> The modern record of atmospheric carbon dioxide levels began with observations recorded at Mauna Loa Observatory in Hawaii.

> The seasonal cycle of highs and lows (small peaks and valleys) is driven by Northern Hemisphere summer vegetation growth, which reduces atmospheric carbon dioxide, and winter decay, which increases it.

https://www.climate.gov/news-features/understanding-climate/...

Likely reflects the seasonal cycle (vegetation cycle) and before 1958 likely was no frequent measurements to see it.

it starts 1958 because it changes the source [1]. Before 1958 law dome & ice cores. After 1958 air measurements at Mauna Loa Observatory, Hawaii.

[1] https://www.co2levels.org/#sources

Basically the northern hemisphere has more land/forest. When it's summer there, trees turn CO2 into wood much faster. Until mushroom season. (It's more complex of course, and not just the trees.)

Likely continuous measurements rather than ice cores. We can clearly see the difference between winter and summer (what you call the zigzag pattern), when a small portion is absorbed by the biomass.

I suspect it's the earliest date from when they have more granular data. The zigzag was still there pre-1958, but it was never recorded, so the smooth line is the best you're going to get.

More granular data capture + variance in seasons (?)

Seasonality nature of CO2 with greater ability to measure

That's fear