I'm looking for comments on the form and substance of the following,
in case I've committed a blunder or even just a mistake.
====================
Climate sensitivity, Shaviv and Tung et al.
(1) Climate sensitivities.
There are several climate senstivities, depending upon time to
measurement. The shorest commonly used is transient climate
sensitivity (TCS). This is measured in GCMs as the change in
temperature due to added 1% CO2, compounded yearly, until 2xCO2 is
reached; 70 years. The most commonly used climate sensitivity is
Charney's equilibrium climate sensitivity (ECS). This is defined as
the change in temperature when the climate reaches equilibrium after a
sudden jolt of 2xCO2, requiring a mellinnium or more. Hansen et al
have defined another, longer and higher one; Spencer has implicitly
defined a very short (and small) climate sensitivity in some of his
writings.
Here we need another, the solar cycle climate sensitivity (SCCS).
This is the change in temperature from solar minimum to solar maximum
for the average solar cycle. Although Tung et al. in [1] treat this
as if it is the same as TCS, I doubt this since the time is only 5.224
years (on average), not the 70 yers required for TCS; it ought to be
smaller. Fortunately, this short time agrees fairly well with the
fast response time to an instant doubling of CO2 and from Figure 1 in
[2] this is close to 60% of ECS, not the two thirds of ECS that is
about right for TCS.
(2) Ocean Mixed Layer Depth; Isothermal Layer Depth.
Whle probably Shaviv's estimate of 400 meters for MLD in [3] is about
right, this is vastly too deep for the Isothermal Layer Depth (ILD)
appropriate to solar forcing. For this reason Shaviv obtains much too
small a value for his estimate of the SCCS. On the other hand, Tung &
Camp in [4] use an ILD of zero, too shallow, and so obtain a value of
SCCS in [1] which is larger than the usual TCS estimates. Clearly an
intermediate value should be used. Tung & Camp state that the solar
influence does not extend beyond 100 meters deep; we know that the top
10 meters mixes, so being isothermal, very rapidly due to wave
action. This suggests using about 10 meters as the appropriate depth,
although somewhat deeper might also be the best estimate. On the othr
hand, it canot be much deeper since the phase lag for the air
temperature response to solar forcing is less than one year according
to Tung & Camp.
(3) Discussion.
It appears that the larger the ITD used in such studies, the smaller
the associated SCCS. I assume this is because more of the forced heat
goes into rapidly heating the top layer of the ocean, not the air.
While heat diffusion into the deep ocean seems appropriate, this is
more problematic for the shallow ocean, which is vertically mixing as
well. Whether this much much difference to the analysis in the
appendix of Tung & Camp is not clear. My thought is tahat the same
formulation can be used to account for both the mixing and the
diffusion, leading to the same equations.
(4) Conclusion.
Since SCCS must be less than TCS and 60% of ECS appears about right,
lets use that. With ECS taken as the most likely value of 3 K, then
SCCS is about 1.8 K. But indeed this is almost the same as Tung &
Camp's measured value of about 1.7 K. So once again we see further
confirmation that ECS is close to 3 K.
(5) References.
[1] K.K. Tung, J. Zhou and C.D.Camp; 2008: Constraining Model
Transient Climate Response using Independent Observations of Solar-
Cycle Forcing and Response, Geophys. Research Lett., 35, L17707,doi:
10.1029/2008GL034240.
[2] Reto Knutti, Stefan Krahenmann, David J. Frame, and Myles R.
Allen; 2008: Comment on ‘‘Heat capacity, time constant, and
sensitivity of Earth’s climate system’’ by S. E. Schwartz, Journal of
Geophysical Research, 113, D15103, doi:10.1029/2007JD009473.
[3] Nir J. Shaviv (2008); Using the oceans as a calorimeter to
quantify the solar radiative forcing, J. Geophys. Res., 113, A11101,
doi:10.1029/2007JA012989.
[4] K.K. Tung and C.D. Camp; 2008: Solar Cycle Warming at the Earth's
Surface in NCEP and ERA-40 data: A linear Discriminant Analysis,
Journal of Geophysical Research, 113, D05114, doi:10.1029/2007JD009164.
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