I was reading an article in Scientific American that stated that an atmospheric carbon dioxide concentration of 560 parts per million (ppm or µL/L) is widely regarded as capable of triggering severe climate changes. Currently the level is at 380ppm. So, I wondered to myself, how do these values compare to historical levels?
The answer depends on how historical you want to go.
Although contemporary CO2 concentrations were exceeded during earlier geological epochs, present carbon dioxide levels are likely higher now than at any time during the past 20 million years and at the same time lower than at any time in history if we look at time scales longer than 50 million years.As the graph shows, over the last 500,000 years we are at a high. CO2 levels were 280ppm before the industrial revolution, about the high of the cycles throughout the last 500,000 years. Now we are way beyond that.
In more recent times, atmospheric CO2 concentration continued to fall after about 60 Myr BP, and there is geochemical evidence that concentrations were <300 µL/L by about 20 Myr BP.
But, if you expand your time horizon "a bit" further it appears that our current levels are on the low side.
While these measurements give much less precise estimates of carbon dioxide concentration than ice cores, there is evidence for very high CO2 concentrations (>3,000 µL/L) between 600 and 400 Myr BP and between 200 and 150 Myr BP.As this graph shows, CO2 levels are much lower (almost ten times lower) than they were when dinosaurs walked the earth 150 million years ago.
I don't know where scientists come up with the fact that 560ppm will cause severe climate change, but it is certainly true that the earth has sustained life before with much higher levels. Some worry that the earth will become like Venus and be completely inhabitable. Seems unlikely given that the dinosaurs and other creatures were able to thrive in the higher levels.
I have written before that we should figure out the best temperature for life on earth and get the earth to that level. But, I overlooked the issue that the rate of temperature change might be more important than the actual average temperature. If the temperature increases gradually it gives species time to adapt while a sudden change doesn't. So, the scientists are probably right to worry about rapidly rising levels of CO2, but I wish we understood more what the impact of higher CO2 levels and temperatures meant for life on earth back in the days of the dinosaurs.