Which feedbacks does it take into account, exactly? As I understand it, the IPCC only included "fast feedbacks," but since it's recent publication (and even before to some) it has become clear that so called "slow feedbacks" are not all that slow and will play a major roll.

You need to address the major voice on climate change in America, James Hansen, who has been saying that the target must be 350 ppm or less. This is backed up by the careful work over at www.carbonequity.info who give a range from 300-350 as the target. It was after we got into this range that the Arctic ice cap started on its path toward total disappearance, something that the IPCC report didn't consider likely till very late in this century, but which now seems inevitable in the next few years. Humans evolved on a planet where concentrations ranged from about 200-300. Going much beyond that seems unwise at best.

Beyond the albedo (loss of reflectivity from snow and ice) that seems to be a major feedback driving the current rapid rise in Arctic temperatures, the melting of the tundra and now apparently of methane hydrates frozen on the ocean floor represents a huge new feedback currently underway--new vast sources of powerful greenhouse gases (in the near term, methane is over a hundred times more powerful than CO2 as a ghg) now being unleashed in a process we may not be able to stop.

We need to right now do everything we can to reduce the one forcing we do still have some ability to control--CO2 emissions--before these and other powerful feedbacks kick in and drive the planet to a much hotter and less hospitable climate. Waiting around for PO to do the work for us strikes me as the height of irresponsibility.

Targets in the range of 450 to 550 are more the result of political and economic considerations than conclusions drawn purely from looking at the science.

Remember that about half the CO2 we've emitted has been absorbed by the oceans, acidifying them in the process and disrupting basic systems that sustain life in the oceans and on earth (and that sequester CO2). The oceans may be reaching saturation point, at which they start becoming net contributors rather than sinks. That is why recently scientists have been saying that our emissions will continue to overheat the planet for tens of thousands of years, at least.

Also remember that about two degrees of warming are being blocked by the aerosols from coal plants, aerosols that drop out of the atmosphere within days. When we shut down these dirty coal plants, as we must eventually, the planet is going to suddenly warm by two degrees. In other words, we are already past your projections.

I will look further at your links about coal and GW, but this dirty ff still seems to me and others to pose a particularly powerful threat even (especially?) in a post-PO world. I do think that it may be because he has read the stats on peak oil that Hansen has been focused mostly on minimizing coal use as a central strategy to bring down CO2 emissions.

And of course as others have pointed out, even if we were to accept your contention that there are low probabilities that temperatures will increase beyond two degrees with concentrations topping out at 450 ppm, low odds are not much comfort when the stakes are this high. Russian Roulette is a game that most non-suicidal people would not care to play, whether the odds are one in six or one in a hundred. And this is playing RR with the viability of the only viable planet we know.