since the beginning of the industrial revolution, about 340 to 420 petagrams carbon (a petagram or pg is 1015 grams) in the form of co2 has been emitted to the atmosphere. how much of this anthropogenic carbon dioxide has been absorbed by the world's oceans? select one: a. about 1/3 of it b. about 1/4 of it. c. about 1/2 of it. d. none