Today’s post is at the Stream: xkcd’s Global Warming Time Series Mistakes.
The popular web cartoon xkcd has provided a wonderful opportunity to plug my must-read (and too expensive) book Uncertainty: The Soul of Modeling, Probability & Statistics. Buy a copy and follow along.
In this award-elligible book, which has the potential to be read by millions and which has the power to change more lives than even the Atkins Diet, I detail (in the ultimate chapter) the common errors made in time series analysis. Time series are the kind of data you see in, for example, temperature or stock price plots through time.
The xkcd post—thanks to the many readers who emailed about it—“A Timeline Of Earth’s Average Temperature” makes a slew of fun errors, but, and I want to emphasize this, it isn’t xkcd’s fault. The picture he shows is the result of the way temperature and proxy data are handled by most of the climatological community. Mr Munroe, the xkcd cartoonist, is repeating what he has learned from experts in his attempt at being humorous (but nobody bats a thousand), and repeating things from experts when you yourself don’t know the subject is a rational thing to do.
The plot purportedly shows the average global temperature, presumably measured right above the surface, beginning in 20,000 BC and ending in the future at 2100 AD. Mr Munroe misspells “BC” as “BCE” throughout the cartoon, incidentally, and leaves out “AD”.
No, I’m kidding. “BC” means “Before Christ”, which some academics, sensitive creatures that they are, find offensive on behalf of people they haven’t met, and so they change it to “Before the Common Era”. And how do they demarcate the “Common Era”? By the birth of Christ, a.k.a. BC. The same people who gave us “BCE” gave us “safe spaces”. Skip it.
Now I’m going to show exactly why xkcd’s plot fails, but to do so is hard work, so first a sort of executive summary of its oddities.
(1) The flashy temperature rises (the dashed lines) at the end are conjectures based on models that have repeatedly been proven wrong—indeed, they’ve never been proven right—by predicting temperatures much warmer than today’s. There is ample reason to distrust these predictions.
(2) Look closely at the period between 9000 BC until roughly 1000 AD, an era of some 10,000 years which had, if xkcd’s graph is true, temperatures much warmer than we had the Internet. And this was long before the first barrel of oil was ever turned into gasoline and burned in life-saving internal combustion engines.
(3) There was no reason to start the graph at 20,000 BC. If xkcd had taken the timeline back further, he would have had to have drawn temperatures several degrees warmer than today’s, temperatures which outstrip the threatened warming promised by faulty climate models. And don’t forget that warmer temperatures are always associated with lush and bountiful periods in earth’s history. It’s ice and cold that kill.
(4) The picture xkcd presents is lacking any indication of uncertainty, which is the major flaw. We should not be looking at lines, which imply perfect certainty, but blurry swaths that indicate uncertainty. Too many people are too certain of too many things, meaning the debate is far from “settled.”
The temperature at 20,000 BC was, Munroe claims, surely after referring to expert sources, about 4.3 C colder than the ad hoc average of temperatures from 1961-1990.
Was it actually 4.3 C cooler? How do we know? Forget the departure from the ad hoc average, which is a distraction. How do we know what the temperature was all those years ago? After all, there were no thermometers.
The answer is—get a pen and write this down, it’s crucial …
Go there to read the answer and discover why you’ve been looking at time series the wrong way.