# 12.1: Background

There are two pieces of background that are important for these activities. The first is how earth scientists tell time, and the second is the major elements involved in recounting the Earth’s climate history.

In regard to earth history researchers customarily resort to something called the geologic calendar. In a nutshell the calendar is a global timeline of the events that have shaped the current structure of the planet. As such it extends back in time to the Earth’s formation, nearly 4.5 billion years and it is divided into major segments on the basis of the appearance and disappearance of major types of life as evidenced by the fossil record. The latter is a term used by paleontologists to refer to the total number of fossils that have been discovered to date. One of the goals of activity A on the following page is to familiarize yourself with the divisions of the calendar and the types of life common to each division.

In regard to the elements that make up a climate history the key ones that appear in Earth Viewer that you will be working with in these activities are as follows…

• Mean global air temperature – An average annual temperature for the entire planet. Largely inferred from oxygen isotopes in marine carbonates.
• Luminosity – The energy from the sun received at the top of the atmosphere measured in percentage of the Suns current output. Based on stellar evolution modeling.
• Day length – The average annual day length. Calculated from modeling of the formation of the Earth’s core and tidal dissipation.
• $$CO_{2}$$ – The amount of carbon dioxide in the atmosphere in parts per million (ppm). Modeled using geochemical data from the carbon cycle.
• $$O_{2}$$ – The amount of oxygen in the atmosphere expressed in percentage of total gas volume. Based on data from carbon and sulfur cycles.