The Case of Global Warming

Unfortunately global warming isn't a very good description of what's happening on our planet. A better way of putting it is more along the lines of "Global climate destabilization."

Sure, the temperature has gone up only a little bit in the last century, and sure it's only likely to go up a couple degrees more in the next few decades... but wait.

It's not the change in temperature that's going to get us, it's how fast the change happens. All life on the planet is highly adaptable; It can be a new predator in the area, a lack of sunlight for a long time, new diseases, or what have you. The issue is, life can only adapt so fast.
If conditions change too rapidly for species to adapt to, they simply go extinct. Natural events, like massive volcanos going off, huge meteors hitting the Earth, and such have caused this a number of times in the past. Scientists have titled these "Extinction Level Events."

In the case of global warming the temperature on the planet is likely to rise rather sharply within our lifetime. This sudden spike will have a very sharp, and sudden impact on our ecosystem; An ecosystem that is already just barely staying balanced in the wake of the human population explosion.

So, what should we do?
Well, I agree with wonderingmind42 in the video above. Total inaction is too much of a risk, we have to move to do something about it; Not just as individual nations, but as the only species on the planet that could actually do anything to change what is coming... before it's too late to do so.