From: Jonathan Overpeck <jto@u.arizona.edu>
To: David Rind <drind@giss.nasa.gov>
Subject: RE: solar MM
Date: Mon, 8 Aug 2005 11:24:37 -0600
Cc: Keith Briffa <k.briffa@uea.ac.uk>, Eystein Jansen <eystein.jansen@geo.uib.no>

   David - sounds promising. So, the bottom line is that a little disagreement is ok - that's
   a reflection of the real uncertainty? But, the discrepancy is not all that big in the end?
   No need to take this to a higher level?

   Keith Briffa is back on line and finishing off Section 6.5, so you might want to send him
   an email w/ suggestions that help keep chap 6 compatible w/ 2 and 9 - for example, with
   respect to solar, we acknowledge the forcing could be less than 0.5 W/m**2, and the
   uncertaintly wrt to trop aerosols and land albedo is significant - we could easily be
   closer to chap 9's estimate. Would you say the key is that our analysis acknowledge the
   uncertainty so as to overlap well with the other chapters?

   Keith - please make sure you send your new 6.5 to David too - while you were out, he was
   working hard w/ chap 2 and 9 to make sure we (the IPCC) avoid saying things that confuse.
   The comparison of radiative forcings from 3 different angles is what assessment is all
   about, and it's great David has had the patience to help figure it all out.

   Thx, Peck

     Hi Gabi,

     The key to your proposed solution is the updated numbers from Chapter 2. If indeed the
     radiative forcing change to 1750 is -1.53, then presumably you have made this consistent
     with the earlier part of Chapter 9. The numbers previously looked like this (I haven't
     seen the latest version of 6.5, but I've included the previous estimates we had in the
     ZOD):

                                     W/m**2

                             Chapter 6               Chapter 9

                             MM              1750

     Greenhouse gases:               -2.4            -2.6

     TROP aerosols:          0.5             0.2

     Solar                   -0.5            -0.1

     Volcanic:                       ?               ?

     Land albedo:            +0.4            0.03

     Trop O3:                        -0.35           -0.4

     Strat O3:                       +0.15           0.10

     1'st indirect aerosol forcing                   1.2

     STRAT H2O                               -0.13

     AVIATION                                -0.02

     TOTAL                   -2.2            -1.7

     There is essentially no change in greenhouse gas forcing from 1750 to 1700 (see for
     example Crowley et al., GRL, 2003), so the difference in the estimated numbers is
     probably due to inclusion of more things or different choices in Chapter 2. A similar
     statement holds for trop aerosols. One can also use these two to presume that the same
     also holds true for land albedo. [The value listed for that in Chapter 9 is quite small
     compared to some other studies; e.g., Govindasamy et al., GRL, 28, 291-294,2001.] So, to
     the extent these numbers are still discussed in Chapter 6, they should be made
     consistent with those in chapters 2 and 9.

     With respect to your proposed paragraph below: I would drop the comments about trace gas
     differences but saying land albedo changes may have been greater, along with the
     additional solar change, could give us the -1.8 W/m**2 forcing.

     Concerning the temperature response: the Moberg et al paper itself claims 1°C difference
     between 1500 and 2000, but the figure seems to show a larger number, perhaps 1.3°C
     (again, just eye-balling it). However, the coldest time period is not in the MM but
     before it. I think therefore a better estimate from that paper for the MM would be 1°C.

     So, with respect to the sensitivity: if 0.85 W/m**2 is unresolved, then we have a total
     forcing of ~0.95 W/m**2, and a climate response varying between 0.45°C and 1°C - or a
     climate sensitivity for 2xCO2 of 1.9°C to 4.2°C, or pretty similar to standard IPCC
     estimates.

     I think this will work!

     David

     At 1:02 PM -0400 8/6/05, hegerl@duke.edu wrote:

