No abstract

Aerosols interact with radiation and clouds. Substantial progress made over the past 40 years in observing, understanding, and modeling these processes helped quantify the imbalance in the Earth's radiation budget caused by anthropogenic aerosols, called aerosol radiative forcing, but uncertainties remain large. This review provides a new range of aerosol radiative forcing over the industrial era based on multiple, traceable, and arguable lines of evidence, including modeling approaches, theoretical considerations, and observations. Improved understanding of aerosol absorption and the causes of trends in surface radiative fluxes constrain the forcing from aerosol-radiation interactions. A robust theoretical foundation and convincing evidence constrain the forcing caused by aerosol-driven increases in liquid cloud droplet number concentration. However, the influence of anthropogenic aerosols on cloud liquid water content and cloud fraction is less clear, and the influence on mixed-phase and ice clouds remains poorly constrained. Observed changes in surface temperature and radiative fluxes provide additional constraints. These multiple lines of evidence lead to a 68% confidence interval for the total aerosol effective radiative forcing of -1.6 to -0.6 W m −2 , or -2.0 to -0.4 W m −2 with a 90% likelihood. Those intervals are of similar width to the last Intergovernmental Panel on Climate Change assessment but shifted toward more negative values. The uncertainty will narrow in the future by continuing to critically combine multiple lines of evidence, especially those addressing industrial-era changes in aerosol sources and aerosol effects on liquid cloud amount and on ice clouds. Plain Language SummaryHuman activities emit into the atmosphere small liquid and solid particles called aerosols. Those aerosols change the energy budget of the Earth and trigger climate changes, by scattering and absorbing solar and terrestrial radiation and playing important roles in the formation of

As individuals serving on the AGU Advances editorial board, we condemn racism, affirm that Black Lives Matter, and recognize that inequality is built into the systems that have allowed us to prosper. We aim to persistently foster discussion about racism, inequity, and the need to make our community more diverse and inclusive. This will help AGU Advances do a better job in publishing important science that inclusively reflects the ideas and contributions of all in our community. These are challenging times. The context for the killing of George Floyd and too many others has compelled people across the world to confront the reality of the racism built into our institutions, including the science and education enterprise. For many, this is their lived experience. For others, thinking about this is uncomfortable: Even the most well-intentioned must come to grips with their role in perpetuating inequalities of access and opportunity.

Earth’s planetary albedo shows a remarkable hemispheric symmetry. We assess to what extent CMIP models symmetrize the hemispheric clear-sky albedo asymmetry and what the role of clouds is for this. Following Voigt et al. (2014) we calculate a reference TOA reflected solar radiation considering the masking of clear-sky asymmetry by symmetric cloud contributions. We use the approach of Donohoe and Battisti (2011) to estimate this benchmark and to separate surface, aerosol and cloud contributions to the compensation of this benchmark. In CERES, tropical clouds enhance the reference asymmetry while extratropical cloud asymmetries balance the reference asymmetry and the additional asymmetry introduced by tropical clouds. CMIP multi-model means show similar results as CERES. Clouds compensate reference asymmetries by 85% (CMIP3), 65% (CMIP5) and 78% (CMIP6) as compared to 98% for CERES. Spatial distributions of hemispheric differences indicate clear improvements across the CMIP phases. Remaining all-sky reflection asymmetries predominantly result from too small partly compensating cloud asymmetries: A too weak enhancement of the reference asymmetry in the tropical Atlantic and eastern Pacific is accompanied by a too weak compensation by extratropical clouds. Thus, tropical clouds and extratropical storm track regions are largely responsible for the compensation of hemispheric clear-sky asymmetries in CERES and CMIP, and for remaining biases in the GCMs. An unexpected result is the magnitude of model biases in the clear-sky asymmetries, which potentially condition systematic cloud biases. Experiments testing cloud-controlling factors influencing hemispheric asymmetry could help to understand what drive hemispheric cloud differences.

