For millions of years before the industrial revolution, CO2 levels in the atmosphere remained steady as a result of a balanced carbon cycle. The Carbon cycle is a process in which carbon is exchanged from the atmosphere to plants, plants to the soil, absorbed by the land and ocean, and cycled back once more into the atmosphere. The Carbon cycle is the Earth’s way to reuse and recycle carbon atoms; however, this process has been disrupted by the introduction of the burning of fossil fuels. According to data, natural CO2 output is about 750 gigatons. Human output via fossil fuels is about 29 gigatons, which may not seem like a lot, but adds up because the land, soil, atmosphere, and plants cannot afford to recycle and absorb the surplus of CO2 in the atmosphere. As a consequence, yearly, human emissions of fossil fuels continue to add up and are stagnantly collected in the atmosphere. Although CO2 gas seems weightless, tons are used to measure the atmospheric gas because the molecules that compose it have weight. Carbon and Oxygen atoms have molar mass, and although it seems that gasses are light and weightless, they are dense and do bear mass. CO2 is measured via the collection of air samples, which are shone under infrared light and their contents are examined. Another way that atmospheric CO2 presence has been quantified is using the measurement of parts per million, which measures the mass of a contaminant per million units of the total mass. According to the resources provided by climate.gov, the total ppm of CO2 in the atmosphere is 412.5, which is a global record. Something interesting to note is that during the COVID-19 pandemic there was a lull in the atmospheric levels of CO2 because people weren’t using their cars as a result of leaving their houses. 

Sources: https://www.climate.gov/news-features/understanding-climate/climate-change-atmospheric-carbon-dioxide#:~:text=Based%20on%20preliminary%20analysis%2C%20the,to%20the%20COVID%2D19%20pandemic.