Skeptics like to point out that Carbon Dioxide (CO2) levels were much higher in the past and therefore such large amounts would have little consequence in the present. Some even say that it will improve our environment.
In order to understand how CO2 levels affected us in the past, we need to understand the difference between the recent past, the ancient past and the deep past. The recent past is thousands to hundreds of thousands of years ago. The ancient past is millions to tens of millions of years ago and the deep past is hundreds of millions of years ago.
The further back in time we go the more different the earth becomes with CO2 levels having different impacts its climate.
In the deep past, hundreds of millions of years ago, CO2 had always had a major effect on the Earth’s temperatures but, just as important, was the intensity of the sun; the position of continents (continental drift); the paths of ocean currents; and the tilting and wobbling of the Earth as well as its distance from the sun (Milankovitch cycles). All of these factors were always changing with different effects on our climate throughout the deep past. Sometimes it was CO2 that was a major contributor to the Earth’s climate. Sometimes it was the other influences just mentioned.
The sun was much cooler back then and the earth’s having more heat retaining CO2 than today made up for what would have been much colder temperatures. Now that the sun is much warmer today than back then it would be unwise to raise its carbon dioxide levels to what they once were.
Also, the position of the continents was always changing and that affected the climate as well. Their location affects the ocean currents by changing them as they travel around the Earth. Ocean currents can carry warm or cold water and where they travel to affects the temperatures and thus climate throughout the Earth.
All these changes greatly affected life in general but we did not mind because we were not there.
In the ancient past, millions of years ago, continents barely moved but what little movement there was created major changes in our climate.
The isthmus of Panama in the Caribbean Sea was not yet formed. The ocean currents that went through Central America affected the Earth’s temperatures differently than today. When Panama was permanently formed it created a complex situation throughout the Northern Hemisphere where we had a series of ice ages throughout the past million years with warm ages (called Interglacials) in between.
Throughout the recent past, thousands, tens and hundreds of thousands of years ago, continents have barely moved and ocean currents rarely change. The Sun hardly changes at all and the Earth’s wobbling and tilting only cause changes throughout a period of tens of thousands of years. This means that the major effect on the Earth’s warmth or cold in recent decades is going to be the levels of CO2 which have risen by 26% since 1960.
During this period we did not mind changes to the Earth’s climate because if the weather did not suit us or the animals we hunted migrated, we could pick up our spears and gathering baskets and move elsewhere. There were very few of us and we could change where we lived without much of a problem.
However, we became civilized thousands of years ago by virtue of having extensive amounts of land turned from forest to cultivated land – that is agriculture. This made our population rise up dramatically. Our agriculture was reliable due to a moderate and predictable environment. There were occasional famines brought on by droughts but they were localized and temporary.
Today there are billions of us and it would be impossible for a billion people to move from one area to another assuming that global warming would leave any area unaffected.
CO2 and the Big Picture
With a hotter sun than a hundred million years ago,* the same amount of CO2 that there were tens to hundreds of millions ago would increase temperatures much more so than back then; change to the point of being intolerable. That’s why we need to keep CO2 levels the same as they have been in the past several thousand years when warmth allowed our civilizations to exist. That would be 280 parts per million instead of our current 400 ppm.
*This increase in solar luminosity was 1 percent per hundred million years. This does not contribute to our current increase in heat since it would come out to one-millionth of a percent in the last one hundred years.