From: David Rind <drind@giss.nasa.gov>
To: Stefan Rahmstorf <rahmstorf@pik-potsdam.de>
Subject: Re: 6.5.8 on climate sensitivity and last millennium
Date: Fri, 14 Jan 2005 14:23:47 -0500
Cc: David Rind <drind@giss.nasa.gov>, Tim Osborn <t.osborn@uea.ac.uk>, Jonathan Overpeck <jto@u.arizona.edu>, Keith Briffa <k.briffa@uea.ac.uk>, Eystein Jansen <eystein.jansen@geo.uib.no>, FortunatJoos@email.arizona.edu

   Here are my responses to the comments concerning 6.5.8d. With respect to Stefan's main
   concern: I too am sensitive to the possible mis-use of words that appear in a cavalier
   manner in the text. I think the way to avoid that is to be as precise as possible about
   what is being said. I also feel that hand-waving should be minimized - just because there
   are uncertainties, does not mean IPCC will throw up its hands. Thus the attempt to quantify
   these numbers are precisely as they will be done in other IPCC chapters. Again, the
   responses are in red, and the text alterations (or in this case, some entire text) are in
   blue.

     I'm not working on this topic myself so I'm by no means an expert. But I am still quite
     concerned with the wording in 6.5.8 on the last millennium.
     First, to avoid misunderstandings, I would like to suggest again to describe forcings
     and climate changes going forward in time, rather than going backwards in time. Even
     colleagues here that I discuss it with misunderstand the present version with backwards
     reasoning - it leads to phrases like "deforestation warming" (used by David in his last
     mail), although deforestation caused cooling - backwards in time you can see this as a
     warming, but should you call it "afforestation warming" if you look back in time? I
     suggest to use the physical, forwards, time arrow in the discussion.

   In all the other sections of 6.5.8 we discuss the temperature change and the radiative
   forcing relative to the present - when it was colder than the present, the temperatures
   were indicated to be colder, and the radiative forcing more negative. To alter that for
   this section alone would cause added confusion. I have therefore in each case tried to make
   it perfectly clear what is being said. In particular, I agree that in the case of
   deforestation the terminology does become confusing so the text has been changed to be more
   communicative; it now reads,

   Warming of 0.35°C due to the existence of  vegetation and forests that have since been cut
   down was found by (Bauer et al., 2003) ...

     The section states:

     If one takes mid-range estimates of solar and anthropogenic forcings, and assumes that
     volcanic, tropospheric ozone and land albedo changes cancel out, the resulting radiative
     forcing change is ~-2.4 Wm-2.

     I don't think we should give a "mid-range" of the forcing like this; the assumption that
     ozone, land albedo and volcanic changes cancel is hard to justify in any case. For the
     forcing we need to give a range in my opinion, not one number. If we give a range, it
     will become clear that the forcing is too uncertain for drawing conclusions on climate
     sensitivity from this time period.

   The problem with giving a maximum range for this time period is the same as giving one for
   the 20th century - the inclusion of the potential indirect effects of aerosols means you
   can wipe out all climate forcing entirely. It becomes a 'reductio aud absurdum'. The issue
   in particular for the Late Maunder Minimum time period, and the specific reason for
   including it, is that it potentially says something about SOLAR forcing.

   In writing this section, we are not simply doing a core dump of everything people have
   done, we are supposed to use our brains to assess the likely situation. Having already
   provided the range of uncertainty,we can give a 'best estimate' for the various forcings
   that we can use in a meaningful way if we are careful - and which show the importance of
   the uncertainty in the solar forcing. I do agree that what existed in the text especially
   for the third paragraph needed improvement. Therefore, after several talks with people
   here, I've altered (especially) the first and third paragraphs accordingly. Rather than
   just stating the conclusion that climate sensitivity can't be well defined, the paragraphs
   now show quantitatively that is the case. The specificity, I believe, gives people a real
   feeling for the uncertainties, and in the way it is done here, especially the uncertainty
   in the solar forcing and actual climate response. (This rewrite obviates the need for a
   direct response to several of Stefan's other comments.)

