Answer: Both release carbon dioxide into the atmosphere.
Explanation: Combustion is a process which releases carbon dioxide and water in the presence of oxygen .
For eg- Combustion of methane in presence of oxygen will lead to the release of the carbon dioxide and water.

Respiration is the process taking place in living organism which involves the production of the energy. It is mainly proceeded in the body through lungs with the intake of the oxygen and release of the carbon dioxide.