Carbon naturally cycles through our environment in many ways: From the carbon in the carbohydrates we eat and metabolize into energy, to the wood-chip mulch on garden beds that gets metabolized by soil organisms. Generally, nature is a system that keeps carbon-based energy tied up in cycles.
For hundreds of thousands of years, human activity remained in balance with this carbon-energy cycle. Even when we burned plant material, there was still enough new plant material to pull that carbon back out of the atmosphere. Modern society, as we all know, has fallen out of balance — producing too much carbon by burning fossil fuels on a massive, continuous, and global scale.
Because we extract and burn reserves of ancient hydrocarbons faster than the environment can cycle them back into living tissue, or feed active creatures, we have this imbalance. Too much carbon dioxide stored in the atmosphere (“greenhouse gas”) traps excessive heat, which leads to Earth getting warmer.