The Madden-Julian oscillation (MJO), as represented by the Max Planck Institute for Meteorology Earth System Model (MPI-ESM), is analyzed for the first time over time periods ranging from decades to more than a millennium. Particular attention is paid to the behavior of the MJO index as calculated from the leading pair of empirical orthogonal functions (EOFs) derived from a multivariate EOF analysis. The analysis of 1000 year simulations with the MPI-ESM and its predecessor reveals significant interannual (2-6 years) to interdecadal (10-20 years) internal variability of the MJO but relatively little evidence of significant variability at longer timescales in unforced runs. A 1200 year experiment forced by the best estimates of solar variability, volcanism, and changing atmospheric composition indicates that the MJO simulated in the twentieth century is very similar to the MJO simulated since AD 800. The analysis of sensitivity experiments shows the influence of different external forcings: solar variability may contribute to MJO variability on 11 and 22 year periods, but this is difficult to separate from internal variability; and there is a hint of enhanced decadal variability associated with volcanic forcing. Land use change and changes associated with anthropogenic forcing over the twentieth century have no detectable effect on the simulated MJO. An increase of the CO 2 concentrations by 1% per year starting in AD 1850 leads to an increase in the MJO strength in the twenty-first century, as does the warming associated with an abrupt quadrupling of the atmospheric CO 2 concentration, suggesting that the MJO may intensify with warming.

No abstract

In 2020, we all faced the enormous and unexpected challenges of the Covid-19 pandemic, with its host of new and competing demands on our time. Thus, we are especially grateful to the 154 people who provided reviews for AGU Advances and helped our fledgling journal complete its first year. Peer-review is essential to the process of doing and publishing science, and our reviewers have helped define our new journal by indicating papers expected to have broad impact that advance a discipline, have broad impact across disciplines, or have policy relevance. All papers submitted to AGU Advances first go through an editorial consultation. We are committed to respecting reviewers' time and only send papers for review that the consulting editors agree meet our criteria. Sometimes this means we send papers back to the authors with suggestions how to improve the fit to our journal. Another way we try to streamline the review process is by giving the authors the option to transfer reviews if after review we decide the paper is better suited to another AGU journal. As a relatively new journal, we still have few enough reviewers that we do not want to identify them by name. Nonetheless, you know who you are. Please accept our sincere thanks for generously sharing your expertise and working to improve AGU Advances.

Clouds are one of the most fascinating, important, and complex components of Earth's climate system (Siebesma et al., 2020). Despite their importance, theoretical understanding of what controls planetary-wide cloudiness is largely absent. For example, while we have a good understanding of how clouds form and interact with radiation (Cotton et al., 2014;Houze, 2014;Siebesma et al., 2020), it is difficult to use these theories to make claims about global cloudiness. Earth System Models (ESMs) and other bottom-up approaches do couple simple models of cloud formation to the global circulation. However, so far they have not been proven effective in constraining global cloudiness variability (Sherwood et al., 2020;Zelinka et al., 2020). This makes it difficult to transparently establish links between variability in global cloudiness and Earth's energy balance, or how such a link would change in a changing climate.Conceptual models could be useful in elucidating how clouds, circulation, and energy balance, are tied together. Existing theoretical work has linked cloudiness to circulation, and most examples of such work focus on particular circulation systems, like the tropical overturning circulation (Betts & Ridgway, 1989;Pierrehumbert & Swanson, 1995), or the Walker cell (Peters & Bretherton, 2005), or individual cyclones (Carlson, 1980). What is missing is a conceptual framework that both closes the top-of-atmosphere energy budget (and hence by necessity considers the planet as a whole), but also includes clouds. A suitable candidate for such a framework would be an energy balance model (Budyko, 1969;Ghil, 1981;North & Kim, 2017;Sellers, 1969) that explicitly represents dynamic cloudiness, likely as an implicit function of circulation measures or other state variables.