Global temperatures have gone up and down in cycles thats true. And those with pro-fossil fuel agendas can throw doubt on the adverse impact humans have had on the temperature. That's because a fair amount of research so far has shown an increase in temperature, but within the statistically expected norm. There's also the argument that the earth naturally can accomodate and buffer changes in amounts of carbon.
But just like any of you who've taken chemistry knows, any buffer has a limited capacity. Once its overshot, then we'll see some real significant changes. Will we overshoot the buffering capacity? I don't know. But just because we're not obviously at that point doesn't mean we should carry on care free as the pro-fossil fuel advocates want us to.