Climate energy balance and long-term carbon cycling
The global energy balance links climate and carbon feedbacks
The silicate weathering feedback is thought to keep our planet habitable on geologic timescales by sequestering more atmospheric CO2 when the planet warms. Therefore, the strength of the silicate weathering feedback depends on the sensitivity of temperature and runoff (the variables most relevant for weathering) to atmospheric CO2. Climate feedbacks can influence this sensitivity, but their complexity and uncertainty has made them difficult to incorporate in long-term carbon cycle models. How can we robustly study the interactions of climate and carbon feedbacks?
Energy balance climate models have been used to study climate feedbacks for decades, but recent work shows they are surprisingly effective at capturing complex, real-world dynamics (e.g. Flannery, 1984; Roe et al., 2015; Boos & Korty, 2016; Siler et al., 2018; Peterson & Boos, 2020). Because these models are computationally efficient, they can be used to test how a range of climate feedbacks affect long-term carbon cycling through geologic time.
We are using energy balance climate models to study how climate feedbacks affect steady state and transient carbon cycle dynamics on geologic timescales. By prescribing climate feedbacks and their strength, our spatially-resolved simulations can constrain the emergent behavior of long-term climate-carbon dynamics such as the “memory” of the coupled system and the feasibility of multiple stable states.
Work on this topic is ongoing . . .
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Coupled model mechanics
The above simulations link a moist energy balance model of climate (Siler et al., 2018) to a model of the long-term, exogenic carbon cycle (modified from Rugenstein et al., 2019). One insight that emerges from the coupled models is that the strength of the silicate weathering feedback varies over space in a predictable way.
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Silicate weathering feedback strength decreases with warming in our model
As temperature becomes less sensitive to CO2, water runoff becomes less sensitive as well. This decreases the weathering response to warming. Additionally, the tropics (where the silicate weathering feedback is strongest in these idealized simulations) are weathering near the thermodynamic limit. As warming and water runoff increases, the change in weathering decreases due to limitations in the amount of fresh rock to weather and the weathering reaction rates. Both effects decrease the feedback strength with warming.
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Temperature and runoff sensitivity to CO2 tends to increase weathering
Positive radiative feedbacks in the climate system generally increase the silicate weathering feedback strength because they make temperature (and therefore runoff) more sensitive to CO2. The effect is shown in the top panel where an idealized warming-albedo feedback increases temperature sensitivity. In the bottom panel, the runoff sensitivity to CO2 increases due to a “CO2 fertilization” effect whereby plants need less water per unit carbon fixed as CO2 increases. This has no direct effect on temperature in our model, but it does increase silicate weathering fluxes by modulating runoff sensitivity.
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The impact of a "darker" subtropics on climate and silicate weathering feedback strength
Darkening the subtropics increases global temperature a similar amount to nearly doubling CO2. About half of the increase in silicate weathering is owed to warming, while the other half is due to more rainfall over land. In a transient simulation, subtropical vegetation would strengthen the weathering feedback and, ultimately, lead to a net decrease in atmospheric CO2.
