Direct climate forcing by anthropogenic sulfate aerosols: The Arrhenius paradigm a century later

The so-called ''greenhouse gases'', or ''hothouse'' gases as Arrhenius called them, are not the only radiatively active substances being added to the atmosphere by humans. A different class of matter, aerosols, consisting of small particles of condensed matter suspended in the air, has several climatic effects besides a small amount of absorption of infrared radiation. The particles that constitute the smoke and haze of industrial regions arise both as smoke from combustion and as the products of atmospheric chemical reactions, e.g. of sulfur dioxide from burning of coal. The direct effect of aerosols on the Earth's heat balance consists of their reflection of solar radiation back to space. Their indirect effect derives from their roles as the nuclei around which cloud droplets form. Adding more of these cloud condensation nuclei increases the reflectivity of clouds, a very important determinant of climate as discussed by Arrhenius. Aerosols may also influence the longevity of clouds and thus the fraction of earth covered by them-the ''nebulosity'' mentioned by Arrhenius. An additional direct effect of aerosols is light absorption by black soot particles, which can influence both clear and cloudy skies. Although quantitative assessments of these effects of aerosols have been developed only in recent years, there have been extensive studies and observations of their roles in climate, particularly the ancient observation that large volcanic eruptions caused large-scale cooling to occur. Interestingly, several of the early key studies were also conducted in Sweden, notably by Angstrom and Bergeron, The present understanding is that man-made aerosols have a cooling effect, but that it is somewhat smaller in the global mean than the warming effect of the man-made greenhouse gases and is spatially and temporally very different. Thus, no simple cancellation of the two effects can occur. This paper reviews the historical development of the understanding and quantification of direct aerosol effects on climate. Particular attention will be paid to comparing the parallel approaches to quantifying the effects of greenhouse gases and anthropogenic sulfate aerosols, respectively, which were both developed in Sweden, albeit almost a century apart.