AP BIOLOGY

Detailed explanation-1: -The concentration of carbon dioxide in Earth’s atmosphere is currently at nearly 412 parts per million (ppm) and rising.

Detailed explanation-2: -Since 2005, however, total U.S. greenhouse gas emissions have decreased by 20 percent. Carbon dioxide accounts for most of the nation’s emissions and most of the decrease since 1990.

Detailed explanation-3: -Carbon dioxide concentrations are rising mostly because of the fossil fuels that people are burning for energy. Fossil fuels like coal and oil contain carbon that plants pulled out of the atmosphere through photosynthesis over many millions of years; we are returning that carbon to the atmosphere in just a few hundred.

Detailed explanation-4: -Carbon dioxide levels decreased because of processes that included: dissolving in the oceans. use by plants for photosynthesis. formation of fossil fuels as plants died and their carbon compounds became locked up underground.

Detailed explanation-5: -Carbon dioxide concentrations dropped from 4, 000 parts per million during the Cambrian period about 500 million years ago to as low as 180 parts per million during the Quaternary glaciation of the last two million years.