Option 1: The gradual increase of the temperature of the ionosphere does not describe global dimming. The ionosphere is a layer of the Earth`s atmosphere, and global dimming refers to a different phenomenon.

Option 2: The gradual loss of biodiversity hot spots is unrelated to global dimming. Global dimming specifically refers to a reduction in direct sunlight reaching the Earth`s surface, not the loss of biodiversity.

Option 3 (Correct): Global dimming is the gradual reduction in the amount of global direct irradiance at the Earth`s surface. It occurs due to the presence of aerosol particles and pollution in the atmosphere, which scatter and absorb sunlight, reducing the amount of direct sunlight that reaches the planet`s surface.

Option 4: The gradual increase in the melting of ice in polar regions is not the definition of global dimming. While global dimming may have some impact on climate patterns and the melting of ice, it specifically refers to the reduction of direct sunlight rather than the melting of ice.

In conclusion, global dimming refers to the gradual reduction in the amount of direct sunlight reaching the Earth`s surface, caused by aerosol particles and pollution in the atmosphere. It does not refer to an increase in temperature, loss of biodiversity, or melting of ice in