How much heat is needed to break CO2 down to CO?
Heating CO2 at atmospheric pressure:

At 1000K it is still essentially all CO2.

At 2000K: 98% CO2, 1.4% CO, 0.7% O2

At 3000K: 44% CO2, 36% CO, 16% O2, 4% O

At 5000K: 50% CO, 50% O

only at even higher temperature does significant atomic C appear (9% at 6000K).  

See Thermal decomposition of carbon dioxide in an argon plasma jet for more information.