Ancient Climates2 Million To 46 Billion Years

Climate change of ancient Earth has been determined primarily through the study of plate tectonics and the reconstruction of the locations of the Earth's landmasses, oceans, and waterways as well as geologic evidence of atmospheric CO2. Much of the paleoclimatic reconstruction of the

Earth's ancient climate has been accomplished through the mapping of past positions of the continents and plotting of the distributions of rock types that form in specific climate regions. Certain formations, such as coal, need certain conditions in which to form. Coal needs abundant rainfall and generally forms in tropical rain forest or temperate forest conditions, which provide the necessary biomass, heat, and moisture.

In general, sedimentary rocks form in areas where water is present. Through a knowledge of rock types and the physical conditions necessary for formation, it is possible to step back in time and reconstruct the physical conditions that must have existed on the Earth at the time the rock was formed. By mapping the past distribution of thousands of rock types, scientists have been able to map the distribution of ancient climatic belts. Over the last 2 billion years, the Earth's climate has been constantly changing between hot and cold conditions, as shown in the figure.

The Earth's most ancient climates include the following geologic time span:

Precambrian eon 570 MYA-4.6 billion years ago Phanerozoic eon 0-570 MYA Palaeozoic era 240-570 MYA

Mesozoic era 65-240 MYA

Cenozoic era 2-65 MYA

Within this geologic time span, the Earth experienced a wide range of climate variability. At a period around 635 million years ago, many scientists believe the Earth was completely frozen over into what clima-tologists refer to as Snowball Earth. After that, 300 million years ago the planet became Hothouse Earth (also referred to as the mid-Cretaceous Greenhouse World).

The Precambrian—the oldest period on Earth—covers about 85 percent of the Earth's history, but because of the extreme time span, there is not much reliable evidence. Scientists have found evidence of the following two major periods of glaciation: one at 2.3 to 2.7 billion years ago and another at 0.9 to 0.6 billion years. The three intervals during this time period that scientists do have information on and that have provided some insight to the Earth's past climate are next

Paleoclimate 6billion

Average Global Temperature (°C)

G Infobase Publishing

Average Global Temperature (°C)

G Infobase Publishing

The graph illustrates the way climate has varied over geologic time from hot to cold; today, the Earth is in an interglacial period.

discussed: (1) the Faint Young Sun Paradox, (2) Snowball Earth, and (3) Hothouse Earth.

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