cc: Thomas R Karl , "'Philip D. Jones'" , "Thorne, Peter" , carl mears date: Fri, 15 Feb 2008 08:53:36 -0800 from: Ben Santer subject: [Fwd: Additional calculations] to: Richard W Reynolds 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 ---------------------------------------------------------------------------- X-Account-Key: account1 Return-Path: Received: from mail-2.llnl.gov ([unix socket]) by mail-2.llnl.gov (Cyrus v2.2.12) with LMTPA; Wed, 13 Feb 2008 18:34:52 -0800 Received: from smtp.llnl.gov (nspiron-3.llnl.gov [128.115.41.83]) by mail-2.llnl.gov (8.13.1/8.12.3/LLNL evision: 1.6 $) with ESMTP id m1E2YMTv008791; Wed, 13 Feb 2008 18:34:52 -0800 X-Attachments: LAST_IJC_figure04.pdf X-IronPort-AV: E=McAfee;i="5200,2160,5229"; a="26979778" X-IronPort-AV: E=Sophos;i="4.25,349,1199692800"; d="pdf'?scan'208";a="26979778" Received: from dione.llnl.gov (HELO [128.115.57.29]) ([128.115.57.29]) by smtp.llnl.gov with ESMTP; 13 Feb 2008 18:34:51 -0800 Message-ID: <47B3A8CB.90605@llnl.gov> Date: Wed, 13 Feb 2008 18:34:51 -0800 From: Ben Santer Reply-To: santer1@llnl.gov Organization: LLNL User-Agent: Thunderbird 1.5.0.12 (X11/20070529) MIME-Version: 1.0 To: santer1@llnl.gov, Peter Thorne , Stephen Klein , Susan Solomon , John Lanzante , Melissa Free , Dian Seidel , Tom Wigley , Karl Taylor , Thomas R Karl , Carl Mears , "David C. Bader" , "'Francis W. Zwiers'" , Frank Wentz , Leopold Haimberger , "Michael C. MacCracken" , Phil Jones , Steve Sherwood , Tim Osborn , Gavin Schmidt , "Hack, James J." , peter gleckler Subject: Additional calculations References: <200801121320.26705.John.Lanzante@noaa.gov> <478C528C.8010606@llnl.gov> <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> Content-Type: multipart/mixed; boundary="------------060600010907080200090109" 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. 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