WHAT DOES THE GEOLOGIC PAST TELL US ABOUT THE EARTH'S FUTURE?
Rocks contain the evidence of past conditions on the planet, such as the temperature and the chemical composition of the atmosphere. And there is a remarkably detailed record of those changes for the past sixty-six million years, a time period known as the Cenozoic Era.
For most of the Cenozoic, the planet has been cooling. This is a consequence of the growth of the Himalayas, which increased the rate of chemical weathering which, in turn, pulled more and more carbon dioxide out of the atmosphere, and of the change in the pattern of ocean currents as South America and Australia moved northward, isolating the continent of Antarctica. Because of that isolation, by thirty-two million years ago, an ice cap began to form over Antarctica, further cooling the planet. And, just three million years ago, the northward movement of South America closed the seaway between it and North America forming the Isthmus of Panama and preventing circulation of ocean water between the tropical Pacific and Atlantic Oceans. That further cooled the planet and led to an Arctic ice cap.
And so, for the past few million years, two polar ice caps have been present on the planet. As the geometry of the Earth’s orbit around the Sun has changed, mostly a consequence of lunar tides, the two ice caps have advanced and retreated at regular intervals. The most recent advance of the northern ice cap occurred more than ten thousand years ago. Since then, the planet has been in an interglacial warm period.
But that advance-and-retreat of the ice caps—the periodicity of the ice ages—has been disrupted. About ten thousand years ago, through cutting of forests and the expansion of agriculture, the Earth began to absorb more sunlight. That heated the planet and the climate has been remarkably stable. Then, about two hundred years ago, fossil fuels began to be burned at an increasing rate. That greatly increased—at an exponential rate—the amount of carbon dioxide in the atmosphere.
Today there is more carbon dioxide in the atmosphere than there has been in the last few million years. Both the atmosphere and the oceans are heating. By the end of the current century, there will be no perennial Arctic ice cap. There will be an expansion of deserts—in places, daily temperatures will be above 140° F, too hot for people and other large lifeforms to exist. There is already an increase in extreme storms, such as hurricanes. And an increase in snow fall and rainfall, which will lead to more flooding. That is the immediate future of the planet. But there is a greater concern.
The last time there was a sudden injection of carbon dioxide into the atmosphere, comparable to what is happening today, was fifty-six million years ago during an event called the Paleocene-Eocene Thermal Maximum, or PETM.* The rock record for this event is on display at Fossil Butte National Monument in southwestern Wyoming. It is one of the most intense and abrupt intervals of global warming in the rock record. By the standard of the last ten thousand years of climate stability (and the development of civilization), the PETM was a climate catastrophe with dry regions such as the interior of North America becoming drier and wet regions like East Asia getting wetter.
In short, the PETM was a hyperthermal, a period of extremely warm climate. It lasted a few thousand years. And it took, through removal of carbon dioxide from the atmosphere through chemical weathering, about a hundred thousand years for the Earth’s climate to return to its pre-hyperthermal condition.
*The source of the carbon dioxide may have been the release of methane from the seafloor, possibly triggered by eruption of the Lac de Gras kimberlite field in northwestern Canada.