The natural warming of both the Earth's lower atmosphere and the Earth's surface is called the greenhouse effect.

Here's a breakdown:

* Greenhouse gases: Certain gases in the atmosphere, like carbon dioxide, methane, and water vapor, act like a blanket, trapping heat from the sun.

* Natural process: This process is essential for life on Earth. Without the greenhouse effect, our planet would be too cold to support life.

* Balance: The Earth's temperature is maintained through a balance between incoming solar radiation and outgoing infrared radiation.

* Enhanced greenhouse effect: Human activities, such as burning fossil fuels, have increased the concentration of greenhouse gases in the atmosphere, leading to an enhanced greenhouse effect and global warming.

So, the blank you're looking for is greenhouse.