When work is done, energy is extracted, Carbon is released.
This can be through digesting food, where digestive enzymes break apart Carbon molecules. Or it can be from burning wood or “Fossil Fuels”.

When the Carbon bond is broke, and Carbon is separated, a LOT of energy is released. For digestion, we call this “Calories” that our body uses. When this Carbon is from Fossil Fuels (which is a very cheap fuel source), it is released into the atmosphere.

When Carbon is released, floating around, it looks for a new mate.
Oxygen. Carbon is very attracted to Oxygen, but primarily Oxygen in pairs. Carbon wants to bind to two Oxygen molecules.

So, Carbon binds to Oxygen in the atmosphere, forming Carbon Dioxide (CO2). This is happening at an alarming rate due to the industrialization of the modern world. Fossil Fuels are very cheap to transform to energy (cheaper than Wind, Solar, Nuclear, etc. is, at least in recent times).

The biggest hope to eating up these extra Carbon molecules are TREES.

Trees eat Carbon Dioxide.
But, we are tearing down more and more trees, and not planting enough. This is causing a large imbalance in the amount of Carbon in the atmosphere. All of which contributes to Climate concerns.

INTERESTING SIDE POINTS:

Interesting, while humans breathe in Oxygen, and exhale Carbon Dioxide, tree “breathe” in Carbon Dioxide and “exhale” Oxygen. In this way, oxygen-breathing creatures get a symbiotic relationship with trees. Even more interesting, the Lungs of these oxygen-breathing creatures take on the shape of an upside tree (when look beneath the brownish-pink fleshy outsides of the Lungs).

Carbon tends to trap heat inside the atmosphere, leading to Global Warming concerns. Related to this, Methane is about 25 times more potent at trapping heat in the atmosphere, hence the Climate concerns that have arisen.