today, the earths atmosphere contains a small, almost constant amount of carbon dioxide. Burning fossil fuels releases huge amounts of carbon...

The level of carbon in the atmosphere varies throughout the year, with a maximum typically occurring in the winter months in the Northern Hemisphere. The amount of carbon in recent geologic time has been around 300 parts per million (ppm). However, within the last 150 years the amount has increased to close to 400 parts per million. Note that as a percent of the atmosphere this is a small change (from .03% to .04%) even though the amount of carbon increased by 100 ppm.

Another thing to consider is that not all carbon released into the atmosphere remains in the atmosphere. The ocean absorbs a lot of the carbon, contributing to ocean acidification. Also, there can be an increase in algae and plankton which sequesters more carbon.

There is also a sequestration effect in terrestrial environments as plants take up a lot of carbon -- in long lived plants the carbon is held out of the atmosphere (at least until the plant dies and rots or is burned).