Why Curry, McIntyre, and Co. are Still Wrong about IPCC Climate Model Accuracy
Posted on 4 October 2013 by dana1981
Earlier this week, I explained why IPCC model global warming projections have done much better than you think. Given the popularity of the Models are unreliable myth (coming in at #6 on the list of most used climate myths), it's not surprising that the post met with substantial resistance from climate contrarians, particularly in the comments on its Guardian cross-post. Many of the commenters referenced a blog post published on the same day by blogger Steve McIntyre.
McIntyre is puzzled as to why the depiction of the climate model projections and observational data shifted between the draft and draft final versions (the AR5 report won't be final until approximately January 2014) of Figure 1.4 in the IPCC AR5 report. The draft and draft final versions are illustrated side-by-side below.
I explained the reason behind the change in my post. It's due to the fact that, as statistician and blogger Tamino noted 10 months ago when the draft was "leaked," the draft figure was improperly baselined.
What's Baselining and Why is it Important?
Global mean surface temperature data are plotted not in absolute temperatures, but rather as anomalies, which are the difference between each data point and some reference temperature. That reference temperature is determined by the 'baseline' period; for example, if we want to compare today's temperatures to those during the mid to late 20th century, our baseline period might be 1961–1990. For global surface temperatures, the baseline is usually calculated over a 30-year period in order to accurately reflect any long-term trends rather than being biased by short-term noise.
It appears that the draft version of Figure 1.4 did not use a 30-year baseline, but rather aligned the models and data to match at the year 1990. How do we know this is the case? Up to that date, 1990 was the hottest year on record, and remained the hottest on record until 1995. At the time, 1990 was an especially hot year. Consequently, if the models and data were properly baselined, the 1990 data point would be located toward the high end of the range of model simulations. In the draft IPCC figure, that wasn't the case – the models and data matched exactly in 1990, suggesting that they were likely baselined using just a single year.
Mistakes happen, especially in draft documents, and the IPCC report contributors subsequently corrected the error, now using 1961–1990 as the baseline. But Steve McIntyre just couldn't seem to figure out why the data were shifted between the draft and draft final versions, even though Tamino had pointed out that the figure should be corrected 10 months prior. How did McIntyre explain the change?
"The scale of the Second Draft showed the discrepancy between models and observations much more clearly. I do not believe that IPCC’s decision to use a more obscure scale was accidental."
No, it wasn't accidental. It was a correction of a rather obvious error in the draft figure. It's an important correction because improper baselining can make a graph visually deceiving, as was the case in the draft version of Figure 1.4.
Curry Chimes in – 'McIntyre Said So'
The fact that McIntyre failed to identify the baselining correction is itself not a big deal, although it doesn't reflect well on his math or analytical abilities. The fact that he defaulted to an implication of a conspiracy theory rather than actually doing any data analysis doesn't reflect particularly well on his analytical mindset, but a blogger is free to say what he likes on his blog.
The problem lies in the significant number of people who continued to believe that the modeled global surface temperature projections in the IPCC reports were inaccurate – despite my having shown they have been accurate and having explained the error in the draft figure – for no other reason than 'McIntyre said so.' This appeal to McIntyre's supposed authority extended to Judith Curry on Twitter, who asserted with a link to McIntyre's blog, in response to my post,
"No the models are still wrong, in spite of IPCC attempts to mislead."
In short, Curry seems to agree with McIntyre's conspiratorial implication that the IPCC had shifted the data in the figure because they were attempting to mislead the public. What was Curry's evidence for this accusation? She expanded on her blog.
"Steve McIntyre has a post IPCC: Fixing the Facts that discusses the metamorphosis of the two versions of Figure 1.4 ... Using different choices for this can be superficially misleading, but doesn’t really obscure the underlying important point, which is summarized by Ross McKitrick on the ClimateAudit thread"
Ross McKitrick (an economist and climate contrarian), it turns out, had also offered his opinion about Figure 1.4, with the same lack of analysis as McIntyre's (emphasis added).
"Playing with the starting value only determines whether the models and observations will appear to agree best in the early, middle or late portion of the graph. It doesn’t affect the discrepancy of trends, which is the main issue here. The trend discrepancy was quite visible in the 2nd draft Figure 1.4."
