For 22 million years during the Eocene, Earth had been a hothouse. Then 34 million years ago, the Earth experienced a sudden dramatic drop in global temperature. The corresponding extinction event is called the Grande Coupure, which is French for the “Great Break” in the continuity of life. The Grande Coupure is only part of a long-term cooling trend that began 47 million years ago. Then, like now, it appears atmospheric CO 2 concentration is the principal factor controlling the climate. The warm climate enigma has now become the cool climate enigma. Several causes for the abrupt cooling of the Grande Coupure have been proposed. Some think it might be associated with the rise of the Himalaya Mountains that formed as India collided with Eurasia. The mountains increased weathering and erosion, which in turn reduces atmospheric CO 2 concentrations. Others believe cooling could be associated with the formation of the Antarctic circumpolar current, the most powerful ocean current on Earth. For millions of years South America, Antarctica, and Australia formed one huge continent. This is how for instance, marsupials migrated from South America, where they evolved, to Australia. As Australia and South America migrated northward, they eventually detached from the Antarctic. No longer impeded by land the circumpolar ocean current was formed. The current races around Antarctica isolating it, causing it and the entire planet to cool. Still others think the cooling is related to a series of asteroid impacts about 34 million years ago. One hit what is now Chesapeake Bay, Massachusetts creating an impact crater 85 km (53 miles) across. Another even larger asteroid hit Russia creating the 100 km (62 mile) wide Popigai crater. Finally, there is the 22.5 km (14 mile) wide Toms Canyon impact crater off New Jersey. Whatever the cause, glaciers formed in the Antarctic for the first time in more than half a billion years. Places where palm trees had grown were now covered in ice; soon the entire continent would be. With their escape route now blocked by an ocean, countless unknown plants and animals living in Antarctica died. The true scale of the Antarctic extinction remains unknown. With so much water locked up on land, ocean levels fell 55 m (180 ft), creating more familiar continental coastlines. As ocean levels fell, most of what we call Egypt was revealed. Soon an ancestral Nile River began to flow across the land. The geologic story of the Sphinx now switches from deposition to erosion.

Mediterranean Sea: Formation,

Disappearance, and Reemergence

For more than 100 million years, Africa had been an island continent, slowly drifting northward towards Eurasia. Then, beginning about 20 million years ago, the continents collided. By 14 million years ago Africa was firmly attached to Eurasia. The collision slowed, then eventually pinched off, a powerful ocean current that ran through the strait between Africa and Eurasia. This produced a major reorganization of global ocean currents that cooled the planet. The collision allowed land animals, including primates that first evolved in North America during the PETM, to reach Africa. Millions of years later, humans would retrace that route in reverse, eventually making the trek back to North America, reintroducing primates to their home land.

Messinian Salinity Crisis

The collision formed the Mediterranean Sea. With an area of nearly 2,600,000 km 2 (1,000,000 miles 2 ), more water evaporates from the surface than is replaced by the rivers that feed it. If not for the connection with the Atlantic Ocean, the Mediterranean would dry up. And that is exactly what happened 6 million years ago as Africa continued its northward migration, forming a natural dam at the Strait of Gibraltar. That resulted in what is known as the Messinian Salinity Crisis. In 1,000 years or less the Mediterranean was gone. In its place was a mega Death Valley reaching 5 km (3 miles) below sea level. With day-time highs reaching 80°C (175°F), it was the hottest place on Earth. In response, the rivers that fed the Mediterranean cut deep canyons. In Egypt, the Nile cut a canyon 5 times longer and 25 percent deeper than the Grand Canyon. The Sphinx sits just a few meters west of the rim of that chasm 2 km (1.25 miles) deep.

Zanclean Flood

Then about 5.3 million years ago the Gibraltar dam breached. Currently the consensus is the dam was breached by the headward migration of a river. At first just a trickle of ocean water, it grew to become the world’s largest flood known as the Zanclean Flood. Two years later the Mediterranean Sea was back. As the Mediterranean Sea refilled it drowned what must have been one of the most beautiful river canyons the world has ever seen. With little time to respond, most of the animals that lived throughout the Mediterranean Valley drowned. The lucky ones reached mainland. Others not quite as fortunate were stranded on islands. With limited resources, most starved to death while a few managed to survive. As with the Galápagos Islands, evolution was free to experiment. Every island developed its own unique array of animals, including dwarf mammoths, elephants, and hippopotamus. There were fully grown adult elephants smaller than newborn African Elephants. Living alongside were giant flightless swans that towered over the dwarf elephants. Exactly how and when the animals first reached each island remains a mystery. Some were trapped by rising flood waters; a few swam, some flew, others walked when sea levels were lower. On Crete, fossil evidence indicates elephants go back at least 3.5 million years. One by one the exotic animals on each island went extinct, apparently timed with the arrival of humans. First seeded over 5 million years ago, by 10,000 years ago the last of the exotic Mediterranean island animals were gone.

The Mid-Pliocene Warm Period

From 3.3–3.0 million years ago, CO 2 climbed to 350–450 ppm. Considered by many as an analog to human-induced climate change, Earth experienced a warming of 2–3°C (3.5–5.5°F) -- comparable to temperatures predicted by the end of the century. Sea levels climbed 24 m (80 ft), flooding the Nile Valley all the way to Aswan, some 800 km (500 miles) south of Cairo. Comparable flooding today would submerge Cairo beneath 9 m (30 ft) of water, while the Sphinx itself would be a small island within the Mediterranean Sea.

Coolhouse Earth Oligocene-Pliocene: 34-2.6 million years ago