cc: firstname.lastname@example.org, tar_tsatXYZxyzo.gov.uk
date: Tue, 17 Oct 2000 15:09:23 +0100
from: "Mitchell, John FB" <jfbmitchellatXYZxyzo.gov.uk>
subject: RE: 'balance' Issue for TS and SPM
to: 'Joyce Penner' <penneratXYZxyzch.edu>, "Mitchell, John FB" <jfbmitchellatXYZxyzo.gov.uk>, 'Michael Prather' <mpratheratXYZxyz.edu>, Michael_Oppenheimer@environmentaldefense.org, John Stone <John.Stone@ec.gc.ca>, griggs <djgriggsatXYZxyzo.gov.uk>
I have picked out your two questions and tried to respond to them
WHAT DOES THIS MEAN??
Detection in principle could mean that the signal could explain only
0.0000001C of the the observed change.
The detection studies show typically 0.5K attributable to
anthropogenic effects, with presumably a larger contribution from greenhouse
gases- we chose to be extremely conservative, choosing subsatantial to mean
"on the order of 20% or more". If you can think of a better word, then let
BUT THIS IS CIRCULAR REASONING. THE DETECTION ONLY WORKS IF THE
FORCING IS SMALLER THAN THE GHG FORCING. YOU CAN'T CONCLUDE IT HAS
IF SULFATE FORCING IS AS LARGE AS THE LARGEST NEGATIVE UNCERTAINTY.
ONLY SAY THAT THE OBSERVED PATTERNS ARE ONLY CONSISTENT WITH A
FORCING THAT IS SMALLER THAN THE GHG FORCING
It is not circular reasoning- if we assess that the observed warming
is too large to be explained by natural factors, then there must be a net
anthropogenic warming (NB temperature response). Then, as you say, it
implies that there is a net positive forcing due to all anthropogenic
factors.(unlress there are perverse tricks of heat storage such as we
discussed in NY).
Detction and attribution is a consistency excercise. I don't know
what "CONCLUDE IT HAS WORKED
IF SULFATE FORCING IS AS LARGE AS THE LARGEST NEGATIVE UNCERTAINTY".
I don't think this phrase is used in chapter 12.
The detection makes no assumption about the magnitude of the
simulated response to sulphates - it scales the simulated "sulphate plus
other non well mixed GHG factors) response to give a best fit. In the case
of HadCM3, an approximate calculation based on optimal detection gives an
estimated radiative forcing -0.5 to -1.5 Wm-2 for the indirect effect (5
to 95% uncertainty range) whcih implies the forcing (Non well mixed GHG and
aerosols) actually calculated in the model is consistent with observations
(The estimate will differ with models). If we had got a larger sulphate
cooling in our model, then to be consistent with the observations, the
detection procedure would have to scale down the sulphate response ( in
which case the estimate of the GHG response is unchanged and there is an
implication that the non-ghg forcing or response is over-estimated) , or
scale up the greenhouse gas contribution, in which case the larger sulphate
forcing is balanced by a larger greenhouse gas warming as we imply in the
chapter. Either way, we still detect a substantial GHG contribution.
I am grateful for attempts to clarify the findings of our chapter. I
think it might be helpful if you looked at chapter 12 if you are still
concerned about this issue - detection and attribution has developed
considerably since the SAR.
With best wishes
Hadley Centre for Climate Prediction and Research
The Met. Office, Bracknell
RG12 2SZ UK
Tel +44 1344 856613/6656
Fax+44 1344 856912
> -----Original Message-----
> From: Joyce Penner [SMTP:email@example.com]
> Sent: Tuesday, October 17, 2000 1:51 PM
> To: Mitchell, John FB; 'Michael Prather';
> Michael_Oppenheimer@environmentaldefense.org; Joyce Penner; John Stone;
> Cc: firstname.lastname@example.org; tar_tsatXYZxyzo.gov.uk
> Subject: RE: 'balance' Issue for TS and SPM
> SEE BELOW
> At 12:03 PM +0100 10/17/00, Mitchell, John FB wrote:
> >Dear Michael,
> >As you were not in New York, let me comment briefly.
> >First, the statement is not a closed statement - because of the huge
> >uncertainty in indirect aerosl forcing- necessarily negative, we cannot
> >an upper bound to the greenhouse gas contribution. We only say that it is
> >substantial- ie at least non negligible.
