cc: Thomas R Karl <Thomas.R.Karl@noaa.gov>,  "'Philip D. Jones'" <p.jones@uea.ac.uk>, "Thorne, Peter" <peter.thorne@metoffice.gov.uk>,  carl mears <mears@remss.com>
date: Fri, 15 Feb 2008 08:53:36 -0800
from: Ben Santer <santer1@llnl.gov>
subject: [Fwd: Additional calculations]
to: Richard W Reynolds <Richard.W.Reynolds@noaa.gov>

<x-flowed>
Dear Dick,

I'm forwarding an email that I sent out several days ago. For the last 
month, I've been working hard to respond to a recent paper by David 
Douglass, John Christy, Benjamin Pearson, and Fred Singer. The paper 
claims that the conclusions of our CCSP Report were incorrect, and that 
there is a fundamental discrepancy between simulated and observed 
temperature changes in the tropical troposphere. Douglass et al. also 
assert that models cannot represent the "observed" differential warming 
of the surface and troposphere. To address these claims, I've been 
updating some of the comparisons of models and observations that we did 
for the CCSP Report, now using newer observational datasets (among them 
NOAA ERSST-v2 and v3). As you can see from the forwarded email, the 
warming rates of tropical SSTs are somewhat different for ERSST-v2 and 
v3 - ERSST-v3 warms by less than v2. Do you understand why this is?

With best regards, and hope you are well!

Ben
----------------------------------------------------------------------------
Benjamin D. Santer
Program for Climate Model Diagnosis and Intercomparison
Lawrence Livermore National Laboratory
P.O. Box 808, Mail Stop L-103
Livermore, CA 94550, U.S.A.
Tel:   (925) 422-2486
FAX:   (925) 422-7675
email: santer1@llnl.gov
---------------------------------------------------------------------------- 


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Date: Wed, 13 Feb 2008 18:34:51 -0800
From: Ben Santer <santer1@llnl.gov>
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To: santer1@llnl.gov, Peter Thorne <peter.thorne@metoffice.gov.uk>,
        Stephen Klein <klein21@llnl.gov>,
        Susan Solomon <Susan.Solomon@noaa.gov>,
        John Lanzante <John.Lanzante@noaa.gov>,
        Melissa Free <melissa.free@noaa.gov>,
        Dian Seidel <dian.seidel@noaa.gov>, Tom Wigley <wigley@cgd.ucar.edu>,
        Karl Taylor <taylor13@llnl.gov>,
        Thomas R Karl <Thomas.R.Karl@noaa.gov>, Carl Mears <mears@remss.com>,
        "David C. Bader" <bader2@llnl.gov>,
        "'Francis W. Zwiers'" <francis.zwiers@ec.gc.ca>,
        Frank Wentz <frank.wentz@remss.com>,
        Leopold Haimberger <leopold.haimberger@univie.ac.at>,
        "Michael C. MacCracken" <mmaccrac@comcast.net>,
        Phil Jones <p.jones@uea.ac.uk>,
        Steve Sherwood <Steven.Sherwood@yale.edu>,
        Tim Osborn <t.osborn@uea.ac.uk>,
        Gavin Schmidt <gschmidt@giss.nasa.gov>,
        "Hack, James J." <jhack@ornl.gov>, peter gleckler <gleckler1@llnl.gov>
Subject: Additional calculations
References: <200801121320.26705.John.Lanzante@noaa.gov>	 <478C528C.8010606@llnl.gov> <p06230904c3b2e6b2c92f@[172.17.135.52]>	 <478EC287.8030008@llnl.gov>	 <1200567390.8038.35.camel@eld443.desktop.frd.metoffice.com>	 <7.0.1.0.2.20080117140720.022259c0@llnl.gov> <1200995209.23799.95.camel@eld443.desktop.frd.metoffice.com> <47962FD1.1020303@llnl.gov>
In-Reply-To: <47962FD1.1020303@llnl.gov>
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<x-flowed>
Dear folks,

Sorry about the delay in sending you the next version of our manuscript. 
  I decided that I needed to perform some additional calculations. I was 
concerned that we had not addressed the issue of "differential warming" 
of the surface and troposphere - an issue which Douglass et al. HAD 
considered.

