3. After water vapor, CO2 is the next most important atmospheric trace gas which has both natural and anthropogenic sources. a.) What are the dominant natural and anthropogenic sources of CO2? b.) What is the current level of CO2 in the atmosphere (in ppmV and in % vol)? c.) What is the average annual rate of the CO2 increase? Also, why does the atmospheric CO2concentration show a seasonal variability? Solution Answer : 3. (a) There are two primary sources of CO 2 : (i) Natural CO 2 sources : These sources include animal and plant respiration, decomposition of organic matter, forest fire and emissions from volcanic eruption. (ii) Anthropogenic (man-made) CO 2 sources : These sources are those produced from power generation, transportation, industrial sources, chemical production, petroleum production and agricultural practices. (b) Current level of CO 2 in atmosphere = 0.041% by volume of the atmosphere = 410 ppmV (Source : National Oceanic and Atmospheric Administration) (c) Annual rate of CO 2 = 3 ppm (Source : National Oceanic and Atmospheric Administration) Seasonal variability in atmospheric CO 2 concentration : 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. The respiration of plants is not evenly distributed through seasons since the northern and southern hemisphere have a different amount of land and plants. The southern hemisphere has far less land. In the northern winter, the carbon released increases and then by May the northern hemisphere has enough carbon uptake (due to increased photosynthesis) to cause a dip in the CO 2 record. Then summer in the north ends and autumn commences, and net CO 2 starts to rise again. .