In my view it is, especially when compared to any other form of hard landscaping. The vast majority of the timber I use is Scandinavian Redwood and comes from well managed, renewable sources. The manufacturing process uses very little energy compared to the manufacture of cement or bricks. The deck construction is done using manual labour with the aid of battery powered cordless tools, rather than petrol driven cement mixers and such like. And of course, you don’t have a mow a deck every week in the summer (that was a tongue in cheek comment).