Our work thus far shows that there are no fundamental inconsistencies 
between simulated and observed temperature trends in individual 
tropospheric layers (T2 and T2LT). But we had not performed our "paired 
trends" test for trends in the surface-minus-T2LT difference time 
series. This is a much tougher test to pass: differencing strongly damps 
the correlated variability in each "pair" of surface and T2LT time 
series. Because of this noise reduction, the standard error of the 
linear trend in the difference series is typically substantially smaller 
than the size of the standard error in an individual surface or T2LT 
time series. This makes it easier to reject the null hypothesis of "no 
significant difference between simulated and observed trends".

In the CCSP Report, the behavior of the trends in the surface-minus-T2LT 
difference series led us to note that:

"Comparing trend differences between the surface and the troposphere 
exposes potential discrepancies between models and observations in the 
tropics".

So it seemed wise to re-examine this "differential warming" issue. I 
felt that if we ignored it, Douglass et al. would have grounds for 
criticizing our response.

I've now done the "paired trends" test with the trends in the 
surface-minus-T2LT difference series. The results are quite interesting. 
They are at variance with the above-quoted finding of the CCSP Report. 
The new results I will describe show that the "potential discrepancies" 
in the tropics have largely been resolved.

Here's what I did. I used three different observational estimates of 
tropical SST changes. These were from NOAA-ERSST-v2, NOAA-ERSST-v3, and 
HadISST1. It's my understanding that NOAA-ERSST-v3 and HadISST1 are the 
most recent SST products of NCDC and the Hadley Centre. I'm also using 
T2LT data from RSS v3.0 and UAH v5.2. Here are the tropical (20N-20S) 
trends in these five datasets over the 252-month period from January 
1979 to December 1999, together with their 1-sigma adjusted standard 
errors (in brackets):

UAH v5.2         0.060 (+/-0.137)
RSS v3.0         0.166 (+/-0.130)
HADISST1         0.108 (+/-0.133)
NOAA-ERSST-v2    0.100 (+/-0.131)
NOAA-ERSST-v3    0.077 (+/-0.121)

(all trends in  degrees C/decade).

The trends in the three SST datasets are (by definition) calculated from 
anomaly data that have been spatially-averaged over tropical oceans. The 
trends in T2LT are calculated from anomaly data that have been spatially 
averaged over land and ocean. It is physically reasonable to do the 
differencing over different domains, since the temperature field 
throughout the tropical troposphere is more or less on the moist 
adiabatic lapse rate set by convection over the warmest waters.

These observational trend estimates are somewhat different from those 
available to us at the time of the CCSP Report. This holds for both T2LT 
and SST. For T2LT, the RSS trend used in the CCSP Report and in the 
Santer et al. (2005) Science paper was roughly 0.13 degrees C/decade. As 
you can see from the Table given above, it is now ca. 0.17 degrees 
C/decade. Carl tells me that this change is largely due to a change in 
how he and Frank adjust for inter-satellite biases. This adjustment now 
has a latitudinal dependence, which it did not have previously.

The tropical SST trends used in the CCSP Report were estimated from 
earlier versions of the Hadley Centre and NOAA SST data, and were of 
order 0.12 degrees C/decade. The values estimated from more recent 
datasets are lower - and markedly lower in the case of NOAA-ERSST-v3 
(0.077 degrees C/decade). The reasons for this downward shift in the 
estimated warming of tropical SSTs are unclear. As Carl pointed out in 
an email that he sent me earlier today:

"One important difference is that post 1985, NOAA-ERSST-v3 directly 
ingests "bias adjusted" SST data from AVHRR, a big change from v2,
which didn't use any satellite data (directly). AVHRR is strongly 
affected in the tropics by the Pinatubo eruption in 1991.  If the "bias 
adjustment" doesn't completely account for this, the trends could be 
changed".

Another possibility is treatment of biases in the buoy data. It would be 
nice if Dick Reynolds could advise us as to the most likely explanation 
for the different warming rates inferred from NOAA-ERSST-v2 and v3.

