As you consider global warming, do yourself a favor and watch the Weather Channel (there is a point to this, just keep reading). Try to catch a ten-day forecast. Write down exactly what the prediction is. Ten days from now, see what the weather is. As I'm sure you are already aware, weather and climate prediction are far from an exact science.
Our problem is that we deal with a mixture of repeating conditions and non-repeating conditions. Repeating conditions are things like the eleven-year solar cycle, the Earth's magnetic cycle (the magnetic poles shift back and forth, north to south and vice versa, over time), etc. Non-repeating conditions include meteorite impacts, the aging of the sun (aging does not necessarily mean cooling, mind you; nuclear fusion is a terribly complex process), and mankind's impact.
My opinion is this: the Earth, like most things in nature, is remarkably good at maintaining its present state. The Earth's climate is in a state of equilibrium - this is a "resting" state, or the point of where energy is distributed in the most entropically favorable situation (sorry for the thermodynamics terms). In other words, think of a bowl with a marble in it. The marble will always gravitate towards the bottom of the bowl because that is the most energetically favorable state for it to be in. How do we know this? We know that when the Earth is thrown temporarily out of this resting state, it returns to it. Examples: Ice Age, major meteorite impacts, even major volcanic events. What we need to be careful of is not to shake the Earth so hard that it falls into a different resting state - in the bowl example, knocking the marble out of the bowl. We don't know if there is another even more stable climate state. Odds are, there isn't. But considering that all of our lives are on the line, I don't think that's a chance we should take.
This has gotten far longer than I intended, so I'll wrap it up. Humans have most definitely impacted the environment, but we are certainly not the most serious impact in Earth's history (the meteorite strike 65 million years ago released an amount of energy roughly equal to one million times the largest hydrogen bomb ever detonated (an estimate, of course)). We need to do something, but I highly doubt that we are on a doomsday course that will end humanity.