|Carbon dioxide (CO2) comprises a significant percentage of Earth's atmosphere today.||a) True|
No, that is incorrect.
Carbon dioxide is such a small component of Earth's atmosphere (385 ppm or "parts per million") that it shows up on the pie chart below as only a thin line (shown at 2x actual thickness). Compared to former geologic times, when CO2 levels were at times more than 1000% higher than today, Earth's atmosphere is now " CO2 impoverished."
Did you know. . .
In the last 600 million years of Earth's history only occasionally has CO2 levels been less than 400 ppm. Today’s, Quaternary Period is unusual in that way as the average over geologic time has been well in excess of 1000 ppm. The concept of such relatively high levels is supported by research that shows plants function best between 1000 and 1200 ppm, suggesting they have evolved to that level. Commercial greenhouses pump CO2 into their buildings and quadruple the yield as a result.
Global Temperature and Atmospheric CO2 over Geologic Time
Late Carboniferous to Early Permian time (315 million years ago - 270 million years ago) is the only time period in the last 600 million years when both atmospheric CO2 and temperatures were as low as they are today (Quaternary Period). At no point does temperature and CO2 levels relate.
Temperature after C.R. Scotese http://www.scotese.com/climate.htm
CO2 after R.A. Berner, 2001 (GEOCARB III)