Climate changes across the past 24,000 years provide key insights into Earth system responses to external forcing. Climate model simulations
and proxy data
have independently allowed for study of ...this crucial interval; however, they have at times yielded disparate conclusions. Here, we leverage both types of information using paleoclimate data assimilation
to produce the first proxy-constrained, full-field reanalysis of surface temperature change spanning the Last Glacial Maximum to present at 200-year resolution. We demonstrate that temperature variability across the past 24 thousand years was linked to two primary climatic mechanisms: radiative forcing from ice sheets and greenhouse gases; and a superposition of changes in the ocean overturning circulation and seasonal insolation. In contrast with previous proxy-based reconstructions
our results show that global mean temperature has slightly but steadily warmed, by ~0.5 °C, since the early Holocene (around 9 thousand years ago). When compared with recent temperature changes
, our reanalysis indicates that both the rate and magnitude of modern warming are unusual relative to the changes of the past 24 thousand years.
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GEOZS, IJS, IMTLJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBMB, UL, UM, UPUK, ZAGLJ
The Last Glacial Maximum (LGM), one of the best studied palaeoclimatic intervals, offers an excellent opportunity to investigate how the climate system responds to changes in greenhouse gases and the ...cryosphere. Previous work has sought to constrain the magnitude and pattern of glacial cooling from palaeothermometers.sup.1,2, but the uneven distribution of the proxies, as well as their uncertainties, has challenged the construction of a full-field view of the LGM climate state. Here we combine a large collection of geochemical proxies for sea surface temperature with an isotope-enabled climate model ensemble to produce a field reconstruction of LGM temperatures using data assimilation. The reconstruction is validated with withheld proxies as well as independent ice core and speleothem delta.sup.18O measurements. Our assimilated product provides a constraint on global mean LGM cooling of -6.1 degrees Celsius (95 per cent confidence interval: -6.5 to -5.7 degrees Celsius). Given assumptions concerning the radiative forcing of greenhouse gases, ice sheets and mineral dust aerosols, this cooling translates to an equilibrium climate sensitivity of 3.4 degrees Celsius (2.4-4.5 degrees Celsius), a value that is higher than previous LGM-based estimates but consistent with the traditional consensus range of 2-4.5 degrees Celsius.sup.3,4.
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FZAB, GEOZS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Plants influence extreme heat events by regulating land-atmosphere water and energy exchanges. The contribution of plants to changes in future heat extremes will depend on the responses of vegetation ...growth and physiology to the direct and indirect effects of elevated CO2. Here we use a suite of earth system models to disentangle the radiative versus vegetation effects of elevated CO2 on heat wave characteristics. Vegetation responses to a quadrupling of CO2 increase summer heat wave occurrence by 20 days or more-30-50% of the radiative response alone-across tropical and mid-to-high latitude forests. These increases are caused by CO2 physiological forcing, which diminishes transpiration and its associated cooling effect, and reduces clouds and precipitation. In contrast to recent suggestions, our results indicate CO2-driven vegetation changes enhance future heat wave frequency and intensity in most vegetated regions despite transpiration-driven soil moisture savings and increases in aboveground biomass from CO2 fertilization.
Modeling and paleoclimate proxy‐based studies suggest that equilibrium climate sensitivity (ECS) depends on the background climate state, though the reason is not thoroughly understood. Here we study ...the state dependence of ECS over a large range of global mean surface temperature (GMST) in the Community Atmosphere Model (CAM) Versions 4, 5, and 6 by varying atmospheric CO2 concentrations. We find a robust increase of ECS with GMST in all three models, albeit at different rates, which is primarily attributed to strengthening of the shortwave cloud feedback (λcld) at both high and low latitudes. Over high latitudes, increasing GMST leads to a reduction in the cloud ice fraction, weakening the (negative) cloud‐phase feedback due to the phase transition of cloud ice to liquid and thereby strengthening λcld. Over low‐latitude regions, increasing GMST strengthens λcld likely through the nonlinear increase in water vapor, which causes low‐cloud thinning through thermodynamic and radiative processes.
