ABSTRACT
This situation, however, has since changed dramatically, as witnessed in the recent Third Assessment Report of the Intergovernmental Panel on Climate Change (IPCC).3 The global average surface temperature -having increased by about 0.6°C over the 20th century-is projected to increase between 1.4 to 5.8°C over this century, at a rate "very likely [>90%] to be without precedent during at least the last 10,000 years." The threat of an impending ice age has given way to concerns about much more immediate climatic changes in the "opposite direction." The reason is that in the course of the last century, mankind has unintentionally become a force to be reckoned with in influencing the Earth's climatic system. It graduated -or blundered-from "climate-taker" to "climate-maker". Fundamental Distinctions The most general distinction between the causes of the current climatic changes is thus between "natural" on the one hand, and "anthropogenic" ("human-induced", "man-made"), on the other. A paradigm of natural climate variations are the ice-age cycles of geological time scales, some of which prove to be closely correlated with anomalies in the terrestrial orbit.4 Yet there are other natural causes which can lead to changes in regional and global climates.
