The government agency that employs physical scientists and seeks to understand and protect the Earth's environment is the Environmental Protection Agency (EPA).

Here's why:

* Mission: The EPA's mission is to protect human health and the environment.

* Physical Scientists: The EPA employs a wide range of physical scientists, including:

* Chemists: Analyze pollutants and develop methods for their removal.

* Geologists: Study the Earth's structure and resources, and assess environmental risks.

* Meteorologists: Forecast weather patterns and study their impact on the environment.

* Physicists: Develop and apply technologies for environmental monitoring and remediation.

* Environmental Protection: The EPA sets environmental standards, conducts research, and enforces regulations to protect air, water, land, and public health.

While other agencies like the National Oceanic and Atmospheric Administration (NOAA) and the United States Geological Survey (USGS) also employ physical scientists and focus on environmental issues, the EPA is specifically dedicated to protecting the environment.