Great question. If you’re interested in the deeper science around it I thought I’d comment. The underlying process creating the methane in landfill is anaerobic digestion (basically decomposing timber in an environment with no oxygen). In this environment we get CH4 (methane) production rather than CO2 essentially because there’s no O oxygen for the O2 bit of the chemical compound. One kilogram of methane has a global warming forcing factor of 34 to 80 times that of one kilogram of CO2 (depending on the time frame). So if a piece of timber that’s sucked up all this carbon from CO2 through photosynthesis is then decomposed without oxygen it can release way more global warming forcing affect than it originally sucked up (because of the potency of methane verse CO2). Fortunately only a small percentage of timber actually degrades in landfill, hence the impact is pretty small compared to what it could be if it all degraded. This is still an area of research so if you look through the historical eToolLCD datasets you’ll see big shifts in the impacts associated with landfill of timber. Fabiano Ximenes is the guru in this area I think, he’s been digging up old landfills to work out how much timber is decomposing and this information is feeding the LCI data.