by continental configuration through two competing pathways: hemispheric radiation asymmetry and cross-equatorial ocean heat transport. The hemispheric asymmetry of absorbed solar radiation is produced mainly by land-ocean albedo contrast, which can be predicted using only the landmass distribution. The cross-equatorial ocean heat transport is strongly associated with the hemispheric asymmetry of surface wind stress, which is, in turn, controlled by the hemispheric asymmetry of ocean surface area. These results allow the influence of continental evolution on global ocean-atmosphere circulations to be understood through simple mechanisms that depend primarily on the latitudinal distribution of land.

Quantifying climate conditions for the formation of coals and evaporites.

Coals and evaporites are commonly used as qualitative indicators of wet and dry environments in deep-time climate studies, respectively. Here, we combine geological records with climate simulations to establish quantitative relationships of coals and evaporites with temperature and precipitation over the Phanerozoic. We show that coal records were associated with a median temperature of 25°C and precipitation of 1300 mm yr-1 before 250 Ma. Afterwards, coal records appeared with temperatures between 0°C and 21°C and precipitation of 900 mm yr-1. Evaporite records were associated with a median temperature of 27°C and precipitation of 800 mm yr-1. The most remarkable result is that net precipitation associated with coal and evaporite records remained constant across time. The results here have important implications for quantifying climate conditions for other lithologic indicators of climate and for predicting exogenetic ore deposits.

Arctic sea-ice loss is projected to lead to more frequent strong El Niño events.

Arctic sea ice has decreased substantially and is projected to reach a seasonally ice-free state in the coming decades. Little is known about whether dwindling Arctic sea ice is capable of influencing the occurrence of strong El Niño, a prominent mode of climate variability with global impacts. Based on time slice coupled model experiments, here we show that no significant change in the occurrence of strong El Niño is found in response to moderate Arctic sea-ice loss that is consistent with satellite observations to date. However, as the ice loss continues and the Arctic becomes seasonally ice-free, the frequency of strong El Niño events increases by more than one third, as defined by gradient-based indices that remove mean tropical Pacific warming induced by the seasonally ice-free Arctic. By comparing our time slice experiments with greenhouse warming experiments, we conclude that at least 37-48% of the increase of strong El Niño near the end of the 21st century is associated specifically with Arctic sea-ice loss. Further separation of Arctic sea-ice loss and greenhouse gas forcing only experiments implies that the seasonally ice-free Arctic might play a key role in driving significantly more frequent strong El Niño events.

Milankovitch theory and monsoon.

The widely accepted "Milankovitch theory" explains insolation-induced waxing and waning of the ice sheets and their effect on the global climate on orbital timescales. In the past half century, however, the theory has often come under scrutiny, especially regarding its "100-ka problem." Another drawback, but the one that has received less attention, is the "monsoon problem," which pertains to the exclusion of monsoon dynamics in classic Milankovitch theory even though the monsoon prevails over the vast low-latitude (∼30° N to ∼30° S) region that covers half of the Earth's surface and receives the bulk of solar radiation. In this review, we discuss the major issues with the current form of Milankovitch theory and the progress made at the research forefront. We suggest shifting the emphasis from the ultimate outcomes of the ice volume to the causal relationship between changes in northern high-latitude insolation and ice age termination events (or ice sheet melting rate) to help reconcile the classic "100-ka problem." We discuss the discrepancies associated with the characterization of monsoon dynamics, particularly the so-called "sea-land precession-phase paradox" and the "Chinese 100-ka problem." We suggest that many of these discrepancies are superficial and can be resolved by applying a holistic "monsoon system science" approach. Finally, we propose blending the conventional Kutzbach orbital monsoon hypothesis, which calls for summer insolation forcing of monsoons, with Milankovitch theory to formulate a combined "Milankovitch-Kutzbach hypothesis" that can potentially explain the dual nature of orbital hydrodynamics of the ice sheet and monsoon systems, as well as their interplays and respective relationships with the northern high-latitude insolation and inter-tropical insolation differential.

A high-resolution climate simulation dataset for the past 540 million years.

The Phanerozoic Eon has witnessed considerable changes in the climate system as well as abundant animals and plant life. Therefore, the evolution of the climate system in this Eon is worthy of extensive research. Only by studying climate changes in the past can we understand the driving mechanisms for climate changes in the future and make reliable climate projections. Apart from observational paleoclimate proxy datasets, climate simulations provide an alternative approach to investigate past climate conditions of the Earth, especially for long time span in the deep past. Here we perform 55 snapshot simulations for the past 540 million years, with a 10-million-year interval, using the Community Earth System Model version 1.2.2 (CESM1.2.2). The climate simulation dataset includes global distributions of monthly surface temperatures and precipitation, with a 1° horizontal resolution of 0.9° × 1.25° in latitude and longitude. This open access climate dataset is useful for multidisciplinary research, such as paleoclimate, geology, geochemistry, and paleontology.

Comparison of trends in the Hadley circulation between CMIP6 and CMIP5.

There have been extensive studies on poleward expansion of the Hadley cells and the associated poleward shift of subtropical dry zones in the past decade. In the present study, we study the trends in the width and strength of the Hadley cells, using currently available simulation results of the Coupled Model Intercomparison Project Phase-6 (CMIP6), and compare the trends with that in CMIP5 simulations. Our results show that the total annual-mean trend in the width of the Hadley cells is 0.13° ± 0.02° per decade over 1970-2014 in CMIP6 historical All-forcing simulations. It is almost the same as that in CMIP5. The trend in the strength of the Northern-Hemisphere (NH) cell shows much greater weakening in CMIP6 than in CMIP5, while the strength trend in the Southern-Hemisphere (SH) cell shows slight strengthening. Single-forcing simulations demonstrate that increasing greenhouse gases cause widening and weakening of both the NH and SH Hadley cells, while anthropogenic aerosols and stratospheric ozone changes cause weak strengthening trends in the SH cell. CMIP6 projection simulation results show that both the widening and weakening trends increase with radiative forcing.

Large equatorial seasonal cycle during Marinoan snowball Earth.

In the equatorial regions on Earth today, the seasonal cycle of the monthly mean surface air temperature is <10°C. However, deep (>1 m) sand wedges were found near the paleoequator in the Marinoan glaciogenic deposits at ~635 million years ago, indicating a large seasonal cycle (probably >30°C). Through numerical simulations, we show that the equatorial seasonal cycle could reach >30°C at various continental locations if the oceans are completely frozen over, as would have been the case for a snowball Earth, or could reach ~20°C if the oceans are not completely frozen over, as would have been the case for a waterbelt Earth. These values are obtained at the maximum eccentricity of the Earth orbit, i.e., 0.0679, and will be approximately 10°C smaller if the present-day eccentricity is used. For these seasonal cycles, theoretical calculations show that the deep sand wedges form readily in a snowball Earth while hardly form in a waterbelt Earth.