jshore, you are right when you say
[QUOTE=jshore]
[QUOTE=flex727]
I’m wondering, since CO2 is not the only GHG in the atmosphere, shouldn’t this calculation be made based on total incremental increase, rather than just the CO2 increase? For example (made up numbers), if CO2 were 50% of the total GHG and I quadruple the CO2, then the total GHG has only increased by 2X and thus the expected temp rise would be ~1.58X, not 2X. What is the proper way to account for the fact that CO2 is only one GHG component?
[/QUOTE]
Each GHG must be considered on its own merits as they absorb in different parts of the spectrum and such. The way that climate sensitivity is conventional defined is under the assumption that the other gases remain constant. So, when scientists talk about the amount of radiative forcing from the doubling of CO2, they are talking about this under the assumption that the others are constant. (By the way, the fact that the radiative forcing depends logarithmically on the concentration of the gas is not a rigorous law of nature for all concentrations but is true over a fairly broad range of concentrations, and in particular, for the concentration range of interest for CO2.)
I believe that the effects of the major greenhouse gases of interest (CO2, methane, …) in the range of interests are approximately additive…so that, at least to a good approximation, you can compute each independently and add up the radiative forcings due to each one.
[/QUOTE]
In addition, I would add that the radiative forcings for the various gases are calculated by empirical formulas whose patronage is unclear. The result of the calculations are the instantaneous forcings, F(i).
Of particular interest is the difference between the instantaneous forcing F(i) and the equilibrium forcing F(e). F(e) is the net change in forcing after the system has time to respond, so it includes all feedbacks.
Unfortunately, climate models are the only guide to this, and they disagree. And understandably so. The answer depends subtly on the details of the precise implementation. Not an easy puzzle.
Let me see if I can illustrate the difficulty. Consider the view from my window out on to the deep tropical (9°S) Pacific ocean. In the morning, there’s often not a cloud in view.
The ocean, of course, is warmed by both the sun and the “greenhouse effect” during the day, so the surface waters start to warm. As the day heats up, the ocean evaporates a bit more, and a few clouds appear over oceanic hot spots, small clouds, they don’t do much.
As the day gets hotter, and people and the ocean start to sweat more (evaporative cooling, remember), more water vapor goes into the air. And the few clouds turn into a number of small clouds.
And then, if the day is hot enough, which is most days, an odd thing happens.
First one at at time, and then in bunches, the clouds all start to shoot upwards, growing taller and taller, becoming thunderstorms. Soon there’s the familiar high humidity, that hot and sweaty feeling that means rain’s on the way.
And sure enough, the bowl tips over and cooling rain pours back into the ocean.
Now, each of these thunderstorms that reach up to the high troposphere is one of the most amazing heat engines never invented by mankind. They are an emergent property of the climate system, meaning one moment they are not there, and the next they are there. They are “self-organized”, they form by themselves.
A thunderstorm is a reflective heat pipe moving energy way high into the sky. Shielded inside the towering cloud is a stream of warm moist air moving rapidly upwards, like water through a hose. This air cannot radiate to the outside, because it is in the middle of the cloud.
By means of that heat pipe, all of that surface heat (both latent and sensible) is transported way above most of the CO2, where it spreads out and radiates freely out to space. For heat at the surface, a thunderstorm is a tunnel through the wall of greenhouse gases and out to escape..
In addition, the storm returns the hot water evaporated minutes or hours earlier back to the ocean as cool rain.
Next, of course, it reflects hundreds of watts per square metre of solar energy back into space.
In addition, it whips up a wind around the base that increases a) evaporation, b) albedo , and c) cloud nuclei (microscopic salt spray crystals).
I bring all of this up because it relates to the results of increasing forcing in the tropics. A naive view would be that the temperature would react linearly to the increasing forcing. The whole idea of “climate sensitivity” is based on that idea, that forcing times sensitivity equals temperature change.
But look at what happens every day outside my window as the forcing increases. For a while, the ocean temperature rises. But then, clouds start to form, and the rise slows down. When the thunderstorms start to form, they pipe heat directly past the CO2, plus they shade the ocean, plus they provide cold wind and rain. As the sun increases, the number and size of the thunderstorms increases, driven by the increased forcing.
These thunderstorms are very efficient at what they do. Evaporation is linear with wind speed, so the wind-driven evaporation at the base of the thunderstorm is very high. But the winds don’t waste the water on raising the local relative humidity. Instead, the winds sweep the water vapor into the heat pipe, and it is taken aloft, turned into rain, and sent back to the ocean.
Once the thunderstorms start to form, increasing moisture in the air merely makes more clouds, because the air is near saturation. Nor does the temperature rise much further, because of the variety and efficiency of the thunderstorm’s cooling mechanisms (transport of heat aloft, reflection of incoming energy, wind driven evaporative cooling, and cold wind and water from aloft). More forcing just leads to more thunderstorms.
So in response to a linear change in forcing (the sun getting hotter during the day) we have a linear ocean temperature rise for a while. Then as clouds form the temperature rise slows. Finally, when thunderstorms form, the temperature rise slows substantially. Eventually, as the forcing continues to increase, a point will be reached at which the temperature rise stops. This is why open ocean temperatures, no matter how hot the sun, don’t go above about 32°C. Ever.
So, flex727, in answer to your original question as to whether different kinds of forcings can be added together, heck, no. As my example shows, even different amounts of the same forcing can’t be added together. In the morning, an additional 10 watts/m2 of extra forcing might warm the ocean by six degrees. But in the afternoon, an additional ten watts of forcing might change the temperature a quarter of a degree, with the rest going into evaporation.
To me, the whole idea that there is a linear relationship between forcing and temperature, that mythical number called a “climate sensitivity” that linearly relates a change in forcing to a change in temperature, can be shown to be false just by looking out my window. Increased forcing is not at all linear, and climate sensitivity is not a constant.
Nor can one depend on averages to calculate an “average climate sensitivity”. Take the question of the average relative humidity outside my window. It rises until the time of full cloud formation, and then it doesn’t rise any further, even though the forcing is still rising. All that happens is that more and more water goes up in the clouds and comes down again as rain. The atmosphere doesn’t get any wetter, the cycle just runs stronger.
What’s happening outside my window doesn’t depend on the gridcell average relative humidity. It depends on the relative humidity of the wind-whipped air at the base of the thunderstorm, which is very different from the averate.
See, here’s the crucial thing - climate sensitivity depends heavily on temperature. In the morning, climate sensitivity around here is high. But by the afternoon, it is falling fast, and may actually go to zero.
So before the model can tell me the climate sensitivity, it has to tell me the temperature. But before it can tell me the temperature, it has to calculate the climate sensitivity …
w.