> >Second, detection studies work on the largest space scales only -
> >only encompassing a N-S gradient and land-sea contrast -
> > also atmospheric and other processes smooth out regional detail
> >which may be apparent in the forcing pattern. See eg figure 12.3
> >Third, detection and attribution schemes allow for scaling of the
> >of patterns- if the pattern amplitude is wrong then the regression
> >used in optimal detection can scale the signal to correct for this..
> >Fourthly, natural factors are ruled out largely becuase of their time
> >Fifthly, if the aerosol patterns look like greenhouse patterns ito the
> >detection procedure, then attribution of a sizeable greenhouse gas
> >contribution follows from the assertion that the observed warming is too
> >to be explained by natural factors alone - anthropogenic factors must
> >provide a warming and the bigger the (negative) aerosol forcing, the
> >the GHG gas warming required to bal ce it. If the patterns are different,
> >then this makes detection and attribution easier.
> BUT AGAIN, THIS REASONING FORGETS THE OTHER INFORMATION WE HAVE. GHG
> WARMING IS NOT LARGER THAN THE ESTIMATES FROM THE BOTTOM UP STUDIES.
> >Finally, at least two of the studies cited in the chapter include a
> >reperesentation of increases in tropospheric ozone.
> >For more details, I recommend you read the chapter- especially sections 4
> >and 6 - you should consider all the evidence..
> >With best wishes
> >Hadley Centre for Climate Prediction and Research
> >The Met. Office, Bracknell
> >RG12 2SZ UK
> >Tel +44 1344 856613/6656
> >Fax+44 1344 856912
> >> -----Original Message-----
> >> From: Michael Prather [SMTP:email@example.com]
> >> Sent: Friday, October 06, 2000 9:21 PM
> >> To: Michael_Oppenheimer@environmentaldefense.org; Joyce Penner;
> >> Stone; griggs
> >> Cc: firstname.lastname@example.org; tar_tsatXYZxyzo.gov.uk
> >> Subject: Re: 'balance' Issue for TS and SPM
> >> Dear David, John, Joyce, and Michael
> >> My apologies, I have been unable to contribute to this very important
> >> debate
> >> until I cleared my chapter.
> >> The wording in the SPM draft we were discussing (15 Apr draft given
> >> is
> >> far too strong a statement: it removes the fundamental issue that this
> >> finding
> >> is basically still a balance of the evidence. Admittedly what is new
> >> since the
> >> SAR is that more weight has accumulated on the
> >> "have-detected-human-influence"
> >> side of the balance (as Michael O notes). Nevertheless, there are
> >> some
> >> large and open problems (e.g., indirect aerosol effects) that prevent
> >> from
> >> being a closed case.
> >> Today a new SPM draft appeared (6 Oct, below) that chooses more
> >> words
> >> (I only wish that 'balance' could somehow be worked in).
> >> BUT the final bullet in the new section stands out in that it avoids
> >> major
> >> new uncertainties that have been identified - merely by doing a
> >> GHGas+Sulfate
> >> vs. GHGas alone model does not address the uncertainties in "other"
> >> forcings,
> >> such as other aerosols or the history of the increase in tropospheric
> >> ozone -
> >> which cannot be explained well and is certainly not documented. I
> >> that
> >> these studies considered the range of uncertainty in tropospheric ozone
> >> growth
> >> or in OC/BC aerosols and indirect effects. This last bullet cannot be
> >> supported
> >> from what I found in Chapters 4 and 5.
> >> I leave these issues for discussion in NY,
> >> Michael
> >> ------------------------------------------
> >> SPM (15 Apr 2000)
> >> "From the body of evidence since IPCC (1996), we conclude that there
> >> been a
> >> discernible human influence on global climate."
> >> --------------------------------------------
> >> new SPM (6 Oct 2000)
> >> "There is now stronger evidence for a human influence on global climate
> >> than at
> >> the time of the IPCC Working Group I, Second Assessment Report, and it
> >> likely
> >> that increasing concentrations of anthropogenic greenhouse gases have
> >> contributed substantially to the observed global warming over the last
> >> years.
> >> . . .
> >> Uncertainties in other forcings do not prevent identification of the
> >> effect of
> >> anthropogenic greenhouse gases over the last 50 years. The sulphate
> >> forcing,
> >> while uncertain, is negative over this period and changes in natural
> >> forcing
> >> during most of this period are also estimated to be negative."
> Joyce Penner, Professor Office: 2516 Space Research
> Dept. of Atmospheric, Oceanic, Phone: 734-936-0519
> and Space Sciences Fax: 734-764-4585
> University of Michigan E-mail: PenneratXYZxyzch.edu
> 2455 Hayward
> Ann Arbor, MI http://aoss.engin.umich.edu/Penner/