Turns out, horses beat dragons when it comes to methane output.
Previously, Tolkien scholars and climate scientists determined that Lincolnshire is most like Hobbiton, while Texas is more like Mordor. That was done by creating a model of Middle Earth's climate and mapping it atop our world. While in our world we have a few hundred million cars zipping about, emitting carbon, Middle Earth has huge dragons which burninate the countryside. Now, the Climate System Analysis Group at the University of Cape Town has taken a look at the environmental impact of dragons. Turns out, dragons aren't nearly as impactful as you'd think.
The analysis is one of those awesome thought puzzles every grad student wishes to publish one day. It starts by assuming that a Smaug-like dragon has the proportions of a Komodo dragon, but scaled up. They theorize that dragons use methane for fuel, and then make some calculations on exactly how much a single dragon can produce. According to the CSAG, a single dragon can output 1.43 kg of methane a day, or 260.6 tons over its 500 year lifespan. That might sound like a lot, but it's nothing compared the Riders of Rohan, which they estimate have the same environmental impact as 68 dragons.
"Given the dragon species' status as an apex predator, we can conclude that their likely efficiency of combustion, coupled with low population numbers resulted in minimal impact on carbon emissions in Middle Earth," the article concludes. Additionally, it notes that the death of Smaug (spoilers!) doesn't come close to offsetting the industrialization of Isengard, the deforestation of Fangorn forest, and the volcanic activity of Mount Doom. While none of that is particularly good for the future of Middle Earth, I think that in the meanwhile, they might have larger issues to tackle first.