Search for some credible sources for the answer to this question. By credible I don't only mean professionals with Ph.D.'s. Consider if the livelihood of the source is dependent on their research results. Or if it is a politician, former politician, or businessman do they stand to make a lot of money by advocating one position or the other. No one wants to work themselves out of a job and it appears that ethics in the USA (and the whole world) are at an all time low. I am a natural born USA citizen so don't think I am some foreigner primarily bashing the US.
There are so many factors that cause changes over time in the earth's atmosphere. Some of these factors include the Greenhouse effect, damage to the ozone layer, acid rain, volcanic eruptions, variations in Solar Output, and the role of human activity.