Water is one of the strongest greenhouse gases around, and there's plenty of it in the oceans that can evaporate in a runaway process. Once that happens, we're the next Venus.

Reaching that tipping point is quite difficult though, and would require additional warming besides fossil fuels. Methane-based runaway warming maybe? Or even basic thermodynamic heat sink effects (which we'll reach in about 200 years at the current rate of growth in energy supply).

Can you find a "serious climate scientist" who is claiming this "oceans boiling away" scenario is plausible? Alternatively, can you put some basic numbers on that? What will the surface temp need to be to initiate the boiling away?

It looks like currently the TSI at Earth varies between 1315 and 1405 w/m^2 (a range of 90 w/m^2), which apparently has less effect on the temperature than the tilt of the earth. http://lasp.colorado.edu/lisird/sorce/sorce_tsi/index.html

Right. But it is a possible mechanism that has nothing to do with the current thickness of the atmosphere or distance to the sun. Once upon a time, Venus wasn't so bad either.

Is James Hanson not "serious"? I agree that the consensus view seems to be that this is still an unlikely scenario.

http://news.nationalgeographic.com/news/2013/13/130729-runaw...

As far as numbers go, I'm not a climate scientist, but I have done these kind of calculations before. If I figure out the tipover temperature, I'll post it.

If I understood correctly, for that model they varied the "surface temperature" of a 1-dimensional object under constant illumination by hard-coding it, rather than simulating an increase in insolation or pressure to cause warming:

"When surface temperature is below the critical point temperature (647K), the surface pressure is set as the saturation vapour pressure for that surface temperature, and the tropospheric temperature– pressure structure is taken directly from the saturation vapour pressure curve. Where the surface temperature exceeds the critical point temperature, we prescribe surface pressure as 2.6×107 Pa, corresponding to Earth’s surface water reservoir." http://www.nature.com/ngeo/journal/v6/n8/full/ngeo1892.html

So first, they do increase the thickness of the atmosphere, and the increased surface temperature would come from a possible future sun (not adding CO2) I guess. Second, I wouldn't put much stock in such simplified models.