Plain Language Summary
Equilibrium climate sensitivity (ECS) is defined as the equilibrium increase in global mean temperature as a result of a doubling of atmospheric CO2 concentration. The latest assessment by the Intergovernmental Panel on Climate Change reported a likely ECS range of 1.5–4.5°C. Narrowing the ECS range is of paramount importance for prediction of future warming. Earth's surface has experienced prolonged periods of large magnitude warming in the geological past, which provide important empirical information on ECS. To quantitatively use the paleoclimate information, we need a complete understanding of how ECS may depend on the background climate. In this study, we investigate the physical mechanisms responsible for the state dependence of ECS using three climate models that have distinct model physics. In all three models, we find that ECS grows as the background climate warms; that is, a warmer climate is more sensitive to external forcing. We attribute the increase of ECS to both high‐ and low‐latitude cloud processes. Over high latitudes, cloud ice fraction decreases with global warming, weakening the potential for mixed‐phase clouds to reflect solar radiation and amplifying surface warming. Over low latitudes, global warming enhances the efficiency of processes that make clouds less opaque, again, amplifying surface warming.
Key Points
ECS increases with CO2‐induced global warming in CAM6, CAM5, and CAM4 and is primarily attributed to the strengthening of cloud feedback
High‐latitude λcld strengthens with warming due to a decrease of cloud ice fraction and a weakening of the negative cloud‐phase feedback
Low‐latitude λcld strengthening is linked to cloud thinning over subsidence regions likely caused by cloud interactions with water vapor
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The first major build-up of Antarctic glaciation occurred in two consecutive stages across the Eocene-Oligocene transition (EOT): the EOT-1 cooling event at ~34.1-33.9 Ma and the Oi-1 glaciation ...event at ~33.8-33.6 Ma. Detailed orbital-scale terrestrial environmental responses to these events remain poorly known. Here we present magnetic and geochemical climate records from the northeastern Tibetan Plateau margin that are dated precisely from ~35.5 to 31 Ma by combined magneto- and astro-chronology. These records suggest a hydroclimate transition at ~33.7 Ma from eccentricity dominated cycles to oscillations paced by a combination of eccentricity, obliquity, and precession, and confirm that major Asian aridification and cooling occurred at Oi-1. We conclude that this terrestrial orbital response transition coincided with a similar transition in the marine benthic δ
O record for global ice volume and deep-sea temperature variations. The dramatic reorganization of the Asian climate system coincident with Oi-1 was, thus, a response to coeval atmospheric CO
decline and continental-scale Antarctic glaciation.
The Early Eocene, a period of elevated atmospheric CO
(>1000 ppmv), is considered an analog for future climate. Previous modeling attempts have been unable to reproduce major features of Eocene ...climate indicated by proxy data without substantial modification to the model physics. Here, we present simulations using a state-of-the-art climate model forced by proxy-estimated CO
levels that capture the extreme surface warmth and reduced latitudinal temperature gradient of the Early Eocene and the warming of the Paleocene-Eocene Thermal Maximum. Our simulations exhibit increasing equilibrium climate sensitivity with warming and suggest an Eocene sensitivity of more than 6.6°C, much greater than the present-day value (4.2°C). This higher climate sensitivity is mainly attributable to the shortwave cloud feedback, which is linked primarily to cloud microphysical processes. Our findings highlight the role of small-scale cloud processes in determining large-scale climate changes and suggest a potential increase in climate sensitivity with future warming.
Recent rapid Arctic sea-ice reduction has been well documented in observations, reconstructions and model simulations. However, the rate of sea ice loss is highly variable in both time and space. The ...western Arctic has seen the fastest sea-ice decline, with substantial interannual and decadal variability, but the underlying mechanism remains unclear. Here we demonstrate, through both observations and model simulations, that the Pacific North American (PNA) pattern is an important driver of western Arctic sea-ice variability, accounting for more than 25% of the interannual variance. Our results suggest that the recent persistent positive PNA pattern has led to increased heat and moisture fluxes from local processes and from advection of North Pacific airmasses into the western Arctic. These changes have increased lower-tropospheric temperature, humidity and downwelling longwave radiation in the western Arctic, accelerating sea-ice decline. Our results indicate that the PNA pattern is important for projections of Arctic climate changes, and that greenhouse warming and the resultant persistent positive PNA trend is likely to increase Arctic sea-ice loss.
Physical process‐based two‐moment cloud microphysical parameterizations, in which effective cloud particle size evolves prognostically with climate change, have recently been incorporated into global ...climate models. The impacts of cloud particle‐size change on the cloud feedback, however, have never been explicitly quantified. Here we develop a partial radiative perturbation‐based method to estimate the cloud feedback associated with particle‐size changes in the Community Earth System Model. We find an increase of cloud particle size in the upper troposphere in response to an instantaneous doubling of atmospheric CO2. The associated net, shortwave, and longwave cloud feedbacks are estimated to be 0.18, 0.33, and −0.15 Wm−2 K−1, respectively. The cloud particle‐size feedback is dominated by its shortwave component with a maximum greater than 1.0 Wm−2 K−1 in the tropics and the Southern Ocean. We suggest that the cloud particle‐size feedback is an underappreciated contributor to the spread of cloud feedback and climate sensitivity among current models.
