The Team Fortress 2 Website has details on a new "Alpine" art style Valve is introducing into Team Fortress 2 to combat fatigue they've sensed with the desert environments "both internally and externally." They offer a few screenshots showing the new style, and here's the accompanying explanation:

We decided to create a new TF environment after sensing some desert-fatigue, both internally and externally. The new theme had to be a departure from the current look without being so different that the characters felt out of place in it. We also wanted to leverage some content from our existing environments, since crafting every asset from scratch would mean we'd be moving the release beyond Valve time and into the realm of geologic time. Given these goals and constraints, we settled on the Alpine theme for two reasons: Its green foliage and blue-gray rocks let us introduce a new, cooler color palette to the game, and the lumber mill / mining camp buildings fit our existing set of structures. For Arena Lumberyard, the goal was to establish a clear difference between the two sides of the map. One side climbs up toward a large mountain peak, and the other dips down into a vast valley. We're excited to have a new visual theme to play with, and we'll definitely be using it in future maps.