The Earth heated up when its day was 22 hours long

Because most things about Earth change so slowly, it's difficult to imagine them being any different in the past. But Earth's rotation has been slowing due to tidal interactions with the Moon, meaning that days were considerably shorter in the past. It's easy to think that a 22-hour day wouldn't be all that different, but that turns out not to be entirely true.

For example, some modeling has indicated that certain day lengths will be in resonance with other effects caused by the planet's rotation, which can potentially offset the drag caused by the tides. Now, a new paper looks at how these resonances could affect the climate. The results suggest that it would shift rain to occurring in the morning and evening while leaving midday skies largely cloud-free. The resulting Earth would be considerably warmer.

On the Lamb

We're all pretty familiar with the fact that the daytime Sun warms up the air. And those of us who remember high school chemistry will recall that a gas that is warmed will expand. So, it shouldn't be a surprise to hear that the Earth's atmosphere expands due to warming on its day side and contracts back again as it cools (these lag the daytime peak in sunlight). These differences provide something a bit like a handle that the gravitational pulls of the Sun and Moon can grab onto, exerting additional forces on the atmosphere. This complicated network of forces churns our atmosphere, helping shape the planet's weather.

Two researchers, Russell Deitrick and Colin Goldblatt at Canada's University of Victoria, were curious as to what would happen to these forces as the day length got shorter. Specifically, they were interested in a period where the day's length would be at resonance with phenomena called Lamb waves.

Lamb waves aren't specific to the atmosphere. Rather, they're a specific manner in which a disturbance can travel through a medium, from vibrations in a solid to sound through the air.

Although various forces can create Lamb waves in the atmosphere, they'll travel with a set of characteristic frequencies. One of those is roughly 10.5 to 11 hours. As you go back in time to shorter days, you'll reach a point where the Earth's day was a bit shorter than 22 hours, or twice the period of the Lamb waves. At this point, any disturbances in the atmosphere related to day length would have the ability to interact with the Lamb waves that were set off the day prior. This resonance could potentially strengthen the impact of any atmospheric phenomena related to day length.

Figuring out whether they do turned out to be a bit of a challenge. There are plenty of climate models to let researchers explore what's going on in the modern atmosphere. But a lot of these have key features, like day length and solar output, hard coded into them. Others don't let you do things like rearrange the Earth's continents or change some atmospheric components.

The researchers did find a model that would allow them to change day length, solar intensity, and carbon dioxide concentrations to those present when Earth's day length was 22 hours (which was likely to be in the pre-Cambrian). But it wasn't able to reset the ozone concentrations, and ozone is also a greenhouse gas. So, they ran simulations without ozone, which are expected to be an under-estimate, and one where they elevated methane concentrations in order to mimic ozone's greenhouse effect.

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).