What is the impact of tourism toward the great wall?

The Great Wall of China is listed as one of the world's most endangered structures. China as a whole welcomes 130 million visitors a year, and most of them visit the Wall. The government has banned exploration of undeveloped sections to prevent negative impacts. Most of this tourism is just from pent-up reasons- China only opened it's gates for tourism in the 1980's. Tourists have left carvings and graffiti of their names in abundance on the more visited sections of the wall, and the wall is strewn with garbage.