Effective radiative forcing of anthropogenic aerosols in E3SMv1: historical changes, causality, decomposition, and parameterization sensitivities

The effective radiative forcing of anthropogenic aerosols (ERFaer) is an important measure of the anthropogenic aerosol effects simulated by a global climate model. Here we analyze ERFaer simulated by the E3SMv1 atmosphere model using both century-long free-running atmosphere-land simulations and short nudged simulations. We relate the simulated ERFaer to characteristics of the aerosol composition and optical properties, and evaluate the relationships between key aerosol and cloud properties. In terms of historical changes from the year 1870 to 2014, our results show that the global mean anthropogenic aerosol burden and optical depth increase during the simulation period as expected, but the regional averages show large differences in the temporal evolution. The largest regional differences are found in the emission-induced evolution of the burden and optical depth of the sulfate aerosol: a strong decreasing trend is seen in the Northern Hemisphere high-latitude region after around 1970, while a continued increase is simulated in the tropics. Consequently, although the global mean anthropogenic aerosol burden and optical depth increase from 1870 to 2014, the ERFaer magnitude does not increase after around year 1970. The relationships between key aerosol and cloud properties (relative changes between preindustrial and present-day conditions) also show evident changes after 1970, diverging from the linear relationships exhibited for the period from 1870 to 2014. The ERFaer in E3SMv1 is relatively large compared to the recently published multi-model estimates; the primary reason is the large indirect aerosol effect (i.e., through aerosol-cloud interactions). Compared to other models, E3SMv1 features a stronger sensitivity of the cloud droplet effective radius to changes in the cloud droplet number concentration. Large sensitivity is also seen in the liquid cloud optical depth, which is determined by changes in both the effective radius and liquid water path. Aerosol-induced changes in liquid and ice cloud properties in E3SMv1 are found to have a strong correlation, as the evolution of anthropogenic sulfate aerosols affects both the liquid cloud formation and the homogeneous ice nucleation in cirrus clouds. The ERFaer estimates in E3SMv1 for the shortwave and longwave components are sensitive to the parameterization changes in both liquid and ice cloud processes. When the parameterization of ice cloud processes is modified, the top-of-atmosphere forcing changes in the shortwave and longwave components largely offset each other, so the net effect is negligible. This suggests that, to reduce the magnitude of the net ERFaer, it would be more effective to reduce the anthropogenic aerosol effect through liquid or mixed-phase clouds.

Does disabling cloud radiative feedbacks change spatial patterns of surface greenhouse warming and cooling?

The processes controlling idealized warming and cooling patterns are examined in 150 year-long fully coupled Community Earth System Model version 1 (CESM1) experiments under abrupt CO2 forcing. By simulation end, 2xCO2 global warming was 20% larger than 0.5xCO2 global cooling. Not only was the absolute global effective radiative forcing ∼10% larger for 2xCO2 than for 0.5xCO2, global feedbacks were also less negative for 2xCO2 than for 0.5xCO2. Specifically, more positive shortwave cloud feedbacks led to more 2xCO2 global warming than 0.5xCO2 global cooling. Over high latitude oceans, differences between 2xCO2 warming and 0.5xCO2 cooling were amplified by familiar linked positive surface albedo and lapse rate feedbacks associated with sea ice change. At low latitudes, 2xCO2 warming exceeded 0.5xCO2 cooling almost everywhere. Tropical Pacific cloud feedbacks amplified: 1) more fast warming than fast cooling in the west, 2) slow pattern differences between 2xCO2 warming and 0.5xCO2 cooling in the east. Motivated to quantify cloud influence, a companion suite of experiments were run without cloud radiative feedbacks. Disabling cloud radiative feedbacks reduced the effective radiative forcing and surface temperature responses for both 2xCO2 and 0.5xCO2. Notably, 20% more global warming than global cooling occurred regardless of whether cloud feedbacks were enabled or disabled. This surprising consistency resulted from the cloud influence on non-cloud feedbacks and circulation. With the exception of the Tropical Pacific, disabling cloud feedbacks did little to change surface temperature response patterns including the large high-latitude responses driven by non-cloud feedbacks. The findings provide new insights into the regional processes controlling the response to greenhouse gas forcing, especially for clouds.

