Last summer James Hansen wrote an essay, "Silence is Deadly," in which he wrote:
"The tar sands are estimated (e.g., see IPCC AR4 WG3 report) to contain at least 400 GtC (equivalent to about 200 ppm CO2)."That's a much higher carbon number than Swart and Weaver, which is only 22 GtC for the Alberta tar sands proven reserves and 238 GtC for the whole caboodle. Hansen's number is almost 20 times larger.
So where did he get his number? From the IPCC's Third Assessment Report, he wrote. I think he misread that report.
[Aside: I have a paper copy of that report here, because back when I lived on that mountaintop in New Hampshire with the most amazing view of Lake Winnipesaukee, the Fed Ex man showed up one day with a heavy box containing all three volumes of the IPCC 3AR. I never ordered it, so whoever sent that to me, thank you very much -- paper copies are much easier to browse through than online PDFs.]
Anyway, Figure SPM.2 of the IPCC 3AR WG3 (pg 6) shows unconventional oil reserves of about 400 GtC. Table 3.28b of the same volume gives a resource base of "Unconventional Oil" of 440 GtC. But that's all unconventional oil, worldwide, including (from the caption for Figure SPM.2) "tar sands, shale oil, and other heavy oil."
I'm not an expert on oil, so I'm not sure how much unconventional oil there is besides the Alberta tar sands. Wikipedia says Venezuela has 1.2 trillion barrels (Tb) of the stuff, and that oil shale deposits are an estimated 2.8 - 3.3 Tb of recoverable oil. Again, Alberta has an estimated total of 1.8 Tb.
So I think Hansen either misinterpreted the number from the IPCC 3AR, or wasn't clear about what he meant when he wrote "the tar sands...," or Romm/environmentalists didn't take the time to understand his number and mistakenly applied it to the Alberta tar sands.
In any case, his carbon number is almost 20 times too big. Of course, people may well exploit the other sources of unconventional oil, too. But that would only add another 0.6°C or so to whatever warming will occur. When is it too much? Beats me. This 2010 TED talk by Richard Sears says the end of the petroleum age is already at hand, and it's coming about by better technologies (and not constraints on energy usage).