Free Astronomy Magazine January-February 2022

29 JANUARY-FEBRUARY 2022 ASTRO PUBLISHING rocks by weathering, and more. However, the contribution of natural sources is generally offset by pro- cesses such as chlorophyll photosyn- thesis, direct absorption into the oceans, creation of soil and peat, and so on. The dynamic interchange between the geosphere, hydros- phere, atmosphere, and biosphere, known as the “carbon cycle,” is the basis for the amount of carbon diox- ide present in the atmosphere at any given time. Today, around 410 parts per million (ppm) of CO 2 are meas- ured in the atmosphere, or 0.041% of the total atmospheric composi- tion. In the last 800,000 years, which include glaciations and interglacial periods, characterized respectively by less and more atmospheric CO 2 , the values of this gas have never ex- ceeded the threshold of 300 ppm. In the 60s of the last century, human activities had already raised that threshold to 320 ppm, but it is only in the last six decades that we have significantly contributed to the abundance of CO 2 , a growth that in geological terms can be defined as instantaneous. The last time the CO 2 concentration reached levels compa- rable to the current ones was more than three million years ago, during the so-called “mid-Pliocene Warm Period” (mPWP). At that time, aver- age temperatures exceeded those of the pre-industrial era by 2-3 °C and sea levels were at least 7-10 meters higher than they are today.