Answer:
Global warming could be influenced by a variety of factors. Millions of years ago a salt water fern absorbed CO2 from the atmosphere. As the ferns died they sank and that carbon became sequestered on the deep sea floor. The result was a gradual cooling of the climate.
Volcanoes also vent a lot of hydrogen sulfide gas and particulate matter that reflects sunlight, resulting in cooler temperatures.
Since the beginning of the industrial revolution, however, mankind has begun burning coal--much of which was carbon sequestered during the Permian some 300 million years ago, or during the later Mesozoic. In a short time (150 years) atmospheric CO2 levels have risen from 280 parts per million to over 390 ppm today. As we continue to pump 30 billion tons of CO2 into the atmosphere each year from fossil sources, this heat trapping gas will result in an inevitable rise in global temperature.