Why Burn Wood?
Wood can be used as an environmentally sustainable fuel because it is renewable (ie. if responsibly managed, more will grow) and because burning wood results in no NET carbon dioxide emission (unless the trees were cut down with chainsaws and transported using fossil fuels). Of course burning wood does cause CO2 to be added to the atmosphere but it only returns the CO2 that the tree had previously absorbed from the atmosphere in order to grow.
Burning fossil fuels such as oil, gas or coal releases CO2 that was absorbed from the atmosphere over a very long period of time millions of years ago. Releasing that fossil CO2 now in a relatively short period of time is changing our atmosphere and is one of the major causes of global warming. Burning wood instead of fossil fuels therefore avoids contributing additional CO2 and avoids making global warming even worse.
However if everyone cut down trees and burned the wood, the environment could be badly effected. Trees absorb CO2 and give off oxygen, if we cut down all the trees and didn't ensure that others were growing to replace them, we would have released loads of CO2, destroyed habitat for wildlife, and increased the rate of soil erosion and desertification.
Wood is a sustainable and renewable resource only when used and managed responsibly. By using renewable resources now, in a sustainable fashion, instead continuing with the unsustainable use of resources such as fossil fuels, we may provide our children and grandchildren with a better chance of enjoying a clean and healthy environment in the future.
See also: Heating and cooking with wood