The concept of geologic time is difficult to convey to introductory science students at any level. In fact, it is a difficult concept for anyone. The use of an analogy is probably the most effective way to convey this concept, for example, if a person extended his or her arms at their sides horizontally, the time represented by human history on Earth can be represented by passing a nail file one time over the index finger nail. The time represented by the total length of the outstretched arms would represent the length of geologic time. In the new textbook by Farmer and Cook (“Climate Change Science: A Modern Synthesis, Volume 1, The Physical Climate”), geologic time and events are a recurrent theme throughout the text. The second volume in the series will be subtitled “Earth’s Climate History” and the emphasis will be on the most recent climate changes because those are what we know the most about.
Given that Earth is around 4.5 billion years old and the first 4 billion years or so is little known compared to the last .5 billion years, there is spotty evidence for climate change in the geologic record prior to the Cambrian Period (beginning about 540 million years ago). Near the beginning of the Cambrian (slightly before 540 million years ago) rocks of Earth’s crust began to yield fossils of organisms that once lived in certain environments compared to current ones. Many of these organisms, protists (single celled organisms), plants, and animals, lived in shallow marine waters and paleogeographers are able thereby to map ancient shorelines and the distribution of ancient seas.
In an article reposted by John Hartz on this site on 05 March 2012 (a reprint of a news release by the National Science Foundation on 01 March 2012), ocean acidification was discussed as progressing faster at present than at any time in the past 300 million years and the last time it was even close to the present was during the Paleocene-Eocene Thermal Maximum (PETM), beginning about 56 million years ago. During the PETM, in about 5,000 years atmospheric carbon dioxide doubled to 1,800 parts per million (ppm), and average global temperatures rose by about 6 degrees Celsius, according to the article.
A representation of the geologic time scale furnished by John Mason is given below in English and Welsh.
The majority of climate change science papers only go back in time to the start of the Pleistocene, commonly known as the “Ice Age” about 2.5 million years ago. By studying the redundant glacial advances and retreats beginning around 650,000 years ago it is thought that we might better understand the climates of the present and future.
Earth’s global climate has changed rather abruptly in the past; in the case of ice advances and retreats, the demise of the dinosaurs, the Permo-Triassic extinction, and the PETM. It is certainly possible for Earth’s climate to again change abruptly.
Some have postulated that Earth was once a “snowball” and there is some geologic evidence for this. There may have been several time intervals during which the entire Earth was covered in ice but these were far back in Earth history (during the Neoproterozoic) and the evidence is not entirely convincing. Even if true, a snowball Earth does not look like it will be in the planet’s future.
In Volume 2 of the Farmer and Cook textbook series, “Earth’s Climate History,” we will start with a survey of present day climate and continue backward in time to the Huronian glaciation (during the Paleoproterozoic from 2.4 to 2.1 billion years ago). Of course, most of the geologic evidence for climate change is from the Pleistocene. As one travels back in time, the picture of Earth’s climate becomes less and less clear.
Posted by rockytom on Monday, 18 February, 2013
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