Five Important Facts About the Earth's Past
and Its Future

1. THE EARTH IS VERY OLD

The many examples of thick sedimentary layers that can be found around the Earth are ample evidence that the Earth is very old. Examples include the red ribboned chert in Marin County, California, on the northern side of the Golden Gate Bridge, the Castile Formation near the Guadalupe Mountains in Texas and the millions of paper-thin layers of the Green River Formation at Fossil Bluff in southwestern Wyoming. But how old is the Earth? That has been determined by using nature's most reliable timekeeper: the decay of radioactive atoms. That indicates that the Earth reached its current mass and composition 4.51 billion years ago.

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Red ribbon chert in Marin County north of the Golden Gate Bridge in California.

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Castile Formation along State Highway 62 near Guadalupe Peak in southwestern Texas.

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Paper-thin sedimentary layers of the Green River Formation at Fossil Butte in southwestern Wyoming.

2. THE EARTH HAS CHANGED DRAMATICALLY DURING ITS LONG HISTORY

There is a natural tendency to think about the Earth throughout its long history as being similar to what it is today. But the planet has changed dramatically. Not only have the continents shifted positions and have numerous mountains ranges formed and been eroded, but the temperature and chemistry of the atmosphere and the oceans have changed. So have the amount of the Earth's surface covered by the oceans and whether ice existed on the planet. The many thick layers of rock exposed at the Grand Canyon tell of a time when much of what is now the western half of North America was covered by seawater. The petrified wood at Petrified Forest National Park in Arizona tells of a time when rain was heavy and nearly continuous for millions of years. The distinctive outlines of the many towers and mesas of Monument Valley in the Four Corners area record a transition from an extremely wet climate to one that was extremely dry and a return to a wet climate.

3. THE EARTH HAS HAD AN ENVIRONMENT SUITABLE FOR HUMANS ONLY DURING THE LAST FEW HUNDRED THOUSAND YEARS

Human beings are an ice-age animal species. And so they thrive in an ice-age environment, that is, one in which the global climate allows massive volumes of ice to exist on the planet. The Earth has had such a climate for only the last few hundred thousand years. That was preceded by millions of years of higher temperatures across much of the planet.

4. THE EARTH HAS HAD A STABLE CLIMATE SUITABLE FOR CIVILIZATION ONLY DURING THE LAST SEVERAL THOUSAND YEARS

Thousands of years are required for societies to evolve from hunting-and-gathering groups to agricultural ones and, finally, to those with divisions of labor and institutions. For such an evolution to happen, the natural environment must be stable, that is, there must be infrequent episodes of extreme weather and a constant sea level. For most of the last few million years, that was not the case. The climate was unstable and environmental changes were dramatic. But, beginning about ten thousand years ago, the climate stabilized, possibly due to the clearing of forests and to the spread of agriculture. It is during this period of climate stability that modern civilization has evolved.

5. THE CURRENT INCREASE IN GLOBAL HEATING IS CAUSED BY HUMAN ACTIVITY AND IT HAS DESTABILIZED THE CLIMATE

The measurement of oxygen isotopes (oxygen atoms of different atomic weight) in atmospheric carbon dioxide clearly show that the carbon dioxide that has been added recently to the atmosphere has come from the burning of fossil fuels. That has put the chemistry of the atmosphere into a state that has not existed for millions of years, when the global temperature was much warmer than it is today. To what extent world civilization will be able to adjust to this new state is yet to be determined. At the very least, there will be economic disruptions, possible famines and mass migrations of people.