Interesting contribution. I can certainly see how the presence of more water vapour in the atmosphere would increase the heat trapping effect and how the resulting higher temperature would in turn lead to more water vapour in the atmosphere. The obvious question, then, is what prevents this process from leading to a runaway greenhouse effect today. What are the compensating processes that have, historically, allowed equilibrium to be maintained?
I presume that a higher temperature in principle increases the amount of IR radiation from the Earth into space, from cloud tops and whatever windows in the IR are not blocked by water vapour or other greenhouse gas absorption bands. I can only presume that, at least up to now, this increases enough to restore an equilibrium between radiation received and radiation emitted.
If so, then I would imagine the problem with increases in other greenhouse gases may be that these remaining windows for radiating heat become progressively blocked, leading to an increase in temperature before enough radiation is once more emitted, via the remaining channels, to restore equilibrium again.
Would that be your understanding of how these things are related?
I feel it is worth clarifying this point, as the sceptics are otherwise entitled to ask why, if this water vapour +ve feedback loop exists, the Earth did not suffer a runaway greenhouse effect aeons ago.
That is essentially what I’ve figured also. It must be that any measurable temperature change (from a ‘forcing’ on the system) already includes the feedback from any change in water vapor, since water vapor adjusts as quickly as weather changes rather than at a glacial pace.
As you noted, the system radiates away more heat, as the system heats up, until the extra heat leaving will balance with the extra heat of the system. That is of course on average, for the globe over the year, I think, and so it does act like a classic ‘black body’ for the purposes of using physics to understand the system. As a complex system with layers of ocean and atmosphere at different temperatures, the heat transport within an individual layer can experience a lot of horizontal adjustment. And that can occur without changing the vertical structure of all the layers of the system, which still ultimately act like a ‘black body’ radiating to achieve equilibrium.
I mention those horizontal adjustments, or redistribution of heat within a particular layer, thinking this might explain why we don’t get a runaway greenhouse effect when a little extra heating causes more water vapor to enter the system. That extra heat also causes horizontal adjustments in the troposphere, so that extra water vapor also leaves the system more quickly, instead of just building up into a runaway greenhouse effect.
From a textbook on climate (which the professor teaching that class originally learned from) Oxford Monographs on Geology and Geophysics, no. 16: Paleoclimatology; Crowley & North; 1991, they talk about this on pages six and seven as part of the introductory basics.
From: sect.1.2 Energy Balance Models (EBMs) of the Present Climate; part 1.2.1 Radiation and Climate
Part of the upwelling radiation is intercepted and absorbed by layers of the stratified atmosphere primarily through triatomic trace gases such as H2O, CO2, and O3, as revealed in the absorption spectrum (Fig.1.3). Carbon dioxide absorbs at about 2.7, 4, 10, and 14 μm; a water vapor continuum exists from ~12 to 18 μm; and so on. Water vapor is the most important of these infrared absorbers. Its concentration in the atmosphere is highly variable even on short (weather) time scales. Its saturation vapor pressure also increases approximately exponentially as temperatures increase, with the vapor pressure of water roughly doubling for each increment of 10°C in the range of interest. This is called the Clausius-Clapeyron relationship, and it is an important feature of the earth’s climate (and climate models). It is an interesting fact that the relative humidity (ratio of actual concentration to the saturation value) appears to stay at an approximately constant value near 50% as the climate changes.
We’ve been analyzing this from an ‘energy balance’ perspective, but I think it is the ‘general circulation’ within our troposphere that explains why there isn’t a runaway greenhouse effect. Adding more energy to the overall system (climate amping) speeds up the transport of heat from the equator to the poles, since the system is just a big ‘heat engine’ fundamentally, especially in our little tropospheric level.
That same section begins on page six with: “The simplest class of climate models is the energy balance model. As the name implies, the models focus on the required balance between incoming and outgoing radiation at the top of the atmosphere. Even though they consider only temperature, a number of significant insights are possible.”
And that part concludes on page seven by saying: “Climate modelers have learned to model the radiation fluxes and their interaction with the layers of matter in a vertical column of air above the earth’s surface. Not only must the details of the wavelength dependencies of the various absorptivities, emissivities, and scattering properties be included, but allowances for vertical convective motions must also be included. Such convection will occur when the atmospheric thermal layering is buoyantly unstable. The net effect of the convection is to remove heat from the surface and carry it higher in the atmosphere mechanically. Even without including horizontal motions of the atmosphere, these so-called radiative convective models (RCMs) are quite successful in describing the vertical temperature structure of the atmosphere and in leading the way for more accurate and efficient radiation calculations in general circulation models (Ramanathan and Coakley, 1978)." (my emphases)
As Hazelm mentioned, if you’ve got more vapor in the atmosphere, you’ll get more precipitation. Even if it is warm enough (on average) to hold more water vapor, wherever it does get cold enough, precipitation still happens. Consistent with that theory, the records are increasingly showing storms becoming more frequent or more intense and larger, compared to the long-term average over the past century.
“Total annual precipitation has increased over land areas in the United States and worldwide.”
“In recent years, a higher percentage of precipitation in the United States has come in the form of intense single-day events.”
“The prevalence of extreme single-day precipitation events remained fairly steady between 1910 and the 1980s but has risen substantially since then.”
“Nationwide, nine of the top 10 years for extreme one-day precipitation events have occurred since 1990.”
“The occurrence of abnormally high annual precipitation totals has also increased."
Edited by Essay, 24 December 2018 - 05:50 AM.