# A shorter Archean day-length biases interpretations of the early Earth's climate

## TL;DR

This paper explores how a shorter Archean day-length, due to faster Earth's spin, could explain warm early Earth temperatures despite a fainter Sun, impacting interpretations of ancient climate records.

## Contribution

It introduces the role of Earth's faster spin rate in influencing early climate models and the geological record, offering a new perspective on the Faint Young Sun Paradox.

## Key findings

- Faster spin enhances equator-to-pole temperature gradients.
- Enhanced gradients bias geological records toward warmer waters.
- Faster spin reduces Earth's sensitivity to ice-albedo feedbacks.

## Abstract

Earth's earliest sedimentary record contains evidence that surface temperatures were similar to, or perhaps even warmer than modern. In contrast, standard Solar models suggest the Sun was 25% less luminous at this ancient epoch, implying a cold, frozen planet-all else kept equal. This discrepancy, known as the Faint Young Sun Paradox, remains unresolved. Most proposed solutions invoke high concentrations of greenhouse gases in the early atmosphere to offset for the fainter Sun, though current geological constraints are insufficient to verify or falsify these scenarios. In this work, we examined several simple mechanisms that involve the role played by Earth's spin rate, which was significantly faster during Archean time. This faster spin rate enhances the equator-to-pole temperature gradient, facilitating a warm equator, while maintaining cold poles. Results show that such an enhanced meridional gradient augments the meridional gradient in carbonate deposition, which biases the surviving geological record away from the global mean, toward warmer waters. Moreover, using simple atmospheric models, we found that the faster-spinning Earth was less sensitive to ice-albedo feedbacks, facilitating larger meridional temperature gradients before succumbing to global glaciation. We show that within the faster-spinning regime, the greenhouse warming required to generate an ice-free Earth can differ from that required to generate an Earth with permanent ice caps by the equivalent of 1-2 orders of magnitude of pCO2. Accordingly, the resolution of the Faint Young Sun problem depends significantly on whether the early Earth was ever, or even at times, ice-free.