Bottom line: The most recent estimates of tropical SST changes over 1979 
to 1999 are smaller than we reported in the CCSP Report, while the T2LT 
trend (at least in RSS) is larger. The trend in the observed difference 
series, NOAA-ERSST-v3 Ts minus RSS T2LT, is now -0.089 degrees C/decade, 
which is very good agreement with the multi-model ensemble trend in the 
Ts minus T2LT difference series (-0.085 degrees C/decade). Ironically, 
if Douglass et al. had applied their flawed "consistency test" to the 
multi-model ensemble mean trend and the trend in the NOAA-ERSST-v3 Ts 
minus RSS T2LT difference series, they would not have been able to 
conclude that models and observations are inconsistent!

Here are the observed trends in the tropical Ts minus T2LT difference 
series in the six different pairs of Ts and T2LT datasets, together with 
the number of "Hits" (rejections of the null hypothesis of no 
significant difference in trends) and the percentage rejection rate 
(based on 49 tests in each case)

"Pair"                            Trend   1-sigma C.I.  Hits  Rej.Rate
HadISST1 Ts minus RSS T2LT        -0.0577 (+/-0.0347)     1   (2.04%)
NOAA-ERSST-v2 Ts minus RSS T2LT   -0.0660 (+/-0.0382)     1   (2.04%)
NOAA-ERSST-v3 Ts minus RSS T2LT   -0.0890 (+/-0.0350)     0   (0.00%)
HadISST1 Ts minus UAH T2LT        +0.0488 (+/-0.0371)    28  (57.14%)
NOAA-ERSST-v2 Ts minus UAH T2LT   +0.0405 (+/-0.0403)    25  (51.02%)
NOAA-ERSST-v3 Ts minus UAH T2LT   +0.0175 (+/-0.0370)    15  (30.60%)
Multi-model ensemble mean         -0.0846

Things to note:

1) For all "pairs" involving RSS T2LT data, the multi-model ensemble 
mean trend is well within even the 1-sigma statistical uncertainty of 
the observed trend.

2) For all "pairs" involving RSS T2LT data, there are very few 
statistically-significant differences between the observed and 
model-simulated "differential warming" of the tropical surface and lower 
troposphere.

3) For all "pairs" involving UAH T2LT data, there are 
statistically-significant differences between the observed and 
model-simulated "differential warming" of the tropical surface and lower 
troposphere. Even in these cases, however, rejection of the null 
hypothesis is not universal: rejection rates range from 30% to 57%. 
Clearly, not all models are inconsistent with the observational estimate 
of "differential warming" inferred from UAH data.

These results contradict the "model inconsistent with data" claims of 
Douglass et al.

The attached Figure is analogous to the Figure we currently show in the 
paper for T2LT trends. Now, however, results are for trends in the 
surface-minus-T2LT difference series. Rather than showing all six 
"pairs" of observational results in the top panel, I've chosen to show 
two pairs only in order to avoid unnecessarily complicating the Figure. 
I propose, however, that we provide results from all six pairs in a Table.

As is visually obvious from the Figure, trends in 46 of the 49 simulated 
surface-minus-T2LT difference series pairs are within the 2-sigma 
confidence intervals of the NOAA-ERSST-v3 Ts minus RSS T2LT trend (the 
light grey bar). And as is obvious from Panel B, even the Douglass et 
al. "sigma{SE}" encompasses the difference series trend from the 
NOAA-ERSST-v3 Ts/RSS T2LT pair.

I think we should show these results in our paper.

The bottom line: Use of newer T2LT datasets (RSS) and Ts datasets 
(NOAA-ERSST-v3, HADISST1) largely removes the discrepancy between 
tropical surface and tropospheric warming rates. We need to explain why 
the observational estimates of tropical SST changes are now smaller than 
they were at the time of the CCSP Report. We will need some help from 
Dick Reynolds with this.

With best regards,

Ben
----------------------------------------------------------------------------
Benjamin D. Santer
Program for Climate Model Diagnosis and Intercomparison
Lawrence Livermore National Laboratory
P.O. Box 808, Mail Stop L-103
Livermore, CA 94550, U.S.A.
Tel:   (925) 422-2486
FAX:   (925) 422-7675
email: santer1@llnl.gov
---------------------------------------------------------------------------- 


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