A Note on the 50-50 Attribution Argument between Judith Curry and Gavin Schmidt

Judith Curry and Gavin Schmidt are arguing once again about how much of the global warming we’ve experienced since 1950 is attributable to human-induced global warming.  Judith’s argument was presented in her post The 50-50 argument at ClimateEtc (where this morning there were more than 700 comments…wow…so that thread may take a few moments to download.)  Gavin’s response can be found at the RealClimate post IPCC attribution statements redux: A response to Judith Curry.

Gavin’s first illustration is described by the caption:

The probability density function for the fraction of warming attributable to human activity (derived from Fig. 10.5 in IPCC AR5). The bulk of the probability is far to the right of the “50%” line, and the peak is around 110%.

I’ve included Gavin’s illustration as my Figure 1.

Figure 1 - RealClimate attribution

Figure 1

So the discussion is about the warming rate of global surface temperature anomalies since 1950. Figure 2 presents the global GISS Land-Ocean Temperature Index data for the period of 1950 to 2013. I’m using the GISS data because Gavin was newly promoted to the head of GISS. (BTW, congrats, Gavin.)  As illustrated, the global warming rate from 1950 to 2013 is 0.12 deg C/decade, according to the GISS data.

Figure 2

Figure 2

For this discussion, let’s overlook the two hiatus periods during the term of 1950 to 2013…whether they were caused by aerosols or naturally occurring multidecadal variations in known coupled ocean-atmosphere processes, such as the Atlantic Multidecadal Oscillation (AMO) and the dominance of El Niño or La Niña events (ENSO).  Let’s also overlook for this discussion any arguments about how much of the warming from the mid-1970s to the turn of the century was caused by manmade greenhouse gases or the naturally occurring multidecadal variations in the AMO and ENSO.

Bottom line, according to Gavin:

The bottom line is that multiple studies indicate with very strong confidence that human activity is the dominant component in the warming of the last 50 to 60 years, and that our best estimates are that pretty much all of the rise is anthropogenic.

Or in other words, all the warming of global surfaces from 1950 to 2013 is caused by anthropogenic sources.  Curiously, that’s only a warming rate of +0.12 deg C/decade. He’s not saying that all of the warming, at a higher rate, from the mid-1970s to the turn of the century is anthropogenic.  His focus is the period starting in 1950 with the lower warming rate.


Climate models are not tuned to the period starting in 1950.  They are tuned to a cherry-picked period with a much higher warming rate…the period of 1976-2005 according to Mauritsen, et al. (2012) Tuning the Climate of a Global Model [paywalled].  A preprint edition is here.  As shown in Figure 3, the period of 1976 to 2005 has a much higher warming rate, about +0.19 deg C/decade. And that’s the starting trend for the long-term projections, not the lower, longer-term trend.

Figure 3

Figure 3

And that’s why, when compared to the observed warming rate for the period of 1950 to 2013, which, according to Gavin, is the period “that our best estimates are that pretty much all of the rise is anthropogenic”, then climate model warming rates appear to go off on a tangent.  The modelers have started their projections from a cherry-picked period with a high warming rate.

Figure 4 shows the warming rates for multi-model ensemble-member mean of the CMIP5-archived models using RCP6.0 and RCP8.5 scenarios for the period of 2001-2030.  RCP6.0 basically has the same warming rate as the observations from 1976-2005, which is the model tuning period, but that’s much higher than the warming rate from 1950-2013.  And the trend of the business-as-usual RCP8.5 scenario seems to be skyrocketing off with no basis in reality.

Figure 4

Figure 4

And in Figure 5, the modeled warming rates for the same scenarios are shown through 2100.

Figure 5

Figure 5


I’ve asked a similar question before:  Why would the climate modelers base their projections of global warming on the trends of a cherry-picked period with a high warming rate?  The models being out of phase with the longer-term trends exaggerates the doom-and-gloom scenarios, of course.

But we purposely overlooked a couple of things in this post…that there are, in fact, naturally occurring ocean-atmosphere processes that contributed to the warming from the mid-1970s to the turn of the century—ENSO and the AMO.  The climate models are not only out of phase with the long-term data, they are out of touch with reality.