In short, Curry deferred to McIntyre's and McKitrick's "gut feelings." This is perhaps not surprising, since she has previously described the duo in glowing terms:
"Mr. McIntyre, unfortunately for his opponents, happens to combine mathematical genius with a Terminator-like relentlessness. He also found a brilliant partner in Ross McKitrick, an economics professor at the University of Guelph.
Brilliant or not, neither produced a shred of analysis or evidence to support his conspiratorial hypothesis.
Do as McKitrick Says, not as he Doesn't Do – Check the Trends
In his comment, McKitrick actually touched on the solution to the problem. Look at the trends! The trend is essentially the slope of the data, which is unaffected by the choice of baseline.
Unfortunately, McKitrick was satisfied to try and eyeball the trends in the draft version of Figure 1.4 rather than actually calculate them. That's a big no-no. Scientists don't rely on their senses for a reason – our senses can easily deceive us.
So what happens if we actually analyze the trends in both the observational data and model simulations? That's what I did in my original blog post. Tamino has helpfully compared the modeled and observed trends in the figure below.
Global mean surface temperature warming rates and uncertainty ranges for 1990–2012 based on model projections used in the IPCC First Assessment Report (FAR; yellow), Second (SAR; blue), and Third (TAR; red) as compared to observational data (black). Created by Tamino.
The observed trends are entirely consistent with the projections made by the climate models in each IPCC report. Note that the warming trends are the same for both the draft and draft final versions of Figure 1.4 (I digitized the graphs and checked). The only difference in the data is the change in baselining.
This indicates that the draft final version of Figure 1.4 is more accurate, since consistent with the trends, the observational data falls within the model envelope.
Asking the Wrong (Cherry Picked) Question
Unlike weather models, climate models actually do better predicting climate changes several decades into the future, during which time the short-term fluctuations average out. Curry actually acknowledges this point.
This is good news, because with human-caused climate change, it's these long-term changes we're predominantly worried about. Unfortunately, Curry has a laser-like focus on the past 15 years.
"What is wrong is the failure of the IPCC to note the failure of nearly all climate model simulations to reproduce a pause of 15+ years."
This is an odd statement, given that Curry had earlier quoted the IPCC discussing this issue prominently in its Summary for Policymakers:
"Models do not generally reproduce the observed reduction in surface warming trend over the last 10 –15 years."
The observed trend for the period 1998–2012 is lower than most model simulations. But the observed trend for the period 1992–2006 is higher than most model simulations. Why weren't Curry and McIntyre decrying the models for underestimating global warming 6 years ago?
This suggests that perhaps climate models underestimate the magnitude of the climate's short-term internal variability. Curry believes this is a critical point that justifies her conclusion "climate models are just as bad as we thought." But as the IPCC notes, the internal variability on which Curry focuses averages out over time.
"The contribution [to the 1951–2010 global surface warming trend] ... from internal variability is likely to be in the range of −0.1°C to 0.1°C."
While it would be nice to be able to predict ocean cycles in advance and better reproduce short-term climate changes, we're much more interested in long-term changes, which are dominated by human greenhouse gas emissions. And which, as Curry admits, climate models do a good job simulating.
It's also worth looking back at what climate scientists were saying about the rapid short-term warming trend in 2007. Rahmstorf et al. (2007), for example, said (emphasis added):
"The global mean surface temperature increase (land and ocean combined) in both the NASA GISS data set and the Hadley Centre/Climatic Research Unit data set is 0.33°C for the 16 years since 1990, which is in the upper part of the range projected by the IPCC ... The first candidate reason is intrinsic variability within the climate system."
Data vs. Guts
Curry and Co. deferred to McIntyre and McKitrick's gut feelings about Figure 1.4, and both of their guts were wrong. Curry has also defaulted to her gut feeling on issues like global warming attribution, climate risk management, and climate science uncertainties. Despite her lack of expertise on these subjects, she is often interviewed about them by journalists seeking to "balance" their articles with a "skeptic" perspective.
Here at Skeptical Science, we don't ask our readers to rely on our gut feelings. We strive to base all of our blog posts and myth rebuttals on peer-reviewed research and/or empirical data analysis. Curry, McIntyre, and McKitrick have failed to do the same.
When there's a conflict between two sides where one is based on empirical data, and the other is based on "Curry and McIntyre say so," the correct answer should be clear. The global climate models used by the IPCC have done a good job projecting the global mean surface temperature change since 1990.