Carbon or carbon dioxide equivalents (CO2e)? Trees store carbon in their wood by taking in carbon dioxide through photosynthesis. Often we speak of tons of carbon currently held in trees but carbon dioxide when referring to the change in carbon over a year. This is because emissions are reported in terms of CO2 equivalents. CO2 is not the only greenhouse gas. The others are methane (CH4), etc, which have different warming properties and longevity in the atmosphere compared to CO2. To report all the greenhouse gases in terms of one number, these gases of differing properties are converted to the amount of CO2 that would have the same warming potential. That is, the gases are made equivalent.
CO2 =44/12*carbon because 12 is the molecular weight of carbon and the weight of 2 oxygen molecules is 32. The ratio (12+32)/12 =44/12.
What’s a teragram? Tg=1X10 to the 12th grams. This is the same as a million metric ton.
A metric ton contains 2204 pounds. Other common units are Megagram (Mg; metric ton); Petagram (Pg; billion metric tons); and Gigatonne (Gt; billion metric ton).
Metric tons per hectare? Metric tons per acre? Tons per acre? All these units may be used when examining carbon densities—that is, the amount of carbon in a forest per unit area.
Negative or positive? Atmospheric scientists have used positive carbon change numbers to mean CO2 is increasing in the atmosphere, and negative numbers to mean CO2 is decreasing in the atmosphere. When forests are taking more CO2 from the atmosphere than they are releasing, these estimates would then be reported as negative. This standard is used by greenhouse gas inventories because most reported activities are emissions and are listed as positive; only a few sequester carbon. Forest scientists, however, may report increases into the forest as positive. We have adopted the standard used by the Intergovernmental Panel on Climate Change, with sequestration listed as negative.
Measuring, modeling, and surveying carbon in forests can be thought of in terms of carbon per area, and area. Total forest carbon = Carbon per area * Area. The change in forest carbon can be estimated by obtaining total forest carbon at two different times, subtracting them, and dividing them by the years between the surveys to determine annual average change. Note that the source of the carbon per area may be FIA field data, but it could be a modeled estimate. Areas may also come from various sources.
Last Modified: 08/21/2014