Earth has run hot and cold for 4.5 billion years
The earth is a blue jewel floating in the dark vacuum of space. A stunningly beautiful planet we can see in pictures from satellites and the space station. We live on a water planet with 71 percent of its surface area covered by oceans, which absorb most of the color from the light spectrum and reflect predominately blue wavelength radiation. However, blue-earth has not always been the case. At times, the earth has looked like a sparkling white diamond suspended against the deep, black background of the Universe. A planet covered in white is indicative of when the earth runs cold.
The Cryogenian
Far back in the reaches of geological history, some 720 million years ago, the earth slipped into a deep freeze lasting for 85 million years — the Cryogenian Period, sometimes known as snowball earth. The land was glaciated, and oceans froze as ice extended from the poles to the equator. The earth appeared from space as a dazzling white sphere orbiting an ancient sun. But why did the planet cool?
The most plausible answer to this question is; carbon dioxide (CO2) disappeared from the atmosphere. Today, without greenhouse gases — CO2 being the primary component — the earth’s average temperature would be about -18 degrees Celsius. This answer is somewhat unsatisfactory, however, since it doesn’t tell us why the CO2 disappeared. Explanations for the disappearance murky.
The onset of icy Cryogenian conditions coincides with the breakup of the Rodinia supercontinent. Most of the planet’s continents were welded together into this single landmass at the beginning of the Cryogenian Period. Some scientists speculate a massive outpouring of Laurentian flood basalts from the Rodinia breakup, sparked snowball earth. This explanation involves vast amounts of new magma spewing onto the earth’s surface, which resulted in increased weathering. Weathering of surface rocks does indeed pull CO2 from the atmosphere, but I am skeptical of this volcanic theory. Historically, large flood basalt events release significant amounts of CO2 into the atmosphere. These flood basalts should have provided a warming effect.
Other explanations
A more subtle explanation for the lack of CO2 involves the formation of Rodinia. Unusual mineral assemblages are found in Rodinia age rocks. These findings point towards an original supercontinent formation via a process called extrovert assembly — implying the individual continents drifted around for a long time before eventually joining together and forming the supercontinent. This process gave them lots of time to erode and weather surface rocks, thus removing atmospheric CO2.
There is also a possibility the biosphere was culpable for snowball earth. The Cryogenian was long before life crawled out of the sea, and the predominant life-forms were cyanobacteria (blue-green algae). These organisms live by photosynthesis, using sunlight to produce energy and grow. The photosynthetic process gives off free oxygen as a byproduct. Somewhere around 2.5 billion years ago, cyanobacteria created the planet’s oxygen-rich atmosphere. But these algae are carbon-based life, and they fix carbon as they grow. Perhaps their runaway success in reproducing and populating the oceans, helped earth sequestered its way into a deep freeze. I’m sure there are also other reasonable explanations, but the real answer remains a mystery.
Something more modern, please
The Cryogenian freeze is very ancient history, and we can move closer to today for other examples of when earth ran cold. About 20,000 years ago — just yesterday in geological time — our ancestors struggled through the Last Glacial Maximum (LGM). The world was a chilly place, and glaciers had reached as far south as they would go. From there, they retreated, humans discovered civilization, and we sent people into space to take pictures of blue-earth. The earth still would have looked blue during the LGM. But you would have noticed a considerably larger amount of white from ice sheets covering much of the northern hemisphere.
Recent research published in Nature maintains that between 19,000 and 23,000 years ago, the earth was 11 degrees F (6 degrees C) cooler than today. Not a snowball earth, but a significantly colder one. The world was a different place at 46 degrees F, as opposed to the average global temperature of 57 degrees F today.
Sea levels were about 410 feet (120 meters) lower. You could walk from Siberia to Alaska, and people did. Florida had a lot of extra space, and Indonesia was less island and more land. Also, atmospheric CO2 levels, which are now over 400 ppm, were about 190 ppm.
This latest big chill started 34 million years ago at the Eocene-Oligocene boundary. Before then, there were no polar ice caps, because the world was too hot. It’s ironic to think climate change could undo 34 million years of cooling in just several hundred years. “Sorry, GOP, that just ain’t natural.”
Understanding what causes the earth to cool is just as important as knowing why it warms. Hot-earth and cold-earth are two sides of the same coin.
Lessons?
The term ppm stands for parts per million. So, the approximate 200 ppm rise in CO2 since the Last Glacial Maximum (190ppm to 400ppm) resulted in a temperature rise of 11 degrees F. A little CO2 goes a long way. When polar ice started forming in the early Oligocene CO2 levels were between 500ppm — 1000 ppm. At the rate we are adding CO2 to the atmosphere, we will soon be in the range of the last hot-house earth period, which occurred during the early Cenozoic (34–65 million years ago).
Several significant greenhouse gases contribute to global warming and climate change. But CO2 is the most critical non-condensable greenhouse gas. Its importance is why we use it as a reference point for understanding where we are in the climate change process.
The earth naturally runs through cycles of hot and cold conditions, but these cycles track temperature fluctuations over thousands to millions of years. The geological record holds no examples equivalent to the Anthropocene rates of change we are currently experiencing. Climate change is happening, and the change is human-made. We should stop debating the facts and move on to discussing the solutions.
Sources:
Cryogenian Period (By John P. Rafferty; Britannica)
Scientists Project Precisely How Cold the Last Ice Age Was (By Nora McGreevy; Smithsonian)
One of The Supercontinents Is Different from the Others (It’s Rodinia) (Source: Carnegie Science)
Glacial cooling and climate sensitivity revisited (By Jessica E. Tierney, Jiang Zhu, Jonathan King, Steven B. Malevich, Gregory J. Hakim & Christopher J. Poulsen; Nature)
