Now this is scary.
The linear rate of global warming, in and of itself, is scary only in the somewhat long term (100+ years or so).
The real risk with global warming is runaway feed-forward loops. E.g., one that you hear about often is that if the polar ice melts it will make the earth darker and absorb more heat (to the tune of maybe a few percent more).
Much more frightening than that particular feed-forward loop is the frozen methane trapped in clathrates at the bottom of the sea bed, in the frozen tundra in places like Siberia, and (not frozen, but dissolved) in peat bogs.
Methane is a green house gas orders of magnitude more powerful than CO2. It has a dramatic effect on heating (though it doesn’t last long in the atmosphere).
Now there is evidence that the tundra is outgassing. That is pretty scary.
Any methane released would go away fairly quickly, but it could lead to an escalating spiral of heating and trigger other feed-forward effects that would lead to warming of actually civilization-threatening levels.
Ramez Naam, a Fellow of the IEET, is a computer scientist and the author of More than Human: Embracing the Promise of Biological Enhancement. His next book, The Infinite Resource: The Power of Ideas on a Finite Planet is due out in Fall of 2012. He writes at the Unbridled Speculation blog.