'Climatic variations' and 'Human activities' can be regarded as the two main causes of desertification. removal of the natural vegetation cover（by taking too much fuel wood）, agricultural activities in the vulnerable ecosystems of arid and semi-arid areas, which are thus strained beyond their capacity.
Answer:
the inner core is solid because S-waves stop at the inner core. The strong magnetic field is caused by convection in the liquid outer core.
Explanation:
In stage 1 the two rates are balanced. In stage 2 they diverge , as the death rate falls relative to the birth rate. In stage 3 they converge again, as the birth rate falls relative to the death rate. Finally in stage 4 the death and birth rates are balanced again but at a much lower level.
