IPCC = Intergovernmental Panel on Climate Change, a United Nations-sponsored group comprising about 2,000 scientists from dozens of countries.

CDIAC = Carbon Dioxide Information Analysis Center

From CDIAC :
When looking at CO2 emissions estimates, it is important to look at the units in which they are expressed.

The numbers are sometimes expressed as mass of CO2 but are listed in many of our estimates only in terms of the mass of the C (carbon).

Because C cycles through the atmosphere, oceans, plants, etc. and changes the ways in which it is combined with other elements, it is often easier to keep track only of the flows of carbon.

Emissions expressed in units of C can be easily converted to emissions in CO2 units by adjusting for the mass of the attached oxygen atoms, that is by multiplying by the ratios of the molecular weights 3.67.