Fact 1: Carbon dioxide concentrations in the air have increased over the last decade dramatically.

Fact 2: Decades have 10 years.

Given the two facts above, answer the question "Concentrations of what in the air have increased over the last 10 years dramatically" with the following options: 
- chlorofluorocarbons
 - fossil fuels
 - ozone
 - Greenhouse gases
 - Greenhouse gasses
 - oxygen
 - carbon dioxide
 - air molecules


 

       carbon dioxide  