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U. Lohmann

). The global mean decrease of net radiation at the top of the atmosphere, the total anthropogenic aerosol effect, amounts to −1.24 W m −2 in the control simulation ( Table 1 ). This difference includes the direct, indirect, and semidirect aerosol effect. As shown by Lohmann and Feichter (2001) , the net direct and the semidirect effects are rather small with a contribution of only 0.1 W m −2 . Thus, this difference is mainly attributable to the first and second indirect aerosol effects. The net

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Michael Ghil

666 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME38tion that anthropogenic aerosols will, in fact, causea global heating. We cannot pass judgment on that.The point of our note was to show that an anthropogenic increase in CCN, one component of theaerosol which is not even well monitored on a globalbasis at present, could cause a global cooling. It ispossible for the (non CCN) aerosol to cause a

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Rong Zhang, Thomas L. Delworth, Rowan Sutton, Daniel L. R. Hodson, Keith W. Dixon, Isaac M. Held, Yochanan Kushnir, John Marshall, Yi Ming, Rym Msadek, Jon Robson, Anthony J. Rosati, MingFang Ting, and Gabriel A. Vecchi

indirect effects that modify net surface shortwave radiation. On the basis of these results, B12 concluded that aerosols are a primary source of this multidecadal variability. However, B12 mainly compared the evolution of modeled and observed area-averaged NASST. In this study we show that there are major discrepancies between the HadGEM2-ES simulations and many observed changes in the North Atlantic. The discrepancies cast doubt upon the main conclusion of B12 . Fig . 1. Area-averaged NASST

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William W. Kellogg

, 229-241.Kellogg, W. W., 1979: Trends in anthropogenic aerosols and their effects on climate. Proc. WMO Tech. Conf. Regional and Global Observations of Atmospheric Pollution Relative to Climate. WMO, Geneva. , 1980: Aerosols and climate. Interactions of Energy and Climate. w. Bach, J. Pankrath and J. Williams, Eds., Reidel, 281-296.Liou, K.-N., 1976: On the absorption, reflection, and transmis sion of solar radiation in cloudy atmospheres. J. Atmos. Sci., 33, 798-805.Toon, O. B., and J

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Ulrike Lohmann

1. Introduction Anthropogenic aerosols such as sulfate and carbonaceous aerosols have substantially increased the global mean burden of aerosols from preindustrial times to the present day. The indirect aerosol effect, where anthropogenic aerosols act as cloud condensation nuclei, thereby determining the initial cloud droplet number concentration, albedo, precipitation formation, and lifetime of warm clouds, is estimated to be between 0 and −2 W m −2 ; for example, in Rotstayn (1999) and

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Guoxing Chen and Wei-Chyung Wang

stored every 3 h. Two cases—polluted and clean (including and excluding effects of anthropogenic aerosols, respectively)—were simulated. Both cases determined the aerosol size distribution and initial concentration through fitting the mean aerosol spectrum below cloud base (<500 m) observed by C130-RF04 ( ) during the VOCALS campaign with a trimodal lognormal distribution as in Chen et al. (2015) . All aerosols were assumed to be ammonium sulfate

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Connee S. Mitchell, Gerald L. Potter, Hugh W. Ellsaesser, and John J. Walton

parameter influencesthe effects of the others.11. gntroduction As one tool for analyzing the sensitivity of theLLNL SDM we have used a method similar to thatdeveloped by Manabe and Wetherald (1980) andWetherald and Manabe (1980) for evaluating therelative importance of factors such as temperature,water vapor, cloud cover, surface albedo and carbon dioxide on the solar, longwave and net fluxesat the top of the atmosphere resulting from adoubling of COs. As a simple example of our technique

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Timothy J. Garrett, Lawrence F. Radke, and Peter V. Hobbs

increase in cloud droplet concentration of 1 cm −3 ( Twomey 1991 ). The effects of anthropogenic aerosols on maritime stratiform clouds have been measured from satellites and aircraft. For example, diesel-powered commercial ships, burning low-grade marine fuel oil, emit ∼1 × 10 16 particles per second ( Hobbs et al. 2000 ). Approximately 12% of these particles act as CCN in overlying stratus and stratocumulus clouds ( Hudson et al. 2000 ). Advection of this pollution downwind of a ship can produce

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S. Christodoulaki, G. Petihakis, N. Mihalopoulos, K. Tsiaras, G. Triantafyllou, and M. Kanakidou

deposition of nutrients has larger impact in oceanic oligotrophic areas and semienclosed seas where nutrients are the dominant ocean productivity limiting factors ( Powley et al. 2014 ; Van Cappellen et al. 2014 ). In particular, areas that are either perennially or seasonally depleted in surface nitrate are affected more by anthropogenic N deposition than high-nutrient low-chlorophyll (HNLC) regions where surface water is relatively rich in nitrates. Recent findings also indicate that increased

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Timothy J. Garrett and Peter V. Hobbs

,b,c). Twomey (1974) suggested that the effects of anthropogenic CCN on cloud microstructures should increase the albedo of clouds, and Twomey et al. (1984)pointed out that marine stratiform clouds should beparticularly susceptible to such modification. Subsequently, the phenomenon of "ship tracks" (Conover1966; Coakley et al. 1987; Radke et al. 1989; King etal. 1993) provided excellent verification of these suggestions. Since the earth's radiation balance is sensitive Corresponding author address: Dr

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