re: 15 billion billion always equals 15 billion billion.
15 billion billion tonnes of Carbon does not equal 15 billion billion tonnes of CO2.
Though it is possible that Professor Falkowski meant CO2 when he said carbon.
But if we assume Professor Falkowski was speaking accurately then 15 billion billion tonnes of carbon is equal
to about 50 billion billion tonnes of CO2.
Or an average rate over the period of 2 billion years of about 25 billion tonnes per year.
I think in any case that Nature has some carbon credits due.