The GISS Land-Ocean Temperature Index data are available here, and the CMIP5 climate model outputs are available through the KNMI Climate Explorer, specifically the Monthly CMIP5 scenario runs webpage.


About Bob Tisdale

Research interest: the long-term aftereffects of El Niño and La Nina events on global sea surface temperature and ocean heat content. Author of the ebook Who Turned on the Heat? and regular contributor at WattsUpWithThat.
This entry was posted in CAGW Proponent Arguments, Climate Model Failings, Model-Data LOST. Bookmark the permalink.

8 Responses to A Note on the 50-50 Attribution Argument between Judith Curry and Gavin Schmidt

  1. fhhaynie says:

    The two big flaws in the models are the assumptions that (1) all the rise in atmospheric CO2 concentrations is anthropogenic, and (2) global average atmospheric temperatures are sensitive to small changes in CO2 concentrations. My best guess is that anthropogenic emissions contribute no more than a third of the obseved rise in CO2 and is probably around 5%. It is more likely that atmospheric concentrations of CO2 is being controlled by the temperature in clouds over the tropical oceans rather than CO2 controlling the surface temperature of the tropical oceans.

  2. I have pointed this out repeatedly (admittedly only on the few blogs I visit at all regularly), since the spring of 2010, just a few months after I first became aware of the global warming debate (around October 2009). I thought, for most of that time, that their cherry-picked period was 1976-2000, until this last September, at Steven Goddard’s blog, when it became clear (to me) that the actual core of the alarmists’ belief is based only upon 1976-1989/90:

    My comment at Real Science Sept. 17, 2013

    (See the graph at the link I provided within that comment, from USAToday in 2010. I called it fraudulent science as soon as I saw it presented to the public there.)

  3. …and the best (and earliest) paper I saw showing the clear (detailed) cause-effect relationship between ENSO events and global mean temperature was Prof. Horst Malberg’s March 2009 article, in German, “La Niña – El Niño und der solare Einfluss: Die Klimaentwicklung 1950 – 2008″ (“La Niña – El Niño and the Solar Influence: The Climate Trend 1950 – 2008″).

  4. catweazle666 says:

    Cherry-pick a portion of the positive phase of an obviously periodic function, linear regress it to Armageddon, and run round screeching and flapping your little arms!

    Don’t you just love Cliamte McScientists!

  5. Thanks, Bob. Very good information about GCMs training; the models are specialists.
    Why would the climate modelers base their projections of global warming on the trends of a cherry-picked period with a high warming rate?
    To better scare the money out of our pockets, of course.

  6. It´s important to keep in mind the RCP designations (RCP6, RCP8.5, etc) are the forcing in watts per meter squared they wanted to achieve by 2100. This is an important point which seems to be getting missed all the time. In other words, an IPCC committee picked the forcings to be used for the scenarios.

    They picked RCP8.5 as an extreme outlier (that´s what I was told). If we look over the IPCC report we won´t find RCP8.5 being called “business as usual”.

    As you know I´m just a casual observer, but I took a couple of days trying to figure out where RCP8.5 began to be designated “business as usual”, and it seems to have emerged gradually during “science communication” (ie propaganda) articles being written by IPCC members and their friends. THis is a fairly common practice, which was perfected in the Soviet Union as an agitprop technique, and was later codified by experts such as Abe Shulsky, who worked for the Pentagon´s Office of Special Plans during the W. Bush era (this was documented by retired Air Force Colonel Karen Kwiatowski in a series of articles she wrote for American Conservative magazine).

    Where Am I headed? This type of propaganda (mounting an article or paper to be quoted, and creating a daisy chain of articles which build the propaganda to a crescendo) has been perfected and is being used on YOU. So when you use the term “business as usual ” and link it to RCP8.5 you have fallen in their trap.

    And this is why if you use the RCP6.0 you get a closer answer. The models are still in lala land, but they aren´t as bad as RCP8.5 would indicate. Once the models are tuned properly they should start yielding about 1.2 to 1.6 Transient Climate Response. Now the line in the sand is much more reasonable, and the focus would have to be in understanding the cyclic phenomena (ENSO, etc).

    By the way, while I´m at it, work such as Balmaseda´s needs improvements. These guys don´t include the geothermal heat flux, and because the flux is from underneath and isn´t homogeneous it is going to mess up their data. It does have to be accounted for.

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