I want to echo Joel, # 7, in reminding people to start with the total carbon mass of fossil fuel emissions. (realclimate.org)
«The plausible range of carbon mass input, approximately 4000 — 7000 PgC, strongly suggests a major alternative source of carbon in addition to any contribution from methane hydrates. (skepticalscience.com)
Global warming in the PETM is reasonably well defined at 5 — 6 °C and the plausible range for carbon mass input is approximately 4000 — 7000 Pg C [14]. (rsta.royalsocietypublishing.org)