76. Read the following passage, then answer the question below.

The stratosphere, the second major layer of Earth's atmosphere, extends from approximately 10 to 50 kilometers above the surface and is distinguished by a temperature inversion: unlike the troposphere below it, temperature increases with altitude in this layer. This inversion occurs primarily because the ozone layer, concentrated within the stratosphere, absorbs a substantial portion of incoming ultraviolet radiation from the sun, converting it into thermal energy. Because warmer air resides above cooler air in the stratosphere, vertical mixing is strongly suppressed, resulting in a remarkably stable layer with minimal turbulence. This stability is precisely why commercial aircraft often cruise near the lower boundary of the stratosphere, avoiding the weather disturbances that dominate the troposphere. However, anthropogenic emissions of certain compounds have been shown to deplete stratospheric ozone, reducing its capacity to absorb ultraviolet radiation and posing long-term risks to the thermal structure of this atmospheric layer.

What is the main topic of the passage?