1.Only recently have scientists been able to measure the amount of CO₂ in the atmosphere and how quickly the rate is increasing. Up until 1950 the levels of atmospheric CO2 were pretty steady at 300-‐310 ppm (that's parts per million out of all the molecules in the air).
2.Levels of carbon dioxide in the atmosphere rise and fall each year as plants, through photosynthesis and respiration, take up the gas in spring and summer, and release it in fall and winter. Now the range of that cycle is expanding as more carbon dioxide is emitted from burning fossil fuels and other human activities.
