Human population levels from the time of the agricultural revolution to the present depict a shaped curve.

Exponential

Explanation:

Since the Agricultural Revolution, the human race became sedentary and the population began to grow. Nevertheless, the  population growth from this point up to the industrial revolution (Late 1700's) had been more less stable (Approx 1 billion people).

But since XIX century, the population began to grow faster and faster until nowadays rates (1 billion people more every 12-13 years). This type of growth depicts an exponential curve. Its main feature describes a very low growth rate at the beginning and later on this rate increases considerably, just like human population.