It's an interesting dataset, the 'Keeling Curve', tweeted about here all the time. It's one of the most important to the future of the planet, yet how hard have we looked at it?

https://keelingcurve.ucsd.edu/ 

It measures our atmosphere's carbon dioxide concentration, very precisely, atop a volcano in Hawaii, continuously* since 1958, run for the duration by one father and son team.

* By infrared gas spectrometer, originally on paper chart recorders, in a tin shed. Construction, 1956:

Why on top of a volcano in the middle of a huge ocean? Because that's up in the northeast trade winds, blowing clean air, far from sources of pollution, from vegetation (and up above the thermal inversion to limit volcanic contamination on rare occasions wind doesn't blow).

We measure at other places as well now of course, including our own Cape Grim in Tasmania (nice 'roaring forties' air), but Mauna Loa has the long record. Our atmosphere is a windy place, 'well mixed', so it matters little where you measure, as long as your air is 'clean'.

But the data: There's a long, steady accelerating increase ... with just a few slight multi-year wiggles, particularly one around 1990-95. And there's an obvious annual cycle, which interestingly is smaller in Cape Grim data (tweet above).

That's the Northern Hemisphere terrestrial vegetation 'breathing'. There's much more land in the mid-latitude north, where vegetation only really grows in summer -- in July, when the annual dip in CO₂ concentration is most rapid. Cape Grim is further away, so sees less signal.

But we came in late, despite Charles Keeling's best efforts -- the CO₂ concentration was already rising rapidly by 1958. The data can be extended, less precisely, with measurements on 'air samples' (bubbles) trapped in Antarctic ice (not Greenland ice -- complex reasons).

We started out at about 275 ppm before about 1750 (below). 'ppm' is parts per million, measured by volume (not mass) because that's how we do gasses. In a gas, volume = molecular count, so 275 ppm means 275 CO₂ molecules in every million gas molecules.

https://keelingcurve.ucsd.edu/ 

Not much, but enough. We'd have been a 100% ice world without that original 275 ppm.

But the data: If you're a statistician, first thing you want to do is remove that annual cycle -- you 'seasonally adjust' the data. The famous Scripps Institution of Oceanography in La Jolla, California has already done that for us:

https://scrippsco2.ucsd.edu/data/atmospheric_co2/primary_mlo_co2_record.html

Next, you fit a straight line ... because that's what you do. CO₂ is rising at an 'average' of 1.6 ppm per year. Except problem: It ain't linear. Not even close.

Why do people want to fit a straight line, even to things that obviously aren't linear? We've tweeted about this often. Because it's *what you do*. Because it's what you were taught in school / uni. Because it was drummed into you that *anything else is wicked*.