It's an article of faith among those of us who inventory greenhouse gases that "current account" emissions don't count. For example, if Greenback were to heat our campus by burning wood chips ("current" biomass), the resulting CO2, methane and nitrous oxide wouldn't appear in our GHG inventory. What makes an emission "current" is that the energy comes from burning something which grew fairly recently and which is presumed to be replaced by another like object (tree, cornstalk, whatever) in the near future.
"Current account" emissions are kind of like one of those natural cycles that you studied in middle school science. Think of the "water cycle": water evaporates, condenses, falls out of the sky, runs downhill, and ends up in a lake or ocean just in time to start evaporating all over again. Well, the natural "carbon cycle" is kind of like that. Carbon dioxide exists in the atmosphere until it gets absorbed by plants. The plants get burned, which releases the CO2 back into the air. Or they fall down and rot which releases the CO2 back into the air (some of it as methane, but then the methane breaks down in the presence of oxygen). Or the plant gets eaten by an animal which breathes in oxygen and uses the carbon ingested to convert O2 into CO2, which gets exhaled -- oops, I mean "released" -- back into the air. There are some more scenarios, but you get the picture.
In theory, the whole process maintains some sort of equilibrium, neither increasing nor decreasing the level of CO2 in the atmosphere very much. So when we burn fossil fuels, we count the tons of CO2 we're emitting, but if we burn cornstalks we figure that the emissions are just part of the natural carbon cycle (even if sped up a bit by human intervention).
The flaw in this model lies in the presumption that the cornstalk burned today will be replaced next year when the farmer replants, and that the tree burned (in whole or in small parts) this year will be replaced with another, similar tree within a couple of decades. Sometimes, that's true. Sometimes, it's not.
Now, I don't intend to go off on a rampage about deforestation in the sourthern hemisphere (although that is, in fact, a major ecological disaster right now). What I do want to mention is the aspect of the WBGU report (you knew I was going to get to the WBGU report, didn't you?) that deals with this type of emission. In a nutshell, WBGU recognizes the behaviors (deforestation, etc.) which increase one-time current account emissions as a problem, proposes an international protocol to address them, and -- most important -- proposes a firewall between accounting for current carbon and fossil carbon.
What that means is that nobody -- not Greenback, not Wal-Mart, not nobody -- gets to offset their fossil carbon emissions as a result of doing something which they claim will decrease current carbon emissions. Some sharp operators have been selling offsets based on promises not to cut down forests. Others have sold offsets based on the planting of seedlings -- they've calculated the amount of CO2 the tree will absorb in its lifetime, and used that estimate to create salable offsets.
The problems with current cycle emissions reductions being used to offset fossil carbon emissions are many. First, some of those forests weren't going to get cut down anyways, so no real reduction has taken place. And many of those seedlings were replacing trees recently cut down, so the replanting is, again, not a real reduction. And what's going to happen to those trees at the end of their normal lifespans? Whatever the answer, all the carbon they were storing is going straight back into the atmosphere. I'm not saying that there can't be legitimate GHG reductions achieved by paying attention to current cycle carbon. But any mixing of current and fossil carbon, from an accounting perspective, makes legitimacy extremely difficult to determine. And current carbon offsets are usually an order of magnitude cheaper than true fossil carbon offsets, so Gresham's law applies.
As a result, the WBGU suggests attention to land use, land use change, and forestry (under the unlovable acronym LULUCF), as well as reducing emissions from deforestation and forest degradation (mostly, but not entirely, in developing countries). And it bars offsets generated in either sector (fossil or current) from being used in the other.
It's a realistic approach, it creates two large problems in the place of one gigantic one, and it may help the diplomats in Copenhagen to reach at least some consensus agreements, even if (as seems increasingly likely) they're not going to be able to deal with the elephant in the room.