The living forest, as it grows, is a sink that stores vast amounts of carbon and so helps to regulate global climate. Cutting affects the atmosphere with a double-whammy because cleared forests have released much of their carbon into the atmosphere and they are no longer absorbing carbon. Moreover, the young trees that grow into the clearing are capable of storing only a fraction of the carbon that was contained in the living mass of the mature forest. The overall result of widespread fragmentation is an increase in atmospheric carbon, which contributes to the greenhouse effect and global warming. Taking a global perspective brings to bear a complex set of considerations.
Wood is a renewable resource.
If a forest is harvested there is a temporary increase in atmospheric carbon until the new forest matures. As the forest grows, it once again absorbs and incorporates atmospheric carbon into living tissues. Other, non-renewable resources, such as steel and cement, require tremendous quantities of fossil fuels in their mining, manufacture and transport, which results in a permanent net gain in atmospheric carbon. In addition, when we use locally-grown forest products we alleviate the need for burning fossil fuels to transport logs great distances and we aren't foisting the environmental costs of our consumption onto distant environments and cultures. Since a substantial portion of our lumber and wood products come from ancient forests and rainforests where more than one third of the world's species of plants and animals live, it is essential to choose which forests can be harvested sustainably while having the least impact on global biodiversity.