We should keep in mind that the Earth's warmth is almost completely dependent upon the Sun (other than geo-thermal heating, which is not used very much, and the heat humanity itself creates, which is pretty small as well). If the Sun increased its output by only 1%, the Earth would (within the span of several months to a year or two) become completely uninhabitable. Luckily, the Sun is a very stable fusion engine - it's been going for billions of years, and it will continue to go for several billion more. However, all these temperature fluctuations on Earth could be due simply to the Sun changing its output by a fraction of a percent. That does not mean that humans are completely innocent (I'm sure all these greenhouse gasses don't help the situation any), and it doesn't imply that humans should throw up our hands and say that there's nothing we can do.