You wrote

If the oceans evaporate, I believe the whole Earth will be covered by clouds and its reflectivity, or ‘albedo’, will increase. Right now the (warming) greenhouse effect of clouds is about half their (cooling) albedo effect.

I only looked briefly into the sections you mentioned, they write (3a):

Although it seems quite unlikely, a case of more widespread and larger droplet size high clouds or good gaseous absorbers in the water vapour window and a reduction in low clouds could, in principle, increase the absorbed solar radiation and decrease the radiation limit. Without better understanding whether such changes are possible, we cannot totally rule out a runaway greenhouse.

So it seems rather that if clouds would form at higher altitudes (and if the temperature of the troposphere would rise and the earth surface temperature would stay as it is or get lower then if I understand correcty the clouds would be higher) than these may have larger droplets and thus they may actually have a lower albedo and it may eventually be so low that the greenhouse effect of clouds may get even bigger than the cooling cloud albedo effect, which would result in a runaway greenhouse. :O

Take a look at sections 3a), b) and c).

“What’s more relevant is what they call the ‘moist greenhouse’, in which enough of the oceans evaporate to make water vapor a major constituent of the troposphere.”

On a first glance I couldn’t find a good overview about the reflectivity of different earth surfaces, but I could imagine that if enough of the oceans evaporate then this could lead to quite an increase of the amount of solar radiation absorbed by the Earth from its current value of 240 watts/meter2 to a larger value…..where I hope that this value is not going over the above mentioned 300 watts/meter2……

Did the authors say something on that?

What’s more relevant is what they call the ‘moist greenhouse’, in which enough of the oceans evaporate to make water vapor a major constituent of the troposphere. They estimate this could be caused by raising the CO₂ concentration from its current value of 390 ppm to the much higher value of 10,000 ppm (by volume). This is apparently higher than could be achieved by burning all ‘conventional’ fossil fuels, but perhaps it’s achievable if we burn all tar sands, methane clathrates, and so on.

I have glanced at page 12 and things seem to be explained clearly already over there, but i want to understand Figure 5 better, so i’ll read it.

I have read somewhere that in one billion years or so the sun will shine 10% more, and that will trigger a runaway green house effect resulting in evaporation of the oceans.

Right. People argue about when this will happen—I’ve seen estimates of 1 or 2 billion years—but at some point it’ll happen, unless we do something. And the real bummer is that when this happens, some of the H₂O in the upper atmosphere will photodissociate, and the hydrogen will slowly get lost to outer space, so eventually the planet will lose a lot of its water.

So why are we sure that this will not happen in the next millennium if we burn all available fossil fuels ?

James Hansen believes it could, but most experts believe it’s unlikely. This paper seems to represent the consensus view:

• Colin Goldblatt, Andrew J. Watson, The Runaway Greenhouse: implications for future climate change, geoengineering and planetary atmospheres.

However, they write: “In the event that our analysis is wrong, we would be left with the situation in which only geoengineering could save us.”

You should definitely read this paper, Giampiero. Me too—I just discovered it! It’s quite technical, but it tries to give a clear overview of a rather complex problem. Let me just quote some, as an appetizer:

It is a common misconception that the runaway greenhouse is a simple extension of the familiar water vapour feedback. As the planet warms more water evaporates. Water vapour is a greenhouse gas, so this enhances the warming (a positive feedback). Accelerating this, by implication, would boil the entire ocean. Historically, this possibility was discussed by Sagan (1960) and Gold (1964), but some of the earliest modern-era climate models (Manabe & Wetherald, 1967) showed that while water vapour feedback is an important positive feedback for Earth, it is not a runaway feedback.

In fact, the physics of the runaway greenhouse is rather different to “ordinary” water vapour feedback. There exist certain limits which set the maximum amount of outgoing thermal (longwave) radiation which can be emitted from a moist atmosphere. In the ordinary regime in which Earth resides at present, an increase in surface temperature causes the planet to emit more radiative energy to space, which cools the surface and maintains energy balance. However as a limit on the emission of thermal radiation is approached, the surface and lower atmosphere may warm, but no more radiation can escape the upper atmosphere to space. This is the runaway greenhouse: surface temperature will increase rapidly, finally reaching equilibrium again only when the surface temperature reaches around 1400K and emits radiation in the near-infrared, where water vapour is not a good greenhouse gas. Along the way, the entire ocean evaporates.

In this section we review the physics of the runaway greenhouse in detail. Our theoretical framework follows Nakajima et al. (1992). Whilst Nakajima et al. (1992) was very formal and mathematical, our description here is more qualitative. We begin by describing three different “limits” on the outgoing longwave radiation. Whilst we find it useful to group these together under a single umbrella term we emphasize that the physical character of them is rather different. Limit 1, the black body upper limit, is the maximum radiation that any planet of given surface temperature can emit; this varies with temperature. It does not lead to a runaway greenhouse, but is included for context. Limit 2, the moist stratosphere upper limit, is the maximum amount of radiation which can be transferred by a moist stratosphere; this is invariant with temperature, so can lead to a runaway greenhouse. Limit 3, the moist troposphere asymptotic limit is the amount of radiation which a thick, pure water vapour atmosphere will emit; the radiation from any thick water-rich atmosphere will tend towards this asymptotic limit. It is invariant with temperature, so can lead to a runaway greenhouse. We then describe the changes which occur when increasing temperature in both runaway and moist greenhouses.

Understanding the runaway greenhouse requires familiarity with many aspects of atmospheric radiative transfer and thermodynamics. Given the interdisciplinary interest in this topic, we summarise the necessary background material in Appendix A.