Klimaspirale Part 2: The Baseline Trap – Simply Explained
Part 1 had settled the second check: there is already a maintained climate spiral, and the original idea for the project was gone. Now came the first check: are our own numbers displayed correctly?
The numbers held. The warmest month really was the warmest, the warmest year really the warmest. The problem wasn’t in the data. It was in the scale.
On the build day I had had a translucent cylinder set at 1.5 degrees, because that used to be the climate target. It doesn’t stand where the climate target actually lies.
Two heights for the same mountain
Ask two people how high the mountain in front of them is. One counts from the sea, the other from the valley he’s standing in. Same mountain, both do the arithmetic correctly, both name a different number. The difference is exactly how high the valley sits.
With warming it’s the same. Our scale doesn’t show a temperature. It shows a deviation: how much warmer or cooler a month was compared to the average of the years 1951 to 1980. That average is the zero point, the “valley floor” of our scale. Scientists call that span the baseline period.
The Paris Agreement counts from somewhere else: from before industrialization, calculated as 1850 to 1900. Between the two zero points lie 0.19 degrees. NASA states that number itself, and it comes with a version date because new measurements can still shift it slightly. Claude Code recalculated it against our data and landed on the same value.
The cross-check ran via the IPCC (the Intergovernmental Panel on Climate Change): their report says 1.09 degrees of warming counted from 1850 to 1900, and our series says about 0.90 degrees for the same period counted from 1951 to 1980. The difference: 0.19. Same mountain, two valleys, same gap.
The zero point changes nothing about the measurements. The curve keeps its shape, only the labelling moves.
For my graphic that means something uncomfortably concrete. The Paris 1.5 degree mark sits at 1.31 degrees on our scale, so further in than the ring that had said 1.5 the whole time. A viewer sees the most recent loops pressing against that ring, remembers the number from the agreement, and reads the picture wrong without making a mistake.
The rule I drew from this is narrow but non-negotiable: never call a ring “Paris target” without saying in the same picture which zero point it is counted from.
This sort of thing isn’t unique to our project. Practically every series brings its own zero point. Our one calculates against 1951 to 1980, another against the twentieth-century mean, yet another against 1991 to 2020, Copernicus against 1850 to 1900. Four reference periods for the same atmosphere, and anyone laying numbers from them side by side produces contradictions that aren’t contradictions.
The Paris toggle
The proposal came from Claude Code. The existing rings stay where they are, and a second pair joins them, dashed, at the spots where the Paris marks actually sit on our scale: 1.31 and 1.81 degrees.
This pair is labelled with the Paris numbers. The distance to the identically named solid ring is the offset itself, visible as a gap instead of as a footnote.
“Yes, that would be a good option for our interactive page, a Paris toggle like that, one that visibly shows the shift.”
All four rings sit on the same scale, no second data source comes in. But 0.19 degrees is a narrow band: across the whole series you take that distance for line width, so drawing it alone isn’t enough. It has to be written down as well.
Target and upper limit
Then I had an objection about the labelling:
“Should we even be showing Paris 2.0 degrees? That implies as if there had been a 2 degree agreement.”
There is no two degree agreement in the Paris text. It commits to holding warming well below 2 degrees, and additionally to pursuing efforts to limit it to 1.5 degrees. The 2 is the upper limit, the 1.5 is the target.
Two identically named rings would have turned the upper limit into a second target, one nobody agreed to in that form. So one is now called Paris target and is yellow, the other Paris limit and red. The difference is one word and one colour, and it decides what somebody takes away from a screenshot.
The top-down view
Looking straight down into the funnel is the most obvious move when you have a spiral in front of you. And that was exactly where the picture was useless.
“When the spiral is finished and I tip it so that I’m looking straight down on it, then the lines cover the rings completely”
The annual loops pile on top of each other, the topmost ones look bigger because they’re closer to the camera, and you can’t read anything. But this flat top-down view is really how this spiral became famous. The original clip runs as a flat disc for most of its length. Anyone looking down in the browser is after the view they already know.
So a toggle was built that takes the perspective out: a flat chart where no ring is closer to the viewer than another.
Event markers
Then came an idea from me to add special events as floating signs:
“How about we also add special events with floating labels, like e.g. the Paris decision, 1990 I think it was?”
The year is a guess, and it’s wrong. The agreement was adopted in 2015 and entered into force in 2016. I was a quarter of a century off, and the verification corrected it without comment.
In this project the chain of evidence has corrected my memory more often than the other way round.
The same check delivered facts for each candidate. The eruption of Pinatubo in 1991 cooled the global average by about 0.4 degrees. Kyoto was 1997. With political events a sign runs into trouble, because they have two dates: Paris was adopted in 2015, entered into force in 2016. A floating sign sits at one year. Which of the two you pick is a decision, not a fact.
The trap in my own app
Two days later I stood in front of the running application and took one of these markers for a bug.
“I think there’s a bug here: at the event ‘2024 - first year above 1.5 °C (pre-ind.)’ the curve is very clearly below 1.5 degrees. Is that a different reference space or does the presentation not fit?”
The answer to the question in the last half-sentence is yes. The sign talks about warming counted from before industrialization. The scale underneath it counts from 1951 to 1980. That’s why an event above 1.5 degrees sits visibly below the 1.5 line.
Both statements are correct, together they look like an arithmetic error. That is exactly the trap the Paris toggle exists for. It caught me in my own application, with the conversion still fresh in my head.
I asked a second time right away, because I didn’t understand why 2024 of all years was singled out:
“Um, but why 2024, the years before and after it are each warmer than their previous years too?!”
The embarrassing part is in the quote itself. The sign said “(pre-ind.)”, the abbreviation for pre-industrial. So the answer was already in the picture, and I read straight past it. An abbreviation in brackets apparently explains nothing. You only read it once you already know what it points to.
I didn’t want to drop the sign. It stays, and the number from our own scale goes behind it. That puts both references right next to each other.
There is a further wrinkle: 1.29 plus 0.19 makes 1.48, just under 1.5. On our series, 2024 doesn’t actually reach the mark. So the statement on the sign can’t come from our series but from a different one. Different temperature series arrive at different values for the same year because they measure differently and combine differently. Same effect as the zero point, one level up.
The first viewer to trip over this sign was me.
An explainer page in the application
That same evening I wrote the brief for a page of its own inside the application, one click away from the spiral:
“We still need good, easy to understand explanations on the page, why the Paris targets deviate and by how much and the thing with the different measurement periods”
Out of that came an overlay: what a deviation is, what a zero point is, and why the measurement series and the agreement use different ones.
The section I consider the most important says: a record month on this scale does not mean a climate target has been broken. The offset is only the first reason. The second: the agreement talks about long-term warming, not about a single month. An unusually hot month is an event, not yet a permanent state.
That page sits in the application, one click from the spiral. It isn’t in the videos. Anyone who sees the climate spiral only as an animation sees the cylinder at 1.5 degrees and has no place where the thing about the zero point is written down.
Everything to do with cleanup and publishing, we pushed to later. Build first, clean up after.
Part 2 of 4 in the Klimaspirale series.
← Klimaspirale Part 1: From a Video to My Own Data | Klimaspirale Part 3: The Objection I Missed →