land and water. Other components of the model include the effective heat capacity of the earth-atmosphere column, the isotropic heat diffusion coefficient, the solar constant, the regional distribution of solar radiation, and the albedo of the earth-atmosphere system.

These models yield results for the annual climatic cycle that generally agree with observations. For 250-million-years ago, the model indicates remarkably pronounced seasonality, owing to the accumulation of continental crust into a single giant continent. Simulated subtropical mean temperatures for the Southern Hemisphere range from about 10°C in the winter to about 40°C in the summer, with summer temperatures occasionally reaching the value of 45°C (113°F). These results have profound implications for terrestrial life. A large group of mammal-like reptiles called therapsids—whose fossil record ranges from South Africa and Virginia to Russia—occupied the subtropics of the Southern Hemisphere, where it appears that temperatures underwent large great seasonal oscillations. This occurrence is inconsistent with the idea that therapsids, which have no living representatives, were cold-blooded like their reptilian relatives. The implication is that, like their mammalian descendants, they employed thermoregulation. In fact, fossil trackways of these creatures suggest that they were warm-blooded. Their footsteps were usually far apart, like those of fast-moving mammals and unlike those of cold-blooded reptiles.

A more general conclusion of this energy-balance model applied to the world 250-million-years ago is that continental glaciation in polar regions was favored by positioning of small continental areas near poles. Summer temperatures remained low because of the high heat capacity of neighboring seas, which caused abundant winter snow accumulation to persist throughout the year. If a very large dry continent were situated more centrally over a pole, pronounced seasonality would produce warm summers that could prevent the snow from persisting and glaciers from expanding.

Secular Change in the Global System

The numerous cyclical processes that interact to make up the entire earth system are superimposed on unidirectional, mostly gradual, processes of which radioactive decay and biological evolution are the most fundamental. Catastrophic events that affect large parts of the earth system are capable of interrupting the operation of familiar cycles and can greatly modify the established direction of gradual secular change. During the past decade, in one of the most exciting developments in the study of the Earth, there occurred a deviation from traditional assumptions of general gradualism to consideration of a possible role for catastrophes.

The history of the Earth can be conveniently divided into three major intervals distinguished by important secular changes in the geological record. Many surface processes have changed somewhat throughout the three phases, but the biggest changes are seen in the sedimentary and fossil evidence. The record from the oldest rocks used to be extremely sporadic in quantity and quality, but gradually it has been improved by the accumulation of superior data.

The Precambrian Record

The interval of time that preceded the development of a rich fossil record of invertebrates with skeletons, nearly 600-million-years ago, is called the Precambrian. Representing nearly 4-billion-years of earth history, its unique features have led geologists to use techniques that are given less emphasis in the evaluation of later intervals. One limitation of this most ancient record is that rocks older than about 1.4-billion-years lack any fossils that can be used to correlate strata and establish synchroneity from place to place. Limited correlation using fossils is possible among rocks between 1.4 billion and 600-million-years old. Nonetheless, global developments of environments and life forms during the Precambrian were so significant that many general features have been deciphered and assigned at least approximate dates on the basis of isotopic dating. Among these was a major radiation in phenotypic diversity in eukaryotes, organisms with a cell with a true nucleus, at about 1.6-billion-years ago or possibly even earlier—an event that may have been linked to changes in the proportion of atmospheric carbon dioxide and oxygen.

Both prokaryotic, or prenucleus, and eukaryotic single-celled organisms are remarkably well preserved in rocks of this period. Beginning around 550-million-years ago, preskeletal multicellular animals left an abundant fossil record consisting of tracks, trails, and body imprints.

Study of early environments, and the organisms that evolved in them, is intertwined with the study of secular trends. These earliest secular trends were more profound and influential than any of the changes characterizing the most recent 600-million-years. Not only was the Sun weaker and the greenhouse effect stronger, but calculations suggest that

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