Why Short-Term Trends are Misleading

In a recent post by Willis Eschenbach[1] at the Watts Up With That blog, we are again being told that there has been a recent decline in global temperatures. The argument was that a breakpoint analysis of multiple datasets reveals a break around 2015, and trends following that breakpoint are either flat or cooling. Yet it's completely unsurprising that such a breakpoint was found in 2015, since that was the beginning of a very large El Nino, and we haven't had a significant El Nino since. Instead, La Nina conditions have prevailed. Yet, the following is indisputable from the GMST data we have:

1. El Nino years are warming at about the same rate as La Nina years (both are warming at 0.2 C/decade since 1980).
2. El Nino years average almost 0.2 C warmer than La Nina years (the difference in the Y direction between the red and blue lines above).
3. The AGW warming signal is about 0.2 C/decade since 1980.
4. ENSO cycles between El Nino and La Nina inside of decadal time scales.

That means you can pretty much always cherry pick short term “cooling” trends if you begin with a strong El Nino and end in the next La Nina, and it has nothing to do with whether the AGW warming signal is changing. This is no less true if you use the Bai & Perron algorithm in the “strucchange” package in R. The breakpoint does not indicate that climate trends are changing. It simply identified that 2015 was the beginning of a large El Nino.

And we've been through this before. From 1998 to 2014, trends in GMST seemed to slow, and some people referred to this as a "hiatus" or a "pause." It turns out that this was just a statistical artifact that created the impression of less warming by starting with a very large El Nino year (1998) and then taking short term trends from that year. All that ended in 2015 when the next large El Nino began, and 2016 became by far the warmest year on record. Four years later, 2020 tied 2016 for the warmest year on record without the benefit of being an El Nino year. The 30-year trend in HadCRUT5 is 0.225 C/decade from 1992 to 2021, and that includes both of these so-called “pauses.” Short-term "trends" tell you more about ENSO than they do about climate. You need to use 30 year trends or longer to say anything meaningful about climate.

A Climate Warming Signal of 0.2 C/decade still shows short-term Cooling due to Internal Variability

To illustrate this point, I constructed graph with a climate warming signal of 0.2 C/decade, then added an "ENSO" sin curve such that the "El Nino" portions occur every 16 years and are 0.2 C warmer than "La Nina" portions. I then added a random number for noise on the signal. There are of course what we could call "pauses" following every El Nino portion on the graph. Obviously, this means that short term "pauses" or even short-term "cooling" is not an argument against the long-term warming signal. The so-called "pauses" are simply an artifact of ENSO and other forms of internal variability operating on the warming signal.

References:

[1] Willis Eschenbach, "The Recent Decline." WUWT.
https://wattsupwiththat.com/2022/05/11/the-recent-decline/