Warm sodas do seem to have more fizz, don't they? Actually, bottled soda doesn't gain any carbonation when it's warm. The people that make soda dissolve CO₂ into the soda at cold temperatures. CO₂ (carbon dioxide, a gas) is what gives soda its bubbles.

CO₂ dissolves into cold liquids better than warm liquids. When you warm the soda up, all the CO₂ that was added when the liquid is cold wants to escape, so the soda seems fizzier. One term you might want to look up is vapor pressure to help you understand what is going on even better.