Mars has a thin atmosphere made up of 95 percent carbon dioxide (CO2). If carbon dioxide was such an effective greenhouse gas (where on Earth it only makes up 0.04 percent of the atmosphere) it should keep Mars nice and toasty at 95 percent! However, the average temperature on Mars is -63 degrees Celsius. So if Mars is so cold, despite carbon dioxide being the most prevalent component of its atmosphere, how effective is CO2 going to be as a warmer on Earth where the percentage of CO2 in our atmosphere is 4 hundredths of 1 percent?
As an example, imagine that the atmosphere is all the people in this "Where's Waldo" image and CO2 is Waldo. Where is Waldo's ability to warm up all these other people? It doesn't seem likely does it?