The Earth heated up when its day was 22 hours long

Nothing but sunshine

As the researchers tested day lengths between 16 and 24 hours, the point of resonance was obvious. Things like average temperatures and the difference between day and night pressures started rising when day lengths reached 21 hours and peaked at a 22-hour day length before declining again. And the differences were substantial. The present-day difference between minimal and peak daily pressures is about 110 Pascals. At a day length of 22 hours, it was 330 Pascals, a threefold increase.

The increase in average temperatures is on the order of 2 Kelvin. We're currently trying to avoid that sort of temperature rise due to the potential for widespread ecological disruptions. The addition of methane to mimic ozone leads to an increase of 4 Kelvin. Accordingly, atmospheric moisture goes up by about 10 percent, and the amount of precipitation by five percent.

But why does the Earth get warmer when the days are shorter? In part due to the altered pressure changes, the organization of tropical clouds changes. Now, peaks in cloud cover and precipitation occur in the morning and evening; the daytime in between is largely cloud-free. This means that the peak of absorption of solar energy now coincides with the peak in sunshine.

Deitrick and Goldblatt note that, given how long the Moon has been slowing down the Earth's rotation, a period like this is inevitable at some point in our past. The question is when.

As we noted above, there's a similar atmospheric/orbital resonance that may offset the Moon's drag on our rotation, leading to an extended period where the Earth's day length remains largely unchanged. Deitrick and Goldblatt see no clear evidence of that resonance in this work. And a simple extrapolation back suggests that this resonance may have occurred shortly (in geological terms) before the Cambrian.

And that's an interesting time. Thanks to the contribution of a fainter Sun, the Earth went through a number of global glaciations immediately prior to the Cambrian. The ice is thought to have blocked the exchange of carbon dioxide with the ocean, allowing it to build up in the atmosphere and eventually warm the planet enough to melt it out of the snowball phase. But this resonance has the potential to drive a melt even without the greenhouse gas buildup.

If this holds up in further studies, that may mean that a lot of the complex life on Earth owes its existence to the warming driven by this atmospheric resonance.

Nature Geoscience, 2024. DOI: 10.1038/s41561-024-01469-3  (About DOIs).