When the conditions today are significantly different from conditions in the past, relying on the past to predict the present or the future is an illogical appeal to history.

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EPICA ice core data (image source: antarcticglaciers.org)

There are a lot of illogical arguments made by climate disruption deniers (those who deny the overwhelming scientific evidence supporting the reality of industrial climate disruption). One of the most common illogical arguments is that increasing carbon dioxide (CO₂) today can’t cause global warming because CO₂ increased after global warming had already started when previous ice ages were ending. This argument sounds reasonable, but it’s actually a logical fallacy known as a “predictive appeal to history” (also known as an “appeal to tradition“). It’s the wrong same argument as “we’ve always done it this way, so we have to keep doing this way.”

To illustrate the issue with this argument, let’s look at an apocryphal story told by Richard Quinnell to engineers:

In the early 1940s, so the story goes, the Army wanted a dependable supply of llama dung, as required by specifications for treating the leather used in airplane seats. Submarine attacks made shipping from South America unreliable, so the Army attempted to establish a herd of llamas in New Jersey. Only after the attempt failed did anyone question the specification.

Subsequent research revealed that the US Army had copied a British Army specification dating back to Great Britain’s era of colonial expansion. The original specification applied to saddle leather. Great Britain’s pressing need for cavalry to patrol its many colonies meant bring together raw recruits, untrained horses, and new saddles. The leather smell made the horses skittish and unmanageable. Treating the saddle leather with llama dung imparted an odor that calmed the horses. The treatment, therefore, became part of the leather’s specification, which remained unchanged for a century.

The moral of the story is that, since airplanes don’t need to be calmed down, the llama dung was unnecessary. Something had changed that rendered the historical knowledge irrelevant.

When it comes to science (or my own field, electrical engineering), it’s best to start from how things have been in the past, but not to rely on the past as a guaranteed predictor of the future. For example, let’s say that we’re playing a game of billiards. Throughout the game, the cue ball has been moving in exactly straight lines, so we reasonably expect that it will continue to do so. But let’s say that suddenly, halfway through our game, our shots start to curve. We haven’t changed how we’re hitting the cue, so something changed. When we inspect the table we find that it’s now tilted by a remote-controlled motorized jack that our annoying neighbor secretly placed under one side of the table.

In this example, experience told us that the cue should continue moving in a straight line, and we made our shots as if it would. But when the ball stopped moving in a straight line, we knew something had changed that rendered our prior experience irrelevant.

When a climate disruption denier claims that CO₂ can’t cause global warming because of prior ice ages, they’re assuming, as we would in a game of billiards, that something hasn’t changed so much to render the ice age evidence irrelevant. To use our billiards example, they’re essentially denying that the cue ball has started curving.

Three Milankovich variations (image credit: UCAR COMET)

It’s true that global warming as the Earth transitions from an ice age to an interglacial period is not initially caused by changes in CO₂. Instead, changes in the amount of sunlight hitting the northern hemisphere due to Milankovich cycles serve as the trigger for the transition out of an ice age. But CO₂ still plays a role. Ice cores from Antarctica and Greenland and ocean and lake sediments show that, once the Milankovich cycle drives a small amount of warming, CO₂ is released into the atmosphere by the warming ocean, and that additional CO₂ drives the greater warming needed to end an ice age.

Essentially, ice cores and sediments show that ending an ice age proceeds like this: increased insolation due to Milankovitch cycle (A) causes slight global warming (B) which causes the ocean to release some CO₂ into the atmosphere (C) which in turn causes a lot more global warming (D). A –> B –> C –> D.

What about the global warming being observed today? If we start from what we know about ice ages, we could be in either period B (early warming driven by a warming ocean) or period D (lots of warming driven by CO₂). We know that we’re not in period B for two reasons. First, the Milankovic cycle we’re in is one where the Earth should be cooling down, not heating up. Second, while the oceans are warming up, the amount of CO₂ in the ocean is increasing, not decreasing (which is, in turn, the cause of ocean acidification).

Clearly, as with the case of the llama dung, something has changed, and that something has rendered the historical evidence from ice age deglaciations partially obsolete.

What has changed, of course, is that humans are burning massive amounts of fossil fuels and pouring billions of tons of CO₂ into the atmosphere. So while we don’t have A or B this time, we still have C, and that’s going to cause D unless we change do something about it. The laws of physics require it.

Relying on history and experience is a good starting point for understanding. But concluding that the present is similar to the past can only happen after a thorough investigation of both the similarities and the differences between the present and the past. Appealing to history or tradition cuts out the investigation entirely, resulting in an illogical argument.