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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 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]