On interannual to decadal time scales, the climate mode with many of the strongest societal impacts is the El Niño–Southern Oscillation (ENSO). However, quantifying ENSO's changes in a warming ...climate remains a formidable challenge, due to both the noise arising from internal variability and the complexity of air‐sea feedbacks in the tropical Pacific Ocean. In this work, we use large (≥30‐member) ensembles of climate simulations to show that anthropogenic climate change can produce systematic increases in ENSO teleconnection strength over many land regions, driving increased interannual variability in regional temperature extremes and wildfire frequency. As the spatial character of this intensification exhibits strong land‐ocean contrasts, a causal role for land‐atmosphere feedbacks is suggested. The identified increase in variance occurs in multiple model ensembles, independent of changes in sea surface temperature variance. This suggests that in addition to changes in the overall likelihoods of heat and wildfire extremes, the variability in these events may also be a robust feature of future climate.
Plain Language Summary
Changes in climate variability strongly affect the overall impacts of climate change. In this work, increases in the intensity of heat waves and wildfire driven by El Niño/La Niña in a business‐as‐usual climate scenario are identified in recently produced climate simulations spanning the 20th and 21st centuries. The intensification in temperature extremes occurs mainly over land regions and independently of changes in eastern Pacific sea surface temperature variability. It is argued that land atmosphere feedbacks are likely to play a key role in the simulated amplification, with relevance to impacts such as heat waves and wildfire frequency.
Key Points
Intensity increases in temperature and wildfire extremes driven by ENSO in a warming climate are identified in climate model large ensembles
The intensification occurs mainly over land regions and is influenced by precipitation
Land‐atmosphere feedbacks are likely to play a key role in the projected amplification
As the world warms due to rising greenhouse gas concentrations, the Earth systemmoves toward climate states without societal precedent, challenging adaptation. Past Earth system states offer possible ...model systems for the warming world of the coming decades. These include the climate states of the Early Eocene (ca. 50 Ma), the Mid-Pliocene (3.3–3.0 Ma), the Last Interglacial (129–116 ka), the Mid-Holocene (6 ka), preindustrial (ca. 1850 CE), and the 20th century. Here, we quantitatively assess the similarity of future projected climate states to these six geohistorical benchmarks using simulations from the Hadley Centre Coupled Model Version 3 (HadCM3), the Goddard Institute for Space Studies Model E2-R (GISS), and the Community Climate System Model, Versions 3 and 4 (CCSM) Earth system models. Under the Representative Concentration Pathway 8.5 (RCP8.5) emission scenario, by 2030 CE, future climates most closely resemble Mid-Pliocene climates, and by 2150 CE, they most closely resemble Eocene climates. Under RCP4.5, climate stabilizes at Pliocene-like conditions by 2040 CE. Pliocene-like and Eocene-like climates emerge first in continental interiors and then expand outward. Geologically novel climates are uncommon in RCP4.5 (<1%) but reach 8.7% of the globe under RCP8.5, characterized by high temperatures and precipitation. Hence, RCP4.5 is roughly equivalent to stabilizing at Pliocene-like climates, while unmitigated emission trajectories, such as RCP8.5, are similar to reversing millions of years of long-term cooling on the scale of a few human generations. Both the emergence of geologically novel climates and the rapid reversion to Eocene-like climates may be outside the range of evolutionary adaptive capacity.
The El Niño Southern Oscillation (ENSO) is Earth's dominant source of interannual climate variability, but its response to global warming remains highly uncertain. To improve our understanding of ...ENSO's sensitivity to external climate forcing, it is paramount to determine its past behaviour by using palaeoclimate data and model simulations. Palaeoclimate records show that ENSO has varied considerably since the Last Glacial Maximum (21,000 years ago), and some data sets suggest a gradual intensification of ENSO over the past ∼6,000 years. Previous attempts to simulate the transient evolution of ENSO have relied on simplified models or snapshot experiments. Here we analyse a series of transient Coupled General Circulation Model simulations forced by changes in greenhouse gasses, orbital forcing, the meltwater discharge and the ice-sheet history throughout the past 21,000 years. Consistent with most palaeo-ENSO reconstructions, our model simulates an orbitally induced strengthening of ENSO during the Holocene epoch, which is caused by increasing positive ocean-atmosphere feedbacks. During the early deglaciation, ENSO characteristics change drastically in response to meltwater discharges and the resulting changes in the Atlantic Meridional Overturning Circulation and equatorial annual cycle. Increasing deglacial atmospheric CO2 concentrations tend to weaken ENSO, whereas retreating glacial ice sheets intensify ENSO. The complex evolution of forcings and ENSO feedbacks and the uncertainties in the reconstruction further highlight the challenge and opportunity for constraining future ENSO responses.
We conducted the first synchronously coupled atmosphere-ocean general circulation model simulation from the Last Glacial Maximum to the Bølling-Allerød (BA) warming. Our model reproduces several ...major features of the deglacial climate evolution, suggesting a good agreement in climate sensitivity between the model and observations. In particular, our model simulates the abrupt BA warming as a transient response of the Atlantic meridional overturning circulation (AMOC) to a sudden termination of freshwater discharge to the North Atlantic before the BA. In contrast to previous mechanisms that invoke AMOC multiple equilibrium and Southern Hemisphere climate forcing, we propose that the BA transition is caused by the superposition of climatic responses to the transient CO₂ forcing, the AMOC recovery from Heinrich Event 1, and an AMOC overshoot.
