“[There] is overwhelming evidence that the planet has warmed during the past century. But could this warming be due to natural dynamics? Given what we know about the complexity, long-term persistence, and non-linearity of the climate system, it seems the answer might be yes.” Wait, did scientists really say that? Yes they did, in a peer-reviewed 2005 study by statistical experts at the US Geological Survey. Now you might think 2005 is a long time ago and we know so much more today. But remember the science was all settled back then too. Climate scientists were already pointing to significant upward trends in global temperature. And the article we quoted didn’t dispute that there was a trend, or that by conventional methods it was statistically significant. What they said was that the conventional methods ignore evidence of long-term persistence (LTP) in the climate which is jargon for long, slow natural cycles. In short, (they concluded), “the presence of LTP in a stochastic process can induce a significant trend result when no trend is present, if an inappropriate trend test is used.” Scientists say.
The article, by Timothy Cohn and Harry Lins of the US Geological Survey, had the whimsical title “Nature’s style: Naturally trendy.” Much of the piece is not so much whimsical as mathematically dense, as one might expect and indeed require of a deep dive into statistical methodology, but they made sure readers would understand the importance of the topic. (To which end our small contribution, before quoting them, is to explain that the term “null hypothesis” is used to describe the scenario in which the key variable being investigated does not change.)
While trend magnitude can be determined with little ambiguity, the corresponding statistical significance, sometimes cited to bolster scientific and political argument, is less certain because significance depends critically on the null hypothesis which in turn reflects subjective notions about what one expects to see.
Ah yes, using trends to support a political argument. Seems they had the measure of their field even back then. After explaining how to correct the model for LTP when measuring the significance of a trend, the authors applied the method to the global surface temperature record.
By standard measures the temperature trend is very significant, meaning it can’t be attributed to natural variability. (At least if the various “adjustments” have not manufactured it out of whole cloth.) But those measures assume natural variability operates only over short periods of time. By applying the corrected statistical method, in which natural fluctuations can occur over centuries, the trend was still just as big, but nevertheless it was no longer significant. It indicated that recent warming was within the bounds of what nature could concoct through random variability alone, with no help needed from CO2. And this conclusion, they noted, has both scientific and political implications given the uses to which significance findings are put in the discussion of climate changes.
These findings have implications for both science and public policy... that reported trends are real yet insignificant indicates a worrisome possibility: natural climatic excursions may be much larger than we imagine. So large, perhaps, that they render insignificant the changes, human-induced or otherwise, observed during the past century.