   (d)Last 1000 years

   We concentrate here on the Late Maunder Minimum time period in which sunspots were
   generally missing (approximately 1675-1715), but outside of the estimated solar irradiance
   change, the discussion is applicable for the pre-industrial climate in general. The primary
   forcings relative to today are (1) a decrease in various greenhouse gases, with a forcing
   of approximately -2.4±0.25 Wm-2 (not including tropospheric ozone changes); (2) reduced
   tropospheric sulfate aerosols, whose direct effect is estimated by IPCC (2001) as +0.4±0.3
   Wm-2 with an indirect effect ranging from +0.5 to +2 Wm-2  (3) a solar forcing reduction
   estimated as ranging from -0.12 to -1.56 Wm-2 (0.05% to 0.65%) ((Hoyt and Schatten,
   1993);(Lean, 2000);(Foukal and Milano, 2001); (Reid, 1997)); and (4) volcanic aerosol
   forcing either similar to today ((Robertson, 2001)), lower than today ((Robock and Free,
   1996)), or higher ((Crowley, 2000)). Large uncertainties therefore exist for all of the
   forcings except the trace gas values (again excluding tropospheric ozone). The cooling
   effects are offset to small degree by land albedo changes, estimated to contribute +0.4
   Wm-2  ((Hansen et al., 1998)). Reduced tropospheric ozone has been estimated to cause an
   additional forcing of -0.3 to -0.8Wm-2 (Mickley et al., 2001), while increased
   stratospheric ozone produced a positive forcing of -0.09 to -0.25 Wm-2 (IPCC, 2001). If one
   takes the most widely used  or mid-range estimates of solar (-0.5 Wm-2 from (Lean, 2000))
   and anthropogenic forcings (-2.4Wm-2  from reduced trace gases, other than tropospheric
   ozone; +0.5 Wm-2  from reduced sulfate aerosols), land albedo changes (0.4 Wm-2), decreased
   tropospheric ozone (-0.35 Wm-2 (IPCC, 2001)) and increased stratospheric ozone (+0.15 Wm-2
   (IPCC, 2001)), the net radiative forcing for this time is estimated as -2.2 Wm-2. [For this
   exercise we ignore the effects of volcanoes, the indirect effects of sulfate aerosols, and
   the effects of carbon and organic aerosols.]. Including these additional components (except
   for volcanic aerosols for which even the sign of the change is not well know), Hansen
   (personal communication) calculates a value close to -2 Wm-2.
   How cold was this time period? Different reconstructions (Fig. X1) provide different
   estimates of cooling, ranging from -0.45°C ((Mann et al., 1999), annual value for the
   Northern Hemisphere), to about -0.7°C ((Esper et al., 2002)for 20-90°N in the growing
   season, and (Briffa and Osborne, 2002)(from borehole temperature records). Model studies
   (Fig. X2) for this time period have generally employed significant solar reductions (-0.2%
   to -0.4%), which by themselves have resulted in cooling of about -0.5°C ((Cubasch et al.,
   1997);(Bauer et al., 2003);(Rind et al., 2004)). Utilizing a forcing of -1.5 to -2 Wm-2
   from the combined influence of preindustrial trace gases and aerosols results in additional
   cooling of about -1 to -1.5°C ((Fischer-Bruns et al., 2002);(Rind et al., 2004);(Zorita et
   al., 2004)). If volcanic aerosols were actually more extensive during this time period,
   then additional cooling would arise from this factor as well (on the order of -0.4°C found
   by (Hegerl et al., 2003)using the (Crowley, 2000)reconstruction). Warming of 0.35°C due to
   the existence of vegetation and forests that have since been cut down was found by (Bauer
   et al., 2003), on the same order but of opposite sign to the tropospheric ozone forcing
   (Mickley et al., 2004). Adding these effects from model simulations together produces a
   total cooling on the order of  -1 to -1.5°C or greater, significantly larger than any of
   the paleo-estimates. For the ~50 year time period associated with the Maunder Minimum,
   without large forcing trends, the model results are essentially in radiative balance, and
   while the influence of past solar variations could still be in acting, in at least one
   study they were shown to be unimportant (Rind et al., 2004).

   The climate sensitivity from the GCMs used for these studies in on the order of 0.6°C/ Wm-2
   (or higher, depending on the model used). To calculate the sensitivity from the
   observations, we first use the estimated forcing of -2.2 Wm-2 and recognize that ~0.85 Wm-2
   of this is unresolved (Hansen, personal communication) due to the rapid trace gas changes
   of the last few decades.  Therefore, only 1.35 Wm-2 of the radiative forcing should have
   been expressed in the system. Were this to have resulted in a temperature change of about
   -0.5°C (as in the Mann et al reconstruction), it would imply a climate sensitivity of 0.37
   Wm-2, i.e. at the low end of the IPCC range for doubled CO2 response. Using the higher
   estimated cooling of -0.7°C results in a climate sensitivity of 0.52 Wm-2. Alternatively,
   if the uncertain solar forcing change was at the estimated minimal value (-0.12 Wm-2), then
   the radiative forcing change would be reduced accordingly, and climate sensitivity for the
   two reconstructions increases to 0.5 Wm-2 and 0.7 Wm-2 (near 3°C for doubled CO2)
   respectively, for the different temperature reconstructions. This exercise can be carried
   on ad infinitum; considering the actual uncertainty in many of the forcings, and in the
   actual temperature response of the climate system, we conclude that we cannot properly
   constrain climate sensitivity for this time period (and to some extent the results are
   similar for other preindustrial time periods compared to the present).

   As an aside: if one uses the minimal estimate of solar forcing in the example presented,
   one gets a range of temperature response to 2xCO2 of 2-3°C, not too much different from
   that concluded in the paper Stefan just sent around (which was 2.5 to 3C).

     Then you state the Mann et al. data are 0.5 ºC below the 1990s in the Maunder Minimum. I
     can see they are 0.4 ºC below the reference level (I believe this is 1961-1990). The
     mean of the 1990s is 0.3 ºC above this level (I calculated this from the Jones data) -
     so I find that the Mann data are in fact 0.7 ºC below the 1990s in the MM. The
     difference between model expectation for 2.4 W/m2 and the actual found in the Mann data
     is almost gone then. Add to that the possibility that the Mann data may somewhat
     understimate the variability, and I do not see any significant discrepancy between
     models and data, which we should mention and which we could defend as real - even for
     "best guess" sensitivity and forcing, let alone considering the uncertainty in those.

   The easiest way to see this is to note that the Mann et al reconstruction has the late
   1600s slightly warmer than the late 1800s. It is widely acknowledged that the late 1800s
   were 0.6C colder than today (taking into account the heat island effect) (and the radiative
   forcings, a la IPCC 2001, are all with respect to the 1990s.) That puts the late 1600s at
   less than 0.6C colder, close to the value indicated in the text.

   David

--

   ///////////////////////////////////////////////////////////////////////////
   ///////////////////////////////////////////////////////////////////////////

