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This paper is an introduction to the special issue of Ambio (Ambio Vol. 26 No 1, February 1997) commemorating the 100-year anniversary ofthe publication of Svante Arrhenius' landmark paper on the greenhouse effect. Arrhenius' most significant achievements in this particularfield of science are briefly summarized and an attempt is made to provide a realistic explanation of the fundamental physical processes contributing to the greenhouse effect. [ABSTRACT FROM AUTHOR]

In his 1896 paper, Svante Arrhenius laid the foundation for the modern theory of the greenhouse effect and climate change. The paper is required reading for anyone attempting to model the greenhouse effect of the atmosphere and estimate the resulting temperature change. Arrhenius demonstrates how to build a radiation and an energy balance model directly from observations. Arrhenius was fortunate to have accessto Langley's data, which are some of the best radiometric observations ever undertaken from the surface. The successes of Arrhenius' model are many, even when judged by modern-day data and computer simulations: the suggestion of the diffusivity factor including its correct numerical value; the remarkably accurate simulation of the total emissivity of the atmosphere which seem to agree within 5% of modern-day values; the logarithmic dependence of the CO{sub}2{end} radiative heating effect; and others documented in the text. We uncover two aspectsof the model which, most likely, were not recognized by earlier studies: First, Arrhenius included the water vapor feedback by introducing the fixed relative humidity assumption, which amplified the surfacewarming in his model by about 30%. Second, his model overestimated the surface warming, primarily because the radiation model overestimates the opacity of the CO{sub}2{end} bands in the 6 to 8 mu m region. In constructing his model, Arrhenius had to account for the magnitudeof the solar radiation absorbed within the atmosphere -- a topic that is currently pursued with renewed vigor. The second part of the paper addresses this topic, including the controversy that surrounds it.Observed values of solar absorption, since the 1950s, have almost always exceeded theoretical and model values. The magnitude of this excess absorption, i.e., observed-theoretical absorption, on climatologically relevant time and spatial scales was quantified recently (1990s) by six independent studies to be about 25 W m{sup}-2{end} or larger. We [ABSTRACT FROM AUTHOR]

The agenda of global carbon cycle research features two fundamental problems. The first concerns the direct response of the carbon systemto the anthropogenic perturbation, i.e. how is the excess carbon dioxide emitted by fossil fuel burning and from changes in land use (i.e. deforestation) distributed between atmosphere, land and the ocean, and how is this distribution changing as a function of time? The second problem addresses the feedbacks, i.e. how do changes in climate feedback on the natural carbon cycle and may thus lead to variations in the atmospheric CO{sub}2{end} concentration? This paper reviews some of the observational constraints to addressing these problems: (i)The direct atmospheric observations and the ice-core measurements which document the CO{sub}2{end} increase during the last 100 years; (ii) the role of the ocean in the current atmospheric CO{sub}2{end} budget as determined from model simulations and by new methods based on observations of oxygen and carbon isotopes; and (iii) the spatial information revealed by the global CO{sub}2{end} monitoring networks. One hundred years ago, Arrhenius and his geologist colleague Hogbom were both aware of the major processes which control the atmospheric CO{sub}2{end} concentration, however, they were more interested in the long-term aspects and did not perceive the industrial emissions as an immediate threat to the global climate. Today, we can quantify and model the direct response to anthropogenic perturbation, although important uncertainties still exist, in particular with respect to terrestrial biospheric processes. However, our understanding of the multitude of climatic feedbacks on the carbon cycle is very limited. This problem constitutes a challenge that will remain on the scientific agenda for the next decades. [ABSTRACT FROM AUTHOR]

The life and work of Svante Arrhenius provides an interesting windowon changes in the role of science in society. One hundred years ago,Arrhenius too served as science adviser to government, and he emphasized the importance of internationalism. World War I ruptured the social relations of science, and marked the beginning of a complex intertwining of research with the realm of politics. This paper considers Arrhenius' response to this trend, and asks what he would have thought of today's situation when climatology has become a megascience. Some hints of a possible answer are given against the background of a review of Arrhenius' ideas on method, and his Darwinian perspective on the growth of scientific knowledge. In the course of this, the authormakes some observations on how policy determination of scientific agendas introduces an organized social dimension, where the drive for consensus may contradict the traditional ideal of a "Darwinian struggle of hypotheses", Arrhenius advocated. At the same time it is noted how the traditional ideal of science as neutral "truth speaking to power" is constantly being resuscitated, among others by the leading echelon of the Intergovernmental Panel on Climate Change (IPCC). It is suggested that the social dynamics of global-climate-change-science (or "greenhouse research") may be understood in terms of a model of mutually reinforcing credibility cycles, linking science and politics. [ABSTRACT FROM AUTHOR]

The treatment of radiation in contemporary climate models is considerably more complex than that used by Arrhenius. While this offers many more possibilities for investigations than were available a centuryago, the very complexity of the models now in use demands that they be subjected to wide-ranging evaluation. This paper discusses some ofthe ways in which the radiation codes and models may now be comparedwith observations and some of the issues which those comparisons have raised. The opportunities for studies of the clear-sky greenhouse effect offered by data from the re-analysis project performed at the European Centre for Medium-range Weather Forecasts are highlighted, and some preliminary results from comparisons with the Hadley Centre Climate Model are shown. [ABSTRACT FROM AUTHOR]

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 absorptionof infrared radiation. The particles that constitute the smoke and haze of 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 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 bep [ABSTRACT FROM AUTHOR]