Plain Language Summary
Effects of clouds on Earth's radiation budget vary with their spatial and temporal distribution and their physical properties, including water content and its partitioning between liquid and ice, and cloud particle size. Changes in cloud distribution and physical properties can amplify or damp anthropogenic global warming and is the largest source of uncertainty in predictions of future climate. The simulation of cloud physical properties in climate models is limited due to a lack of understanding from theory and observations about what controls these properties. Recent progress has been made in some models to predict cloud particle sizes based on physical processes. In this study, we find an increase of cloud particle size in response to anthropogenic warming and estimate the resulting cloud radiative effects. The larger particles increase scattering of solar radiation in the downward direction leading to an amplification of surface warming. We suggest cloud particle‐size changes play a role in the large spread of warming in model predictions of future climate.
Key Points
Cloud particle size increases with warming in an Earth system model
The associated cloud particle‐size feedback is estimated to be 0.18, 0.33, and −0.15 Wm−2 K−1 for net, shortwave, and longwave components
Cloud particle‐size feedback is an underappreciated contributor to the spread of climate sensitivity in current models
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The percentage of oxygen in Earth's atmosphere varied between 10% and 35% throughout the Phanerozoic. These changes have been linked to the evolution, radiation, and size of animals but have not been ...considered to affect climate. We conducted simulations showing that modulation of the partical pressure of oxygen (pO2), as a result of its contribution to atmospheric mass and density, influences the optical depth of the atmosphere. Under low pO2 and a reduced-density atmosphere, shortwave scattering by air molecules and clouds is less frequent, leading to a substantial increase in surface shortwave forcing. Through feedbacks involving latent heat fluxes to the atmosphere and marine stratus clouds, surface shortwave forcing drives increases in atmospheric water vapor and global precipitation, enhances greenhouse forcing, and raises global surface temperature. Our results implicate pO2 as an important factor in climate forcing throughout geologic time.
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BFBNIB, NMLJ, NUK, PNG, SAZU, UL, UM, UPUK
Climate model simulations of the mid‐Holocene (MH) consistently underestimate northern African rainfall for reasons not fully understood. While most models incorporate orbital forcing and vegetation ...feedbacks, they neglect dust reductions associated with greater vegetation cover. Here we simulate the MH climate response to reduced Saharan dust using CESM CAM5‐chem, which resolves direct and indirect dust aerosol effects. Direct aerosol effects increase Saharan and Sahel convective rainfall by ~16% and 8%. In contrast, indirect aerosol effects decrease stratiform rainfall, damping the dust‐induced total rainfall increase by ~13% in the Sahara and ~59% in the Sahel. Sensitivity experiments indicate the dust‐induced precipitation anomaly in the Sahara and Sahel (0.27 and 0.18 mm/day) is smaller than the anomaly from MH vegetation cover (1.19 and 1.08 mm/day). Although sensitive to dust radiative properties, sea surface temperatures, and indirect aerosol effect parameterization, our results suggest that dust reductions had competing effects on MH African rainfall.
Plain Language Summary
Six thousand years ago, changes in Earth's orbit led to greater summer season solar radiation over northern Africa. The increase in energy resulted in higher rainfall amounts, widespread vegetation, and reduced dust aerosols over regions that today are desert. In this study we use a climate model, CESM CAM5‐chem, that accounts for the ways dust aerosols interact with sunlight and cloud droplets to examine how the reduction in Saharan dust during this past humid time affected rainfall. When dust aerosols are reduced in the model, more sunlight reaches the surface, the Sahara warms, and convective rainfall from the West African Monsoon increases. However, through dust‐cloud droplet interactions, the same reduction in dust decreases nonconvective rainfall, which is less prevalent during the monsoon season but still important, and thus dampens the total rainfall increase. Overall, dust reduction leads to a rainfall response that is dependent on rainfall type. Lastly, we compare the rainfall response of reducing dust to that of increasing vegetation cover and find that while important, the response from dust is considerably weaker.
Key Points
Changes in direct dust aerosol effects from reduced mid‐Holocene Saharan dust loading increase convective rainfall in northern Africa
Changes in indirect dust aerosol effects weaken total precipitation increases by limiting stratiform rainfall, particularly in the Sahel
The African rainfall response to total dust aerosol effects is lower than a previous study and substantially less than vegetation forcing
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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