While Earth does go through cycles of warming and cooling, there is almost no doubt at all that current global warming is at least partially anthropogenic.
Carbon dioxide is a greenhouse gas, meaning that it essentially insulates the planet by preventing the escape of heat. Human beings are emitting an enormous of amount of carbon dioxide into the atmosphere, carbon that was trapped on or beneath the surface for millions of years. What we're doing is not natural and has not been done before. Again, it almost certainly contributes to global warming.
This is a bad thing. Rising ocean temperatures can contribute to more severe storms (e.g. hurricanes) and, possibly, a shift in the gulf stream at some future time. A drastic shift could severely reduce temperatures in Northern Europe, which shares latitudes with Northern Canada. Furthermore, melting ice caps in other areas of the world could inundate large sections of the globe. These last two events are possible futures, but they hardly seem unlikely.
We can reduce the severity of global warming by reducing our emission of greenhouse gases -- that is, by burning off less carbon. Pretty simple. International agreements like the Kyoto Protocol are meant to achieve this.