A reader asked that I estimate the trend in the JRA-55 data for global temperature, because it is touted by climate deniers Patrick Michaels and Ryan Maue. Let’s have a look. Here’s the data:
These aren’t observed data, they’re reanalysis data, the output of a computer model which uses observed data of many kinds to guide a weather simulation. Reanalysis data has some advantages, particularly that we can use the laws of physics to derive estimates of things we haven’t been able to observe. But for something like global mean temperature, one is better advised to use that instrument designed to measure temperature: the thermometer.
Undaunted, we forge ahead. Step 1 is to estimate the linear trend, which really represents the average rate of global warming during the 40-year period of record for these data. We have to account for some tricksy aspects like autocorrelation of the noise, but we have tools for that. Least squares regression (corrected for autocorrelation) says the rate is 0.17 +/- 0.03 °C/decade (95% confidence interval).
We can also use a non-parametric trend estimate; I’m fond of Theil-Sen regression, also good is “L1 regression,” and both of them give the same answer as least squares regression: 0.17 +/- 0.03 °C/decade.
But … maybe the true trend isn’t just a straight line. I looked for changes in the rate of global warming, using polynomial regression, changepoint analysis applied to linear splines, and the analysis of variance. All of them give the same result: no evidence of any trend change during this time span. Maybe the trend changed — but there’s no proof, there’s not even any evidence.
The rate estimated from the JRA-55 data is in excellent agreement with the rate estimated from other global data sets (the ones based on thermometers, note rates are in °C/yr).
All data sets — including the JRA-55 data — say that the rate since 1979 cannot possibly be as low as 0.1 °C/decade (0.01 °C/year):
Patrick Michaels and Ryan Maue have this to say about the JRA-55 data:
Figure 2. Monthly JRA-55 data beginning in January, 1979, which marks the beginning of the satellite-sensed temperature record. The average warming rate is 0.10⁰C/decade and there’s a clear “pause” between the late 1990s and the beginning of the recent El Niño.
Patrick Michaels and Ryan Maue proved one thing: that for trend analysis they are incompetent.
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