It's all about greenhouse gases and what not. The Earth has something called the greenhouse effect, which traps gases within the planet's atmosphere and keeps the planet warm. However, when there gets too much of it, the planet will, logically, keep getting warmer. Tons of things cause this... gargantuan forest fires, factory pollution, pollution from driving cars, burning coal, etc.
Unless I'm talking out of my ass. I just woke up and I haven't looked over that stuff in years, I even read news a while back about scientists possibly lying about temperature change over the past 30 years, so I don't know what to believe anymore