Weak Sauce from Climate Deniers

Over at RealClimate, in a post about Jim Hansen’s forecasts from the 1980s, a commenter calling himself “Victor” has bent over backwards to argue that the climate change we’ve seen already is just “natural.” He recently went so far as to say this:

“First of all: what persistent trend? Sorry but I can’t find one. What we see in the actual evidence is a roller coaster pattern of ups and downs. The only persistent upward trends occurred either early in the 20th century when CO2 emissions had only a small effect or late in the 20th century over a period of only 20 years.”

Let’s take a look.

Here’s the data, yearly average global temperature from NASA:

I’ve included a smooth fit (modified lowess smooth) as a red line. It looks to me like the persistent trend has been continuing for over 40 years, not “only 20 years.”

And that’s the way it really is. I’ve analyzed this data using rigorous statistics, many times, and demonstrated that since about 1975 the trend has been upward, with no evidence of any “pause” or “hiatus” or even “slowdown.” I’ve even published my results in peer-reviewed scientific literature. You can see for yourself here (article is on page 6).

Let’s see if we can show it in a way which even “Victor” can understand.

We’ll use the data from 1975 through 1999 and fit a straight line (by least squares regression) to estimate what the trend is:

The blue line is the estimated trend line. The dashed blue lines above and below show the range within 2 standard deviations (of the residuals). Note that all the data points lie withing the 2-standard-deviation limits (which doesn’t always happen, but isn’t uncommon with only 25 data points).

Now let’s use that trend to estimate what would have happened if the trend continued unchanged:

Essentially, we’d expect the data to fall within the dashed red lines. Of course they’ll fluctuate about that line — after all, there is natural variation. But if the trend continued they would mostly fall between the 2-standard-deviation limits.

So … what actually happened? This:

Imagine that! They did fall within the expected range. Only one value is outside the range, and that one (for 2016) is not far outside and is above the limit. As I’ve often said, there’s no evidence that global warming showed a “pause” or “hiatus” or even a “slowdown.”

What if the trend had stopped in 2000? Then we’d expect this:

It turns out that 5 of the 18 years had temperatures outside the “no-trend” limits. All but one of them is above the “no-trend” value itself. And the last three values aren’t just above the “natural variation” limits, they’re way above.

The “trend didn’t continue” hypothesis just doesn’t hold water.

Maybe Victor thinks there was a “pause” in there somewhere. Maybe he sees what he thinks looks like a “pause,” but he certaintly hasn’t provided any analysis to back up that idea. He can’t seem to conceive of the idea that what “looks like” a “pause” — to him — is just natural variation in addition to a continuing trend. Funny how he blames the temperature trend on natural variation, but can’t conceive of its creating the false impression. Does he believe in natural variation, or doesn’t he?

When you actually do the analysis, using rigorous statistics rather than visual impression, you find that there is just no evidence for a “pause” or “hiatus” or even “slowdown.” As for this kind of visual impression, I’ve done it before — my regular readers might be getting bored with it. But since climate deniers continue to push falsehoods, I guess I’ll just have to continue to show how wrong they are. And they continue to push the “pause” idea. Victor pushes his own version, namely “late in the 20th century over a period of only 20 years.” I prefer actual science. And I prefer truth.

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