Indirect relationships important on the global or subglobal scale are often separated from their cause spatially and/or temporally. For example, the dramatic increase in volcanic activity (possibly caused by the impact of an asteroid) at the end of the Mesozoic era is thought to have led to the extinction of dinosaurs, which arguably stimulated the eventual evolution of mammals (including humans). The increased production and use of fertilizers in the 1950s led to the increased phosphate inputs, eutrophication, and decrease in water quality in many lakes, ponds, and reservoirs during the subsequent decades. The increased consumption of fossil fuels in the twentieth century led to the increased emissions of carbon dioxide, which were eventually followed by global warming and an apparent increase in the frequency of natural disasters. This climate change was probably accelerated by the depletion of the planet's ozone layer due to the CFC (chlorofluorocarbon)-containing deodorants and refrigerants.
It should be noted that indirect relationships are not related just to the activities of humanity, but have been important throughout the history of our planet. For example, a gradual development of the modern atmosphere was largely due to the activity of cyanobacteria, which were among the first organisms to produce oxygen as a by-product of their metabolism. The indirect implications of the atmospheric oxygen enrichment were far reaching, and led not only to profound global biological and geo-chemical changes, but also ultimately enabled the development of Homo sapiens and its current civilization.
Last century, the line of thought started by Vernadsky has eventually led to the creation of a new integrative branch of natural sciences, sometimes referred to as 'global ecology'. Essentially, 'global ecology' encompasses methods and scope of virtually all other environmental disciplines, and is predominantly concerned with the dynamics (including past and future) of the global ecosystem - the biosphere. As an example, it is worth mentioning the now classic climatological research carried out by Budiko and co-workers, which led to the creation of a half-empirical model of the thermal regime of the atmosphere. This model was subsequently used to simulate past and future dynamics of the atmosphere, and changes between glaciation and interglacial periods. Furthermore, the results obtained aided interpretation of human evolution, and led to further research aiming to counteract possible global change, for example, by injecting certain substances into the stratosphere, and direct and indirect consequences to which such manipulations may lead.
Currently, global climate change (principally related to the increased concentrations of greenhouse gases) is still one of the most discussed topics in ecology and environmental sciences in general. While the detailed review and the lively controversy of the discussions related to this topic is outside the scope of this publication, it is worth pointing out that the absolute majority of studies dealing with it also inevitably deal with indirect effects (although the exact term is often not mentioned).
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