     On Sat, 6 Aug 2005 hegerl@duke.edu wrote:
     p.s. I modified the text for MM forcing according to below theory
     (please yell if its off!) which would say (and has questions for you):
     During the cool period of the Late Maunder Minimum (approximately
     1675-1715), sunspots were generally missing, and solar irradince is
     believed to have been smaller. This period will be used in Section 9.6 to
     discuss climate sensitivity; therefore we discuss its radiative forcing
     estimates . The estimated difference between present day solar irradiance
     and the late 17th century Maunder Minimum is presently -1.1 W/m2 (best
     estimate, range -0.5 to -2 W/m2 , Chapter 2), but with large

     uncertainties. This leads to a best estimate radiative forcing of -0.2
     W/m2 (-0.1 to -0.35 W/m2 67% confidence interval; note that solar forcing
     from 1750 to the present is estimated having increased by 0.1 W/m2 ,
     chapter 2). Many radiative forcing changes, particularly those associated
     with industrialization,  are very similar from the present to the Maunder
     Minimum as they are from the present to preindustrial (total forcing
     estimated of -1.53 W/m2, see 9.2.1.2). CO2 may have been slightly lower
     (by???), and land cover changes may also have been glightly greater
     between the Maunder Minimum and 1750.  This yields an approximate net
     radiative forcing of-1.8 W m-2 (between the late Maunder Minimum and the
     present, with large uncertainties.
       >
     > > Hi David et al, > > I spent some more time pondering the MM forcing.
     > I think the best place to start is the updated chapter 2 forcing
     > from preindustrial, which is (according to what Joyce pulled out of

     > ch 2, so hope its correct):
     >
     > -1.53 from present to the 1750 period (all included that they deem
     > relevant, so no volcanoes because episodic, but all else in there
     > including contrails and other weird small stuff, I THINK it also
     > includes land cover changes)
     >
     > We would have to add -0.1 for the more reduced solar (given +0.1 1750 to
     > now from ch2, and 0.2 from MM on), and maybe some number for the
     > somewhat lower CO2 between 1700 and 1750 (what would that be)? and
     > maybe another number for additional changes in land cover?
     >
     > Overall, the number you had before of -1.8 (after adjusting solar down
     > to recent wisdom) seems now pretty good to me.
     > Should we keep it, or do you ahve another suggestion?
     > I am glad we didn't loose the forcing from MM to present :)))
     >
     > greetings, let me know what would be good for us to write (and then I'll
     > do the arithmetic for the best guess sensitivity once you guys also
     > check my numbers for high/low estimates of annual temp changes at that
     > period, right now its -0.45 Mann to -1.5 Moberg-readoffplotinahurry by me)
     >
     > Thanks in advance, I think we are very close to resolve this!
     >
     > Gabi
     >
     > On Fri, 5 Aug 2005, David Rind wrote:
     >
     > > As this continuing exchange has clarified, what's in Chapter 6 is
     > > inconsistent with what is in Chapter 2 (and Chapter 9 is caught in
     > > the middle!). Worse yet, we've managed to make global warming go
     > > away! (Maybe it really is that easy...:)
     > >
     > > David
     > >
     > > At 9:49 AM -0600 8/5/05, Bette Otto-Bliesner wrote:
     > > >Gabi,
     > > >
     > > >In Chap 6, we use 2.2 with a range of 1.9 to 2.6 W/m2.  The
     > > >uncertainty range includes both uncertainties in the ice core
     > > >measurements and uncertainties in the radiative transfer
     > > >calculations.
     > > >
     > > >Bette
     > > >
     > > >_
     > >
     > >
     > > >At 2:27 PM -0400 8/4/05, Gabi Hegerl wrote:
     > > >
     > > >David, so with the Judith correction only (solar down by 0.4), we
     > > >get a total forcing of
     > > >0.95 to MM, (after subtracting the 0.85 not realized yet according to Jim)
     > > >
     > > >Then, if the indirect effect and black carbon is added, wouldn't
     > > >this reduce the forcing to nearly nothing?
     > > >(or what am I doing wrong, 2.2 changes to 1.8 with new solar, black
     > > >carbon and ind aerosol takes away
     > > >0.9. yielding 0.9 W/m**2, then Jim says 0.85 of that is unrealized???)
     > >
     >
     > --------------------------------------------------------------------
     > Gabriele Hegerl
     > Dept. of Earth and Ocean Sciences, Nicholas School of the Environment
     > Duke University, Durham NC 27708
     > phone 919-684-6167, fax 919-684-5833
     > email: hegerl@duke.edu   http://www.eos.duke.edu/Faculty/hegerl.html
     > ---------------------------------------------------------------------
     >
     >
     >
     >
     --------------------------------------------------------------------
     Gabriele Hegerl
     Dept. of Earth and Ocean Sciences, Nicholas School of the Environment
     Duke University, Durham NC 27708
     phone 919-684-6167, fax 919-684-5833
     email: hegerl@duke.edu   http://www.eos.duke.edu/Faculty/hegerl.html
     ---------------------------------------------------------------------

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