Auswirkungen von Halogen-induzierten stratosphärischen Ozonänderungen auf den effektiven Strahlungsantrieb und das Oberflächenklima

<p>   Emissionen von anthropogenen FCKW wurden infolge des Montrealer Abkommens von 1987 stark reduziert und entsprechend wird für das 21. Jahrhundert eine Erholung der Ozonschicht erwartet. Eine Änderung der Ozonkonzentration in der Stratosphäre verändert die Energiebilanz der Erde und wird, im Vergleich zum heutigen Tag, zu einem positiven Strahlungsantrieb führen. Daraus resultieren sowohl global als auch regional erwärmende oder abkühlende Einflüsse auf das Klima.</p> <p>    Der effektive Strahlungsantrieb (Effective radiative forcing - ERF) ist definiert als Änderung der Strahlungsflüsse am oberen Rand der Atmosphäre durch bestimmte „Treiber“, unter Berücksichtigung von Rückkopplungen des Klimasystems auf kurzen Zeitskalen, während Rückkopplungen auf langen Zeitskalen (insbesondere die Meeresoberflächentemperaturen) konstant gehalten werden.Einige Studien untersuchten bereits das ERF von troposphärischen Ozonäderungen, nur wenig ist aber bekannt über den Einfluss von stratosphärischem Ozonänderungen auf den effektiven Strahlungsantrieb und dessen Auswirkung auf das Oberflächenklima.</p> <p>    Um die Entwicklung der Ozonschicht und damit einhergehende Klimaänderung in die Zukunft zu projizieren führten wir so genannte Time-Slice Simulationen mit dem Klimamodell ICON-ART durch. Stratosphärische Ozonänderungen wurden mit dem linearisierten Ozonschema (LINOZ) berechnet, mit einem zusätzlichen Verlustterm, um die katalytische Ozonzerstörung in Polarregionen zu berücksichtigen. Das modellierte Ozon war interaktiv und mit der Strahlung gekoppelt.</p> <p>   In der Standardsimulation werden Meeresoberflächentemperatur und Meer-Eisbedeckung, Aerosole und Treibhausgase entsprechend für das Jahr 2000 fixiert. Für Sensitivitätssimulationen verwenden wir den gleichen Modellaufbau wie in der Standardsimulation, aber mit stratosphärischen Halogenkonzentrationen entsprechend dem 1960 Niveau. Unsere Ergebnisse zeigen, dass die Ozonabnahme zwischen den Jahren 1960 und 2000 zwar global zu keinem signifikantem ERF geführt hat. Regional, auf der Südhemisphäre, vor allem über der Antarktis, ist das ERF der Ozonabnahme aber durchaus signifikant. Unsere Ergebnisse zeigen, dass in hohen südlichen Breiten über der Antarktis das negative ERF durch die Ozonabnahme zwischen 1960 und 2000 zu einem wesentlichen Teil das positive ERF durch den CO2 Anstieg kompensiert hat.</p> <p> </p>

Strong control of effective radiative forcing by the spatial pattern of absorbing aerosol

Over the coming decades it is expected that the spatial pattern of anthropogenic aerosol will change dramatically and that the global composition of aerosols will become relatively more absorbing. However, despite this the climatic impact of the evolving spatial pattern of absorbing aerosol has received relatively little attention, in particular the impact of this pattern on global-mean effective radiative forcing. Here we use novel climate model experiments to show that the effective radiative forcing from absorbing aerosol varies strongly depending on their location, driven by rapid adjustments of clouds and circulation. Our experiments generate positive effective radiative forcing in response to aerosol absorption throughout the midlatitudes and most of the tropical regions and a strong ‘hot spot’ of negative effective radiative forcing in response to aerosol absorption over the Western tropical Pacific. We show that these diverse responses can be robustly attributed to changes in atmospheric dynamics and highlight the importance of this previously unknown ‘aerosol pattern effect’ for transient forcing from regional biomass-burning aerosol.

Reduced effective radiative forcing from cloud–aerosol interactions (ERF_aci) with improved treatment of early aerosol growth in an Earth system model

