= 27d567bc2d48d9f40e9ccc20ea370b15)
Mitchell_Porter comments on What is bunk? - Less Wrong
You are viewing a comment permalink. View the original post to see all comments and the full post content.
= 783df68a0f980790206b9ea87794c5b6)
Loading…= b715d02e85f7b3b4ae65ca3d3e450868)
Subscribe to RSS Feed
Powered by Reddit
= f037147d6e6c911a85753b9abdedda8d)
You are viewing a comment permalink. View the original post to see all comments and the full post content.
Comments (101)
This isn't the actual epistemic situation. The usual measure of the magnitude of CO2-induced warming is "climate sensitivity" - increase in temperature per doubling of CO2 - and its consensus value is 3 degrees. But the physically calculable warming induced directly by CO2 is, in terms of this measure, only 1 degree. Another degree comes from the "water vapor feedback", and the final degree from all the other feedbacks. But the feedback due to clouds, in particular, still has a lot of uncertainty; enough that, at the lower extreme, it would be a negative feedback that could cancel all the other positive feedbacks and leave the net sensitivity at 1 degree.
The best evidence that the net sensitivity is 3 degrees is the ice age record. The relationship between planetary temperature and CO2 levels there is consistent with that value (and that's after you take into account the natural outgassing of CO2 from a warming ocean). People have tried to extract this value from the modern temperature record too, but it's rendered difficult by uncertainties regarding the magnitude of cooling due to aerosols and the rate at which the ocean warms (this factor dominates how rapidly atmospheric temperature approaches the adjusted equilibrium implied by a changed CO2 level).
The important point to understand is that the full 3-degree sensitivity cannot presently be derived from physical first principles. It is implied by the ice-age paleo record, and is consistent with the contemporary record, with older and sparser paleo data, and with the independently derived range of possible values for the feedbacks. But the uncertainty regarding cloud feedback is still too great to say that we can retrodict this value, just from a knowledge of atmospheric physics.
Agreed. Nonetheless, as best I can calculate, Really Existing Global Warming (the warming that has occurred from the 19th century up to now, rather than that predicted in the medium-term future) is of similar order to what one would get from the raw, feedback-less effect of modern human CO2 emissions.
The additional radiative forcing due to increasing the atmospheric CO2 concentration from C0 to C1 is about 5.4 * log(C1/C0) W/m^2. The preindustrial baseline atmospheric CO2 concentration was about 280 ppm, and now it's more like 388pm - plugging in C0 = 280 and C1 = 388 gives a radiative forcing gain around 1.8W/m^2 due to more CO2.
Without feedback, climate sensitivity is λ = 0.3 K/(W/m^2) - this is the expected temperature increase for an additional W/m^2 of radiative forcing. Multiplying the 1.8W/m^2 by λ makes an expected temperature increase of 0.54K.
Eyeballing the HADCRUT3 global temperature time series, I estimate a rise in the temperature anomaly from about -0.4K to +0.4K, a gain of 0.8K since 1850. The temperature boost of 0.54K from current CO2 levels takes us most of the way towards that 0.8K increase. The remaining gap would narrow if we included methane and other greenhouse gases also. Admittedly, we won't have the entire 0.54K temperature boost just yet, because of course it takes time for temperatures to approach equilibrium, but I wouldn't expect that to take very long because the feedbackless boost is relatively small.
This might actually be a nice exercise in choosing between hypotheses. Suppose you had no paleo data or detailed atmospheric physics knowledge, but you just had to choose between 1 degree and 3 degrees as the value of climate sensitivity, i.e. between the hypothesis that all the feedbacks cancel, and the hypothesis that they triple the warming, solely on the basis of (i) that observed 0.8K increase (ii) the elementary model of thermal inertia here. You would have to bear in mind that most anthropogenic emissions occurred in recent decades, so we should still be in the "transient response" phase for the additional perturbation they impose...
Now you've handed me a quantitative model I'm going to indulge my curiosity :-)
I think we can account for this by tweaking equation 4.14 on your linked page. Whoever wrote that page solves it for a constant additional forcing, but there's nothing stopping us rewriting it for a variable forcing:
where T(t) is now the change in temperature from the starting temperature, Q(t) the additional forcing, and I've written the equation in terms of my λ (climate sensitivity) and not theirs (feedback parameter).
Solving for T(t),
If we disregard pre-1850 CO2 forcing and take the year 1850 as t = 0, we can drop the free constant. Next we need to invent a Q(t) to represent CO2 forcing, based on CO2 concentration records. I spliced together two Antarctic records to get estimates of annual CO2 concentration from 1850 to 2007. A quartic is a good approximation for the concentration:
The zero year is 1850. Dividing the quartic by 280 gives the ratio of CO2 at time t to preindustrial CO2. Take the log of that and multiply by 5.35 to get the forcing due to CO2, giving Q(t):
Plug that into the T(t) formula and we can plot T(t) as a function of years after 1850:
The upper green line is a replication of the calculation I did in my last post - it's the temperature rise needed to reach equilibrium for the CO2 level at time t, which doesn't account for the time lag needed to reach equilibrium. For t = 160 (the year 2010), the green line suggests a temperature increase of 0.54K as before. The lower red line is T(t): the temperature rise due to the Q(t) forcing, according to the thermal inertia model. At t = 160, the red line has increased by only 0.46K; in this no-feedback model, holding CO2 emissions constant at today's level would leave 0.08K of warming in the pipeline.
So in this model the time lag causes T(t) to be only 0.46K, instead of the 0.54K expected at equilibrium. Still, that's 85% of the full equilibrium warming, and the better part of the 0.8K increase; this seems to be evidence for my guess that we wouldn't have to wait very long to get close to the new equilibrium temperature.
If I knew that little, I guess I'd put roughly equal priors on each hypothesis, so the likelihoods would be the main driver of my decision. But to run this toy model, should I pretend the only variable forcing I know of is anthropogenic CO2? I'm going to here, because we're assuming I don't have 'detailed atmospheric physics knowledge,' and also because I haven't run the numbers for other variable forcings.
To decide which sensitivity is more likely, I'll calculate which value of λ produces a 0.8K increase from CO2 emissions by 2010 with this model and the above Q(t); then I'll see if that λ is closer to the '3 degrees' sensitivity (λ between 0.8 and 0.9) or the '1 degree' sensitivity (λ = 0.3). For an 0.8K increase, λ = 0.646, so I'd choose the higher sensitivity, which has a λ closer to 0.646.