Question: What may cause some of the wall to disappear in the next 20 years?
About the article "Great Wall of China", section "Condition".

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While some portions north of Beijing and near tourist centers have been preserved and even extensively renovated, in many locations the Wall is in disrepair.
Those parts might serve as a village playground or a source of stones to rebuild houses and roads.
Sections of the Wall are also prone to graffiti and vandalism.
Parts have been destroyed because the Wall is in the way of construction.
More than of the wall in Gansu province may disappear in the next 20 years, due to erosion from sandstorms.
In places, the height of the wall has been reduced from more than to less than .
The square lookout towers that characterize the most famous images of the wall have disappeared completely.
Many western sections of the wall are constructed from mud, rather than brick and stone, and thus are more susceptible to erosion.
In August 2012, a section of the wall in north China's Hebei province collapsed after days of continuous heavy rains.
There is no cure for most tremors.
The appropriate treatment depends on accurate diagnosis of the cause.
Some tremors respond to treatment of the underlying condition.
For example, in some cases of psychogenic tremor, treating the patient’s underlying psychological problem may cause the tremor to disappear. 
Whatever the cause, the distortions can recur several times a day and may take some time to abate.
Understandably, the sufferer can become alarmed, frightened, even panic-stricken.
The symptoms of the syndrome themselves are not harmful and are likely to disappear with time. 
The Antonine Wall is a stone and turf fortification built by the Romans across what is now the Central Belt of Scotland, between the Firth of Forth and the Firth of Clyde.
Representing the northernmost frontier barrier of the Roman Empire, it spanned approximately and was about high and wide.
Security was bolstered by a deep ditch on the northern side.
The barrier was the second of two "great walls" created by the Romans in Northern Britain.
Its ruins are less evident than the better known Hadrian's Wall to the south.
Construction began in 142 AD at the order of Roman Emperor Antoninus Pius, and took about 12 years to complete.
Pressure from the Caledonians may have led Antoninus to send the empire's troops further north.
The wall was protected by 16 forts with a number of small fortlets between them; troop movement was facilitated by a road linking all the sites known as the Military Way.
The soldiers who built the wall commemorated the construction and their struggles with the Caledonians in a number of decorative slabs, twenty of which still survive.
Despite this auspicious start the wall was abandoned after only 20 years, and the garrisons relocated back to Hadrian's Wall.
In 208 Emperor Septimius Severus re-established legions at the wall and ordered repairs; this has led to the wall being referred to as the Severan Wall.
However, the occupation ended only a few years later, and the wall was never fortified again.
Most of the wall and its associated fortifications have been destroyed over time, but some remains are still visible.
Many of these have come under the care of Historic Scotland and the UNESCO World Heritage Committee. 
If not thrombosed, external hemorrhoids may cause few problems.
However, when thrombosed, hemorrhoids may be very painful.
Nevertheless this pain typically resolves in 2–3 days.
The swelling may however take a few weeks to disappear.
A skin tag may remain after healing.
If hemorrhoids are large and cause issues with hygiene, they may produce irritation of the surrounding skin, and thus itchiness around the anus.