The question of whether or not burning firewood is bad for the environment, depends on how it is done. People can burn responsibly and minimize the pollution their fire creates, or they can choose to disregard good burning practices and burn fires that make a lot of smoke for their neighbors to breathe. Firewood can also be harvested in a responsible way that enhances forest health, or it can be done in ways that can harm the health of the forest.
Wood smoke is a natural part of the Earth’s environment. Lightning sparked fires have been regularly burning long before humans ever invented fire. Since we started suppressing wildfires, some would argue that there is less wood smoke in the atmosphere now than there was back when wildfires were left to roam free. There is also debate about whether this is a good or a bad thing. But the main point is the smoke from wood burning is a natural component of the atmosphere that the Earth has adapted to long ago.
But on the other hand, burning wet wood or otherwise having an inefficient fire that smokes your neighbors out is just not a nice thing to do, and can contribute to local air pollution and possible health problems. On a worldwide scale, the atmosphere can handle wood smoke just fine, but concentrated levels in poorly ventilated areas can be a problem. There are modern wood stoves that can burn very clean with dry wood and good burning techniques. It all depends on you the operator.
The other environmental concern with firewood is with deforestation and damaging the forests to produce firewood. In some parts of the world, deforestation from cutting fuel wood is a problem. This is mostly in third world countries where people are more concerned about survival than they are about the forest health. But here in North America, Europe, Australia and the rest of the industrialized world, deforestation for firewood is not so much of a problem. But this is not to say there are not problems here.
In most cases, modern forest harvesting in industrialized countries does not lead to deforestation. Even though trees are cut, new trees usually grow to take their place. In many areas, reforestation is required by law. But at the same time, harvesting trees can be done in ways that enhance forest health, and can also be done in ways that are not the best for the forests. It all comes down to who you buy your firewood from and what their harvest practices are.
If you but firewood from people who are cutting down the larger, healthiest, vigorously growing climax species of trees to make firewood, that is probably not the best choice for forest health. But if you choose to buy firewood from people who are cutting it from trees that are being thinned to allow the healthiest and most vigorous trees to grow better, your wood burning can help fund the enhancement forest health.
There are many forest enhancement projects that produce wood that is not suitable for making lumber that is cut into firewood. Much of this would otherwise be left on the ground to rot or burned, which releases CO2 into the atmosphere anyway. So burning this wood for heating your home can be a great way to utilize this biofuel energy source that would otherwise go to waste.
Burning firewood can either be damaging to the environment or it can be helpful. It all comes down to your choices as a consumer and the way you burn your wood and the source you choose to get it from.