According to the data from NASA, it is:
The graph shows yearly averages since 1979, but we get the same result using higher time resolution (like monthly averages). The red line (with pink shading around it) shows the result of fitting a parabolic trend line (with its uncertainty range). That’s the simplest, most direct way to test for acceleration; not a panacea, but a natural first step. It easily confirms that acceleration is “statistically significant” with the rate of acceleration estimated at 0.0006 ± 0.0005 °C/yr/yr.
According to the data from HadCRU (the Hadley Centre/Climate Research Unit in the U.K.), it is not:
Again, the red line shows a parabolic fit — but now it looks like a straight-line fit because the “quadratic coefficient” is too small to make the curvature visible. That’s because the estimated acceleration is too small to be “statistically significant.”
Those are just two of the best-known global temperature estimates. There are at least three others widely used, from NOAA (National Oceanic and Atmospheric Administration), from Cowtan & Way, and from the Berkeley team. NOAA agrees with NASA, that acceleration is statistically significant; Berkeley and Cowtan & Way agree with HadCRU that it is not.
Does this mean that the data sets disagree wildly, that their results are suspect? Don’t be
They can disagree about the “statistical significance” even when their estimates are essentially in agreement. Let me plot their estimated accelerations, with an “error bar” showing how uncertain each estimate is:
Just for fun I threw in the estimates according to two other temperature data sets, from RSS and UAH, but they’re not for temperature at Earth’s surface (where we live), they’re for temperature in the lower troposphere (the bottom ten miles or so of the atmosphere).
The likely range from every data set overlaps the likely range from every other data set. That’s not a proper statistical comparison, but it is a sign that the different data sets essentially agree. If you do the proper statistical comparison, you get the same result.
If they agree so well, why do they give different answers to the “statistically significant acceleration” question? Suppose you and I had to estimate the height of Shaquille O’Neal just from wathching videos of his career as an NBA superstar. You guessed 7 feet 2.5 inches, I guessed 7 feet 1.5 inches. Essentially we are in excellent agreement! But given that Kareem Abdul-Jabbar was 7 feet 2 inches tall, we disagree on the question “was Shaq taller than Kareem?”
All data sets estimate acceleration as upward, not downward. But only NASA and NOAA put it high enough to cross the “statistically significant” threshhold.
Global temperature is affected by things other than human activity. In particular, we know of three natural factors that influence it: ENSO (the el Niño Southern Oscillation), volcanic eruptions, and changes in the output of the sun. Published research shows the method I use to remove their influence. This helps isolate the part due to humanity — maybe it will help us decide whether or not there’s acceleration due to humanity. Here’s the result for NASA data:
As you can see, the amount of fluctuation — the pervasive wiggling up and down — is much smaller now. But the acceleration is still there, at least according to these data.
We can similarly adjust all data sets, and test for acceleration in the revised data. Here are the rates we find:
The results are more precise — the “error bars” are smaller — but the results are unchanged. NASA and NOAA say acceleration is statistically significant, HadCRU, C&W, and Berkely (as well as RSS and UAH) say it is not.
What’s the bottom line?
I’m not ready to declare “acceleration.” Not yet.
But there sure are signs. NASA data show it, NOAA data show it. Other data sets (like the ERA5 reanalysis data) show it.
But there are caveats. HadCRU data, C&W, Berkeley, RSS, and UAH don’t show it, even after removing the confounding influence of ENSO, volcanoes, and solar variations. When looking for something significant, if you try many data sets you increase the chance of finding it just by accident.
But I repeat: there are signs. I expect that by 2025 we’ll know much better. If acceleration has begun, we’re in far worse shape than we thought. Even if is hasn’t, we’re already in trouble and headed for much more. Let’s not wait to start fixing this — and that means, wean ourselves off fossil fuels.
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