What are some mechanisms that have been proposed as causes of mass extinctions?

The possible mechanisms of mass extinctions that have been proposed are:

Asteroid impact: It is one of the proposed mechanisms for the extinction of the dinosaurs.Volcanic eruption: It can lead to accumulation of various gaseous oxides in the air to cause acid rain. Ice age: Ice age is a result of the disturbances in weather pattern. The cold air from the poles get trapped in the air currents around the warm temperature zones of Earth. This affects the survival of various tropical and temperate flora and fauna.Nuclear war: In case of a nuclear war, the residual radioactivity will mutate and kill all the life forms.Solar flares and winds, and sun spots: Our Sun occasionally spurts out solar flares and winds. These are hot charged particles that often get deflected by the magnetosphere of Earth. If the Earth's magnetic field weakens, it will cause catastrophic changes in the plants and animals exposed to it. Sun spots are cold spots on the surface of the Sun. These spots reduce the amount of sunlight reaching the Earth. This can lead to cooling down of the Earth and a possible occurrence of the ice age.Global warming: The warming of the Earth is the leading mechanism proposed as a cause of mass extinction. It can alter the weather patterns, to cause droughts, flood, etc. Shortage of food resources: Loss of forest cover and exploding population of humans affect the availability of food and water resources for various animals. Pollution: Polluted water, air and soil can intoxicate and kill the animals. Ozone layer depletion: Ozone protects life on Earth from the harmful UV rays of the Sun that can cause fatal mutations. Its is depleting due to air pollution.

The answer is constant.