Final answer:
The ocean temperature is kept from rising above 31°C primarily due to negative feedback caused by increased cloud cover, which reflects more solar radiation back into space and cools the Earth's surface.
Step-by-step explanation:
According to the text, the ocean temperature rarely rises above 31°C (88°F) because of negative feedback caused by increased cloud cover. The presence of more clouds as a result of higher temperatures leads to more solar radiation being reflected back into space, which in turn reduces the temperature increase. This process acts as a negative feedback mechanism, where a change leads to an effect that opposes the initial change, hence preventing excessive warming of the ocean.
As ocean temperatures rise, more water is evaporated, forming more clouds. These clouds, having lower emissivity than oceans or land masses, reflect sunlight and heat back to space, cooling the Earth's surface. The greenhouse effect, while warming the Earth by trapping heat from the infrared radiation emitted by the Earth, also plays a role in maintaining the Earth's temperature balance, but it's distinct from the cloud cover's negative feedback on temperature.