Aerosol‐climate interactions constitute one of the major sources of uncertainty in assessing changes in aerosol forcing in the anthropocene as well as understanding glacial‐interglacial cycles. Here ...we focus on improving the representation of mineral dust in the Community Atmosphere Model and assessing the impacts of the improvements in terms of direct effects on the radiative balance of the atmosphere. We simulated the dust cycle using different parameterization sets for dust emission, size distribution, and optical properties. Comparing the results of these simulations with observations of concentration, deposition, and aerosol optical depth allows us to refine the representation of the dust cycle and its climate impacts. We propose a tuning method for dust parameterizations to allow the dust module to work across the wide variety of parameter settings which can be used within the Community Atmosphere Model. Our results include a better representation of the dust cycle, most notably for the improved size distribution. The estimated net top of atmosphere direct dust radiative forcing is −0.23 ± 0.14 W/m2 for present day and −0.32 ± 0.20 W/m2 at the Last Glacial Maximum. From our study and sensitivity tests, we also derive some general relevant findings, supporting the concept that the magnitude of the modeled dust cycle is sensitive to the observational data sets and size distribution chosen to constrain the model as well as the meteorological forcing data, even within the same modeling framework, and that the direct radiative forcing of dust is strongly sensitive to the optical properties and size distribution used.
Key Points
Refined physical parameterizations of dust in the Community Atmosphere Model
Improved soil erodibility, size distributions, wet deposition, and optics
Better representation of dust cycle, size distributions, and radiative forcing
Because of the pervasive role of water in the Earth system, the relative abundances of stable isotopologues of water are valuable for understanding atmospheric, oceanic, and biospheric processes, and ...for interpreting paleoclimate proxy reconstructions. Isotopologues are transported by both largescale and turbulent flows, and the ratio of heavy to light isotopologues changes due to fractionation that can accompany condensation and evaporation processes. Correctly predicting the isotopic distributions requires resolving the relationships between largescale ocean and atmospheric circulation and smallerscale hydrological processes, which can be accomplished within a coupled climate modeling framework. Here we present the water isotopeenabled version of the Community Earth System Model version 1 (iCESM1), which simulates global variations in water isotopic ratios in the atmosphere, land, ocean, and sea ice. In a transient Last Millennium simulation covering the 850-2005 period, iCESM1 correctly captures the latetwentiethcentury structure of δ(exp 18)O and δD over the global oceans, with more limited accuracy over land. The relationship between salinity and seawater δ(exp 18)O is also well represented over the observational period, including interbasin variations. We illustrate the utility of coupled, isotopeenabled simulations using both Last Millennium simulations and freshwater hosing experiments with iCESM1. Closing the isotopic mass balance between all components of the coupled model provides new confidence in the underlying depiction of the water cycle in CESM, while also highlighting areas where the underlying hydrologic balance can be improved. The iCESM1 is poised to be a vital community resource for ongoing model development with both modern and paleoclimate applications.
Speleothem records in southeastern China provide key evidence for past environmental changes. However, the climatic interpretation of these proxies has remained a great controversy. Earlier work ...interprets the cave δ18O signal associated with regional rainfall of the East Asia Summer Monsoon (EASM) or monsoon rainfall upstream of China. Recent isotope modeling supports the latter but show little correspondence between the precipitation δ18O and rainfall in China. Here, we examine the evolution of the climate and precipitation δ18O for the last 21,000 years in models and observations. Recognizing the regional difference of the EASM rainfall, we propose an interpretation of the Chinese δ18O record that reconciles its representativeness of EASM and its driving mechanism of upstream depletion. The δ18O records do represent the intensity of the EASM system. The monsoon intensity is best characterized by enhanced southerly monsoon winds, which correlate strongly with negative δ18O over China and enhanced monsoon rainfall in northern China, as well as the continental scale Asian monsoon rainfall response in the upstream regions.
•Chinese cave δ18O represents the East Asia Summer Monsoon.•Representation of monsoon winds and rainfall in northern China.•Transient and isotope simulation of the last 21,000 years.•Model-data comparison of the EASM of the last 21,000 years.
The ocean thermohaline circulation is important for transports of heat and the carbon cycle. We present results from PMIP2 coupled atmosphere‐ocean simulations with four climate models that are also ...being used for future assessments. These models give very different glacial thermohaline circulations even with comparable circulations for present. An integrated approach using results from these simulations for Last Glacial Maximum (LGM) with proxies of the state of the glacial surface and deep Atlantic supports the interpretation from nutrient tracers that the boundary between North Atlantic Deep Water and Antarctic Bottom Water was much shallower during this period. There is less constraint from this integrated reconstruction regarding the strength of the LGM North Atlantic overturning circulation, although together they suggest that it was neither appreciably stronger nor weaker than modern. Two model simulations identify a role for sea ice in both hemispheres in driving the ocean response to glacial forcing.
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