5:15 PM - 7:15 PM
[PPS06-P07] Theoretical study on the long-term stability of the climate of habitable planets: carbon cycle system and Daisyworld system
Keywords:Earth, Model
A habitable planet like the Earth must satisfy the conditions for the existence of liquid water. It corresponds to the conditions for a warm and wet climate. In order to maintain such conditions, a negative feedback mechanism for a long-term stability of the climate is required.
There is a well-known mechanism for such a negative feedback working on the Earth, a silicate weathering feedback (called “Walker feedback”) in the carbonate-silicate geochemical cycle system. Because chemical weathering reaction of silicate minerals that compose continental crusts depends on the surface temperature, it regulates concentrations of atmospheric carbon dioxide and stabilizes the Earth's climate. This is considered as a universal mechanism on habitable planets that have oceans and continents on the surface, as well as plate tectonics, like the Earth.
There is another well-known mechanism that plays negative feedback in stabilizing planetary climates. It is the Daisyworld, in which the surface of the planet is covered with white and black daisies. The daisies grow depending on temperature, and different albedo of white and black daisies create a negative feedback loop (hereafter called “Daisy-albedo feedback”) that stabilizes the planetary surface temperature over a wide range of solar luminosities.
Although these two negative feedbacks can stabilize planetary climate, they are quite different mechanisms: the Walker feedback stabilizes the climate through controlling greenhouse effect of the atmospheric CO2, while the Daisyworld albedo feedback stabilizes the climate through controlling surface albedo of the planet. However, because both the rate of chemical weathering reactions and the growth rate of daisies depend not only on temperature but also on pCO2, precipitation, land-sea distribution, etc., the two processes should interact with each other. However, there have been no studies so far that have examined their interactions and synergies by considering both processes at the same time. In this study, we developed 0-dimensional (0-D) and 1-dimensional (1-D) climate models which coupled carbon cycle and Daisyworld systems, and discussed the long-term stability of the planetary climate.
First, a coupled model of 0-D climate-carbon cycle-Daisyworld system is used to investigate the effects and interactions of the two feedbacks during the evolution of the planet and the central star. We found that the Daisy-albedo feedback is dominant in the range of solar luminosity in which daisy can grow and the climate can be stabilized owing to the Daisy-albedo feedback. Under these conditions, the surface temperature remain constant at the optimal temperature of the daisies. The Walker feedback is effective over a wider range of solar luminosity, even outside the conditions in which daisies can grow.
Next, a coupled model of 1-D climate-carbon cycle-Daisyworld system was used to examine the interactions and synergies of the two feedbacks. The 1-D model showed that the presence of daisies reduced the difference in the temperature between the equator and poles. As in the 0-D model, we also found that there is a range of solar luminosity where the effect of the Daisy-albedo feedback is dominant, but a range where the synergies between the Walker and Daisy-albedo feedbacks increases the stabilizing effect for the climate.
The relative importance of the Walker feedback and Daisy-albedo feedback depends on the land-sea distribution, or an area ratio of continents and oceans. We found that an area ratio affects both daisy coverage and chemical weathering rate: as land area decreases, the Walker feedback that balances CO2 input increases pCO2 and warms the climate, but limiting Daisy area cover weakens the Daisy-albedo feedback.
There is a well-known mechanism for such a negative feedback working on the Earth, a silicate weathering feedback (called “Walker feedback”) in the carbonate-silicate geochemical cycle system. Because chemical weathering reaction of silicate minerals that compose continental crusts depends on the surface temperature, it regulates concentrations of atmospheric carbon dioxide and stabilizes the Earth's climate. This is considered as a universal mechanism on habitable planets that have oceans and continents on the surface, as well as plate tectonics, like the Earth.
There is another well-known mechanism that plays negative feedback in stabilizing planetary climates. It is the Daisyworld, in which the surface of the planet is covered with white and black daisies. The daisies grow depending on temperature, and different albedo of white and black daisies create a negative feedback loop (hereafter called “Daisy-albedo feedback”) that stabilizes the planetary surface temperature over a wide range of solar luminosities.
Although these two negative feedbacks can stabilize planetary climate, they are quite different mechanisms: the Walker feedback stabilizes the climate through controlling greenhouse effect of the atmospheric CO2, while the Daisyworld albedo feedback stabilizes the climate through controlling surface albedo of the planet. However, because both the rate of chemical weathering reactions and the growth rate of daisies depend not only on temperature but also on pCO2, precipitation, land-sea distribution, etc., the two processes should interact with each other. However, there have been no studies so far that have examined their interactions and synergies by considering both processes at the same time. In this study, we developed 0-dimensional (0-D) and 1-dimensional (1-D) climate models which coupled carbon cycle and Daisyworld systems, and discussed the long-term stability of the planetary climate.
First, a coupled model of 0-D climate-carbon cycle-Daisyworld system is used to investigate the effects and interactions of the two feedbacks during the evolution of the planet and the central star. We found that the Daisy-albedo feedback is dominant in the range of solar luminosity in which daisy can grow and the climate can be stabilized owing to the Daisy-albedo feedback. Under these conditions, the surface temperature remain constant at the optimal temperature of the daisies. The Walker feedback is effective over a wider range of solar luminosity, even outside the conditions in which daisies can grow.
Next, a coupled model of 1-D climate-carbon cycle-Daisyworld system was used to examine the interactions and synergies of the two feedbacks. The 1-D model showed that the presence of daisies reduced the difference in the temperature between the equator and poles. As in the 0-D model, we also found that there is a range of solar luminosity where the effect of the Daisy-albedo feedback is dominant, but a range where the synergies between the Walker and Daisy-albedo feedbacks increases the stabilizing effect for the climate.
The relative importance of the Walker feedback and Daisy-albedo feedback depends on the land-sea distribution, or an area ratio of continents and oceans. We found that an area ratio affects both daisy coverage and chemical weathering rate: as land area decreases, the Walker feedback that balances CO2 input increases pCO2 and warms the climate, but limiting Daisy area cover weakens the Daisy-albedo feedback.