Global temperature is affected by a lot of things, not just humans and their greenhouse gases. We know what some of those things are, and we can even estimate their impact over the last 40 years or so (since 1979 let’s say). Then we can subtract that effect from temperature data, to estimate how hot Earth would have been without those “other things.”
What other things, you wonder? Known factors include the El Niño Southern Oscillation (ENSO), volcanic eruptions, and variations in the output of the sun; published research outlines the method I used (although I’ve tweaked it a bit). Let’s take, for instance, the global temperature from NASA (monthly averages from January 1979 through December 2019):
The red line shows the linear trend (estimated by least squares regression), temperature rising at an average rage of 0.0188 (± 0.0031) °C/yr. But temperature itself (the thin black line) doesn’t just follow the trend, it fluctuates up and down. Quite a lot.
According to the analysis, those other factors were responsible for quite a bit of the fluctuations, influencing temperature in this way:
I’ve included a trend line here, too, but this one slopes downward, very slightly, at a rate of -0.0003 (± 0.0021) °C/yr. That’s -2% of the overall trend since 1979. That leaves the part of the trend that is probably man-made, at 102% of the observed trend.
When we do subtract the “other factors” estimate from the observed temperature data, we get “adjusted” data that better reflects the man-made part of climate change, now that many of the natural fluctuation factors are removed:
It shows a lot less fluctuation than the raw data, and promises us a better estimate of humanity’s influence on the trend, heating up at a rate of 0.0192 (± 0.0016) °C/yr. The part of global warming not due to those “other things” (which is, for the most part, the human influence) is a tiny bit bigger even than we’ve seen. Do note, however, that when it comes to trend the difference is not “statistically significant.”
It’s interesting to look at what the individual factors themselves have been up to. Here’s the estimated strength of each, with the solar influence shown in blue, the volcanic effect in brown, and the impact of ENSO in red:
The solar influence isn’t very big, and its trend is truly tiny, only about -4% of the observed trend. It’s not because the sun can’t affect Earth’s climate, it’s just that the sun’s output hasn’t changed that much.
The ENSO impact goes up and down, but its overall trend during this time span has been downward, at an estimated rate of -0.0008 (± 0.0007) °C/yr. That’s -8% of the observed temperature trend during this time span.
The volcanic effect is dominated by two large eruptions, El Chicon in the early 1980s and Mt. Pinatubo in the early 1990s. Because they cooled the Earth noticeably, the trend in the volcanic impact is upward, at +0.0020 (± 0.0018) °C/yr. That’s 10% of the observed trend.
The bottom line is that yes, those “other things” do affect trend estimates, with volcanoes causing extra heating while ENSO caused extra cooling (with a little help from solar), but the combined effect is just about zero.
We can repeat the exercise with, say, the lower-troposphere temperature from UAH (Univ. of Alabama at Huntsville). I’m not fond of that data set, but let’s see what it has to say. By the same analysis, since 1979 ENSO decreased the trend, changing it by -19%, solar by a further -8%, while volcanoes increased the trend by 27%. Again, the net result is about zero.
It turns out that the “other things” don’t show any trend long-term, and in the not-too-long term (since 1979) have trended oppositely to each other, with a net effect of just about zero. The trend we’ve got now isn’t because of those things.
I have the impression that Roy Spencer wants to blame global warming (since 1979) on volcanoes.
These volcanic effects on the post-1979 warming trend should always be kept in mind when discussing the post-1979 temperature trends.
What he does is compute the trend contribution from volcanoes (alone). He doesn’t use surface temperature, or lower-troposphere temperature, he uses ocean temperature, not from observations but from his climate model. I confess I don’t have much confidence in his calculations, but he ascribes 40% of global warming to volcanoes.
I think his implication is “that only leaves 60% due to mankind, tops.” I think he’s confused.
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