A Very Short History of Climate Change
by Robert Roy Pool
Five years ago I was a skeptic about climate change. I knew the Earth had experienced numerous ice ages during the last 500,000 years. Melting glaciers from the last ice age had formed the Great Lakes, carved out Niagara Falls, gouged out Yosemite National Park, and dug the Hudson River – all in the last 12,000 years. The evidence for these cataclysmic events is overwhelming and irrefutable. Before we could begin to understand contemporary climate change, I thought, we needed to understand what caused the ice ages.
I started reading. Great strides in glaciology have been made over the last 40 years. Scientists now understand not only what initiated the changes that caused the ice ages, but they have also developed persuasive theories about what caused many of the other enormous climate shifts known to have occurred over the last 800 million years of the Earth’s history.
There is strong evidence that the most important variable regulating the Earth’s temperature is carbon dioxide (CO2) in the air. When CO2 goes up, temperatures rise. When CO2 levels decline, temperatures soon follow. But CO2 is not the only variable, and CO2 changes did not cause the ice ages. In fact, changes in the CO2 balance during the most recent glacial and interglacial periods may have helped moderate the temperature swings caused by other factors, preventing the great polar ice sheets from advancing even farther than they did.
Carbon Dioxide – the Elixir of Life
In climate change discussions carbon dioxide usually plays the role of villain, but the truth is that there would be no life on Earth without it. Without CO2 the average temperature on Earth would be below freezing – too cold to support life – and the oceans would be frozen nearly solid. All the continents would be covered with massive ice sheets.
And temperature regulation is only part of the benefits CO2 provides our planet. CO2 literally makes life on Earth possible. Plants require CO2 to breathe and grow. Plants exhale oxygen, which animals must have to breathe. Without CO2 there would be no plants, almost no oxygen, and no animals on this planet. Without CO2 the Earth would be as barren as the Moon.
Snowball Earth
It once was. In Earth’s early history, CO2 levels were often much lower than they are now. This resulted in a much colder planet at times. Between 850 and 550 million years ago – before the dinosaurs and before any multicellular animals existed on Earth – CO2 levels were so much lower that Earth became a giant snowball, totally covered in ice. The Ice Ages during this period – and there appear to have been at least five – were much more severe than any we experienced in the last million years, during the period humans have shared the planet.
Scientists now believe that the giant snowballs melted because the Earth was warmed by enormous volcanoes that spewed massive amounts of CO2 and methane – both powerful greenhouse gasses – into the atmosphere. Some scientists conjecture that the CO2 levels that melted the Snowball Earth were at least 1,000 times greater than the CO2 levels we have now. These extraordinarily high levels of CO2 must have declined slowly as CO2 molecules were absorbed by the liquid oceans, where they made it possible for photo plankton to grow and spread globally – creating the beginnings of a marine ecosystem. The last snowball melted very quickly about 550 million years ago, probably as the result of an enormous series of volcanic eruptions, which once again saturated the atmosphere with CO2.
Plants, breathing the CO2-enriched air, soon proliferated in the oceans and on the land. They sucked up most of the CO2, storing the carbon and exhaling pure oxygen (O2). Oxygen levels in the atmosphere slowly rose. Over time – perhaps 50 million years – the oxygen levels reached a point where our air could support advanced animal life. Soon thereafter multicellular plants and animals evolved very rapidly during a period known as the Pre-Cambrian Explosion. A few hundred million years after the Pre-Cambrian Explosion, dinosaurs ruled the Earth.
Hothouse Earth
But the Earth they ruled was much hotter. During the Jurassic period, at the peak of the dinosaurs’ reign, the Earth probably had ten times as much CO2 in the air as it has now. The higher levels of CO2 are believed to have been sustained by continued volcanic eruptions during that era. For millions of years the Earth was a hothouse – ten or fifteen degrees Celsius warmer than it is today. There were no polar ice caps at all.
Dinosaurs and other advanced life forms adapted to the warm conditions. Sometime after the dinosaurs were wiped out by an asteroid impact, CO2 levels fell more or less permanently. The Earth cooled off. CO2 levels fell because enormous forests spread across five of seven continents, sucking the CO2 out of the air and storing massive amounts of carbon. The oceans also absorbed a great deal of CO2 as photo plankton saturated every ocean in the world, inhaling CO2 and exhaling oxygen. Oxygen is not a greenhouse gas, and has little effect on the Earth’s temperature.
This new, oxygen-rich world was cooler than the world the dinosaurs dominated, and the new dominant species were mammals – animals that could maintain a stable body temperature even in cool conditions. Stable body temperatures allowed brains to grow larger and more organized. Mammals thrived in the new conditions, and reptiles were soon outcompeted in many ecological niches. A handful of these new mammal species began to thrive due to their intelligence and adaptability – the progenitors of modern humans.
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