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