Global Surface Temperature Changes - Model and ActualSee update below.
Bob Tisdale has an article on WUWT about CMIP5 model outputs for RCP 6.0 for the period 1880 to present. He puts up this chart:
Looks not bad, doesn't it. The models have picked up Pinatubo very well. While they appear to be less closely aligned to earlier periods that is where there is greater uncertainty in the record. They are pretty close from the mid-twentieth century onwards. Pity the chart as he shows it has no probability ranges. (Incidentally, I have no idea of whether the chart is an accurate representation of actual temperature or the CMIP5 model outputs. No reason to doubt it but I haven't checked.)
Choosing the trendAnyway, Bob decides to see if he can find a way to make it look bad. So what he does is draw some different coloured 'trend' lines on different sections, one blue line and one red line. He doesn't seem to bother with proper trend analysis or anything fancy. All he does is divide the chart into his own segments (eyeballing only based on his comments). Note how his dividing lines are at peaks and troughs in the record. That pretty well ensures a greater slope for the record than the model output. He picks start and end points for short sections such that the trend lines for actual and modelled have somewhat different slopes. That means (he thinks) he can say "aha" and "gotcha". He seems to be arguing that there should be a perfect alignment at every point on the chart in segments of his choosing.
While some WUWT readers will grab onto the 'gotcha', they'll be the ones who think any article on WUWT (no matter what it is about) means "all the science is wrong".
I think many casual readers of WUWT will be surprised at how close the model output is to the actual record of surface temperature change.
There's a recent article on RealClimate.org discussing models and regional trends - more difficult to model than global trends. Worth a read.
Post Script:This post was meant to be just a dig at Bob's fiddling with 'trends'. But it's probably worth a caution. If you manage to get towards the end of Bob's post you'll read nonsense like this:
Atrocious, horrible and horrendous are words that could be used to describe the performance of the CMIP5-archived climate models during the early warming period of 1917 to 1944. See Figure 7. According to the models, if greenhouse gases were responsible for global warming, global surface temperatures should only have warmed at a rate of about +0.049 deg C/decade. BUT according to the new and improved GISS Land-Ocean Temperature Index (LOTI) data, global surface temperatures warmed at a rate that was approximately 3.4 times faster or about 0.166 deg C/decade.This shows a couple of things. Firstly, that Bob views surface temperature records as being very 'correct' in the early part of this century, ignoring the larger error bars on the actual data (he doesn't show any), as illustrated below from NASA (green bars):
Secondly, despite blogging about climate for quite some years, Bob hasn't learnt what climate studies show. He seems to think that greenhouse gases are said to have been the only forcing since industrialisation. That's not what scientists have found. In fact, science suggests that earlier last century, although greenhouse gas increases contributed to some extent, other forces dominated. Here is a chart indicating likely 'natural' and anthropogenic contributions - from SkepticalScience (click image for larger version). Read more here.