Carbon dioxide is an organic greenhouse gas that is generally colorless, odorless, non-toxic, non-carcinogenic, and non-combustible. Human activities emit 40 billion metric tons of carbon dioxide (CO2) per year, while natural processes emit about 770 billion, or about 20 times human emissions. However, natural processes absorb the equivalent of all natural emissions plus about 52% of man-made emissions, leaving an additional 19 billion tons of CO2 in the atmosphere each year. Since the outset of the Industrial Revolution in the late 1700s, the portion of the earth's atmosphere that is comprised of CO2 has increased from 0.028% to 0.041%, or by about 49%.