Atmospheric CO2 has increased by more than 100 parts per million since the Industrial Revolution in the mid-1700s, when humans began burning fossil fuels like coal and oil. Human activity has increased CO2 to levels not seen in the past 800,000 years.

Figure 1: Atmospheric CO2 concentrations in parts per million (PPM). Levels have peaked throughout time but we’ve seen a steep increase of 100 PPM since the industrial revolution, Climate.Gov
To understand atmospheric CO2 levels, we must look to the carbon cycle. The carbon cycle allows us to track the CO2 being emitted into the atmosphere and absorbed by the planet.

Figure 2: Bathtub Simulation, Climate Interactive
A common analogy for this is a sink or bathtub. When you turn on a faucet, water flows into the basin and out through a drain. As the amount of water in the basin increases, more pressure is put on the drain, and therefore more water releases from the drain. The carbon cycle works in a similar way. As more CO2 (water from a faucet) is released into the atmosphere (the basin), more CO2 is naturally absorbed by the planet (the drain). In addition to the atmosphere, the planet has three main carbon reservoirs which include the solid earth, the land surface, and the ocean. Natural glacial and interglacial cycles over the last 800,000 years have worked to move CO2 between the atmosphere a
— source skepticalscience.com | 19 Nov 2020