At -20 degrees Celsius and 1 ATM pressure, carbon dioxide is in the solid phase, also known as dry ice.

Here's why:

* Sublimation Point: Carbon dioxide's sublimation point (the temperature at which it transitions directly from solid to gas) is -78.5°C.

* Below Sublimation Point: Since -20°C is above the sublimation point, CO2 exists as a solid at this temperature and pressure.