Carbonate-Silicate Cycle Predictions of Earth-like Planetary Climates and Testing the Habitable Zone Concept

TL;DR

This paper models the carbonate-silicate cycle on Earth-like planets to predict how atmospheric CO2 varies with orbital distance, providing a potential observational test for the habitable zone concept.

Contribution

It introduces a coupled climate and weathering model to quantify CO2 variability and proposes an observational method to test the habitable zone hypothesis.

Findings

A log-linear pCO2-flux relationship is predicted.

Scatter in pCO2 is due to geophysical and physicochemical variations.

Detection of the relationship is feasible with at least 83 exoplanet observations.

Abstract

In the conventional habitable zone (HZ) concept, a CO$_{2}$-H$_2$O greenhouse maintains surface liquid water. Through the water-mediated carbonate-silicate weathering cycle, atmospheric CO$_{2}$ partial pressure (pCO$_{2}$) responds to changes in surface temperature, stabilizing the climate over geologic timescales. We show that this weathering feedback ought to produce a log-linear relationship between pCO$_{2}$ and incident flux on Earth-like planets in the HZ. However, this trend has scatter because geophysical and physicochemical parameters can vary, such as land area for weathering and CO$_2$ outgassing fluxes. Using a coupled climate and carbonate-silicate weathering model, we quantify the likely scatter in pCO$_2$ with orbital distance throughout the HZ. From this dispersion, we predict a two-dimensional relationship between incident flux and pCO$_2$ in the HZ and show that it could be detected from at least 83 ($2{\sigma}$) Earth-like exoplanet observations. If fewer Earth-like exoplanets are observed, testing the HZ hypothesis from this relationship could be difficult.