Here is some interesting information on the burning of wood for fuel (from wikipedia)
This is because wood/plant biomass is made up of mostly carbon (and water); carbon came from the carbon dioxide absorbed from the atmosphere by the tree or grass while it grew through the process of photosynthesis, and the carbon returns to the atmosphere when the wood is either burned or left to decompose. It is true that in combustion, most of the carbon joins with oxygen and returns to the atmosphere as carbon dioxide, with a GWP100 of 1. But - if the wood is instead left to decompose through natural processes, the carbon still partially returns to the air - but in the form of the gas methane, which has a GWP100 of 21 - thus causing 21 times more damage to the climate over 100 years than the same quantity of CO2 does over 100 years. Only part of the decaying wood is released as methane; other parts stay within the soil and often improve it substantially over time. This criticism of the climate aspect of wood decay does not apply if the wood is used in a process that fixates the carbon, such as in building materials, or, in the case of sawdust, in particle board, as these media sequester carbon unless or until they are allowed to decay or are consumed by fire.
http://en.wikipedia.org/wiki/Wood_pellet
In the above paragraph they give the numbersGWP100 of 1 and GWP100 of 21 so here is an explanation of what the GWP stands for. (from wikipedia)
Global warming potential (GWP) is a measure of how much a given mass of greenhouse gas is estimated to contribute to global warming. It is a relative scale which compares the gas in question to that of the same mass of carbon dioxide (whose GWP is by convention equal to 1). A GWP is calculated over a specific time interval and the value of this must be stated whenever a GWP is quoted or else the value is meaningless.