If you receive a message stating the Google Earth is running in "OpenGL Software Emulation" mode, you don't have the appropriate 3D graphics drivers installed for the video card in your computer. Install the official NVIDIA or ATI proprietary video drivers for your video card using Ubuntu's built-in Hardware Drivers tool.

click to enlarge
Open the Ubuntu Hardware Drivers utility by clicking "System," "Administration" and "Hardware Drivers."
Click the checkbox next to an NVIDIA or ATI video driver in the hardware drivers window and click "Enable Driver" in the confirmation window. Restart to enable the drivers by clicking "Restart" when prompted to do so.
If there are performance issues, it might help to disable Ubuntu's 3D desktop effects to free up more of the video card's performance for Google Earth.
Open the Appearances configuration tool by clicking "System," "Preferences" and "Appearance."
Click the "Visual Effects" tab at the top of the "Appearances" window.
Disable visual effects by clicking "None" under the "Visual Effects" tab. You can also try clicking "Normal" if it's currently set at "Extra," which might be enough.