Historically, aerosols of anthropogenic origin have offset some of the warming from increased atmospheric greenhouse gas concentrations. The strength of this negative aerosol forcing, however, is highly uncertain – especially the part originating from cloud–aerosol interactions. An important part of this uncertainty originates from our lack of knowledge about pre-industrial aerosols and how many of these would have acted as cloud condensation nuclei (CCN). In order to simulate CCN concentrations in models, we must adequately model secondary aerosols, including new particle formation (NPF) and early growth, which contributes a large part of atmospheric CCN. In this study, we investigate the effective radiative forcing (ERF) from cloud–aerosol interactions (ERFaci) with an improved treatment of early particle growth, as presented in Blichner et al. (2021). We compare the improved scheme to the default scheme, OsloAero, which are both embedded in the atmospheric component of the Norwegian Earth System Model v2 (NorESM2). The improved scheme, OsloAeroSec, includes a sectional scheme that treats the growth of particles from 5–39.6 nm in diameter, which thereafter inputs the particles to the smallest mode in the pre-existing modal aerosol scheme. The default scheme parameterizes the growth of particles from nucleation up to the smallest mode, a process that can take several hours. The explicit treatment of early growth in OsloAeroSec, on the other hand, captures the changes in atmospheric conditions during this growth time in terms of air mass mixing, transport, and condensation and coagulation. We find that the ERFaci with the sectional scheme is −1.16 W m−2, which is 0.13 W m−2 weaker compared to the default scheme. This reduction originates from OsloAeroSec producing more particles than the default scheme in pristine, low-aerosol-concentration areas and fewer NPF particles in high-aerosol areas. We find, perhaps surprisingly, that NPF inhibits cloud droplet activation in polluted and/or high-aerosol-concentration regions because the NPF particles increase the condensation sink and reduce the growth of the larger particles which may otherwise activate. This means that in these high-aerosol regions, the model with the lowest NPF – OsloAeroSec – will have the highest cloud droplet activation and thus more reflective clouds. In pristine and/or low-aerosol regions, however, NPF enhances cloud droplet activation because the NPF particles themselves tend to activate. Lastly, we find that sulfate emissions in the present-day simulations increase the hygroscopicity of secondary aerosols compared to pre-industrial simulations. This makes NPF particles more relevant for cloud droplet activation in the present day than the pre-industrial atmosphere because increased hygroscopicity means they can activate at smaller sizes.

An energy budget framework to understand mechanisms of land–ocean warming contrast induced by increasing greenhouse gases Part I: Near-equilibrium state

Modeling studies have shown that surface air temperature (SAT) increase in response to an increase in the atmospheric CO2 concentration is larger over land than over ocean. This so-called land–ocean warming contrast, φ, defined as the land–mean SAT change divided by the ocean-mean SAT change, is a striking feature of global warming. Small heat capacity over land is unlikely the sole cause because the land-ocean warming contrast is found in the equilibrium state of CO2 doubling experiments.Several different mechanisms have been proposed to explain the land–ocean warming contrast, but the comprehensive understanding has not yet been obtained. In Part I of this study, we propose a framework to diagnose φ based on energy budgets at the top of atmosphere and for the atmosphere, which enables the decomposition of contributions from effective radiative forcing (ERF), climate feedback, heat capacity, and atmospheric energy transport anomaly to φ. Using this framework, we analyzed the SAT response to an abrupt CO2 quadrupling using 15 Coupled Model Intercomparison Project Phase 6 (CMIP6) Earth system models. In the near-equilibrium state (years 121-150), φ is 1.49 ± 0.11, which is primarily induced by the land–ocean difference in ERF and heat capacity. We found that contributions from ERF, feedback, and energy transport anomaly tend to cancel each other, leading to a small inter-model spread of φ compared to the large spread of individual components. In the equilibrium state without heat capacity contribution, ERF and energy transport anomaly are the major contributors to φ, which shows a weak negative correlation with the equilibrium climate sensitivity.

Observational constraints reduce estimates of the global mean climate relevance of black carbon

How emissions of black carbon (BC) aerosols affect the climate is still uncertain, due to incomplete knowledge of its sources, optical properties and atmospheric processes such as transport, removal and impact on clouds. Here we constrain simulations from four climate models with observations of atmospheric BC concentrations and absorption efficiency, and the most recent emission inventories, to show that the current global mean surface temperature change from anthropogenic BC emissions is likely to be weak at +0.03 ±0.02K. Atmospheric rapid adjustment processes are found to reduce the top of atmosphere radiative imbalance relative to instantaneous radiative forcing (direct aerosol effect) by almost 50% as a multi-model mean. Furthermore, constraining the models to reproduce observational estimates of the atmospheric vertical profile reduces BC effective radiative forcing to 0.08 W m-2, a value more than 50% lower than in unconstrained simulations. Our results imply a need to revisit commonly used climate metrics such as the global warming potential of BC. This value (for a 100-year time horizon) reduces from 680 when neglecting rapid adjustments and using an unconstrained BC profile to our best estimate of 160 ±120.