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.
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 hundreds, thousands, and tens of thousands of years ago. The ancient past is millions of years ago and the deep past is hundreds of millions of years ago.
CO2 has always been a major influence on the Earth’s temperatures but just as important were the intensity of the sun; the position of continents (continental drift); the paths of ocean currents; and the tilting and wobbling of the Earth (Milankovitch cycles). All of these factors have been changing with different effects on our climate throughout the deep past. Sometimes it was CO2 that was a major contributor to the Earth’s warmth; sometimes it was the other influences just mentioned.
The position of continents changes how much ice could form in the polar regions which in turn affects the weather throughout the Earth. Continents can take tens of millions of years to travel any significant distance and they were very different hundreds of millions of years ago.
Where the continents are located also affects the ocean currents as they travel around the Earth. Ocean currents can carry warm or cold water and where they travel to also affect the temperatures throughout the earth.
In this time period, 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 formed yet and Indonesian islands were not fully formed. The ocean currents that went through and around them affected the Earth’s temperatures differently than today. This in turn created a situation where we had a series of ice ages throughout the past million years with warm ages (called Interglacial) in between.
Throughout the recent past, continents have hardly moved and ocean currents rarely change. The Sun barely 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.
CO2 and the Big Picture
There were times in the past, millions and hundreds of millions of years ago when CO2 levels were higher than they are today. But CO2 wasn’t the major contributor to Earth’s temperatures. The sun, the position of the continents and ocean currents were also of equal importance. Also, of most importance, life on Earth was adapted to extremes in temperature and precipitation which our civilization would have trouble adapting to in a short time.
Today, with a hotter sun than back then, the same amount of CO2 that there was tens to hundreds of millions ago would increase temperatures much more 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. That would be 280 parts per million instead of our current 400 ppm.