One of the obvious first problems is that pretty much every mountain and most of the hills in the world will experience increasingly frequent landslides as much of their structural strength is eaten, releasing huge plumes of dust that blot out the sun and stay in the atmosphere. Continental shelves collapse into the oceans, causing tsunamis and the oceans fill with the suspended nanobot dust. Biological photosynthesis pretty much ceases, and the mean surface temperature drops below freezing as most of the sunlight power is intercepted in the atmosphere and redirected through the dust to below the surface where half the rocks are being turned into more dust.
If the bots are efficient with their use of solar power this could start happening within weeks, far too fast for humans to do anything to preserve their civilization. Almost all concrete contains at least moderate amounts of feldspars, so a large fraction of the structures in the world collapse when their foundations rot away beneath them.
Most of the people probably die by choking on the dust while the remainder freeze or die of thirst, whichever comes first in their local situation.
One of the obvious first problems is that pretty much every mountain and most of the hills in the world will experience increasingly frequent landslides as much of their structural strength is eaten, releasing huge plumes of dust that blot out the sun and stay in the atmosphere. Continental shelves collapse into the oceans, causing tsunamis and the oceans fill with the suspended nanobot dust. Biological photosynthesis pretty much ceases, and the mean surface temperature drops below freezing as most of the sunlight power is intercepted in the atmosphere and redirected through the dust to below the surface where half the rocks are being turned into more dust.
If the bots are efficient with their use of solar power this could start happening within weeks, far too fast for humans to do anything to preserve their civilization. Almost all concrete contains at least moderate amounts of feldspars, so a large fraction of the structures in the world collapse when their foundations rot away beneath them.
Most of the people probably die by choking on the dust while the remainder freeze or die of thirst, whichever comes first in their local situation.