[cogitator]

You've got me intrigued. Did you find these sites?

Boreal Forest Fire Studies

Excerpt: "The hypothesis we are testing in this interdisciplinary science investigation is that the interannual variations in the fire regime in the boreal forest are responsible for a significant portion of the interannual variations in the seasonal amplitude of the atmospheric CO2 record at high northern latitudes."

Biomass Burning and the Production of Greenhouse Gases

Table 6 is vital. It summarizes the total CO2 released from biomass burning annually as 3546 Teragrams carbon per year. (3.5 petagrams? I always forget what comes after tera-).

Table 8 puts biomass burning at 40% of all CO2 emissions, which are 8700 teragrams.

I also found was U.S. fossil fuel production of CO2 in 1999 (EPA site) at 6746 Teragrams, a little under twice as much as total global CO2 emissions from biomass burning. That would make the U.S. responsible for 77% of global CO2 emissions, and I know that isn't right!

So let's see what else the EPA says:

http://www.epa.gov/globalwarming/emissions/national/co2.html (National Carbon Dioxide Emissions)

Says nothing about "natural" biomass burning!

Argh.

Here's an interesting press release from 1997:

GOVERNMENT INDIFFERENCE FUELS INDONESIAN FOREST FIRE DISASTER

Excerpt: "[1] Up to a million hectares of forest are burning in Indonesia (mainly Kalimantan and Sumatra), releasing 220-290 million tonnes of CO2 (for reference, this amount is equivalent to 50 percent of the UK's annual CO2 emissions.) The fire is also threatening over 1 million hectares of peat forest, and an additional 20 million tonnes of CO2 could be released if just the top ten centimetres of peat were to burn."

So here again we see that forest fires are a substantial source of CO2 to the atmosphere. Now I wonder: why aren't they counted as significant? One answer that I saw in passing is that much biomass burning (not specifically forest fires) is considered not to be a net source of CO2 to the atmosphere if crops are re-grown, because plant growth is a CO2 sink. This probably doesn't apply to major forest fires.

I found a nice diagram here:

Understanding the Global Carbon Cycle

It's Figure 2 in a frame that I can't grab from. The units are in petagrams: I had to check the fossil fuel flux number to be sure. Now here's the key: the net flux to the atmosphere from land for "bacterial respiration" is 60 petagrams. Bacterial respiration of organic matter adds CO2 to the atmosphere the same way that burning organic matter does. So based on the numbers found, that would mean that biomass burning is about 6% of the CO2 released by bacterial respiration, which is in approximate balance with the CO2 taken out of the atmosphere by primary production (photosynthesis).

Whew. So my unanswered question is: when the carbon cycle modelers do their thing, do they just figure biomass burning is another form of bacterial respiration, since the effect on organic matter is the same and the delivery of CO2 to the atmosphere is the same? My guess is that the answer to that question is yes. But I don't really know.

[cogitator]

Replying to myself: after five more minutes of thought,
I think the answer is definitely yes. That's because
the fire only accounts for the release of a small amount
of the CO2 equivalent in the organic matter/wood. But
once the fire has passed, the bacteria get to work on the
dead wood -- and it's that flux of CO2 that is really significant. The weird thing from a climate change perspective is that it's really hard to predict (if it's possible at all) the net effect on fire frequency. Some
areas could dry out and have more frequent fires, while other areas could be wetter and have less frequent fires. So in the whole context of global climate change, I suspect that the amount of CO2 released by fires and subsequently by bacterial respiration of deadwood amounts to no significant change.