|Carbon dioxide (CO2) comprises a significant percentage of Earth's atmosphere today.||b) False|
Yes, you are correct.
There has historically been much more CO2 in our atmosphere than exists today. For example, as seen in the below figure:
During the Jurassic Period (200 mya), average CO2 concentrations were about 1,800 ppm or about 4.7 times higher than today.
The highest concentrations of CO2 during all of the Paleozoic Era occurred during the Cambrian Period, nearly 7,000 ppm -- about 18 times higher than today.
The Carboniferous, Ordovician and Permian eras were the only geological periods during the Paleozoic Era when global temperatures were as low as they are today.
The Late Ordovician Period was also an Ice Age while at the same time CO2 concentrations then were nearly 12 times higher than today-- 4400 ppm. According to greenhouse theory, Earth should have been exceedingly hot. Instead, global temperatures were no warmer than today. Clearly, other factors besides atmospheric carbon dioxide influence Earth temperatures and global warming.
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)