The natural cycles normally keep the atmospheric concentrations of gases constant, but our prodigious burning of fossil fuels is now causing a marked increase in the level of carbon dioxide as shown in this graph. https://climate.nasa.gov/news/916/for-first-time-earths-single-day-co2-tops-400-ppm/
Note that this graph ends at the year 2000. Now 20 years later, 2020 the atmospheric CO2 level is about 415 ppm, and effects on climate are becoming pronounced. Please post questions that kids ask.