Abstract
Abstract. Changes to climate–carbon cycle feedbacks may significantly affect the Earth system's response to greenhouse gas emissions. These feedbacks are usually analysed from numerical output of complex and arguably opaque Earth system models. Here, we construct a stylised global climate–carbon cycle model, test its output against comprehensive Earth system models, and investigate the strengths of its climate–carbon cycle feedbacks analytically. The analytical expressions we obtain aid understanding of carbon cycle feedbacks and the operation of the carbon cycle. Specific results include that different feedback formalisms measure fundamentally the same climate–carbon cycle processes; temperature dependence of the solubility pump, biological pump, and CO2 solubility all contribute approximately equally to the ocean climate–carbon feedback; and concentration–carbon feedbacks may be more sensitive to future climate change than climate–carbon feedbacks. Simple models such as that developed here also provide workbenches for simple but mechanistically based explorations of Earth system processes, such as interactions and feedbacks between the planetary boundaries, that are currently too uncertain to be included in comprehensive Earth system models.
Highlights
The exchanges of carbon between the atmosphere and other components of the Earth system, collectively known as the carbon cycle, currently constitute important negative feedbacks on the effect of anthropogenic carbon emissions on climate change
We describe the dynamics of the land carbon stock, the ocean carbon stock, and atmospheric carbon and temperature in our model
The net transport of carbon to the lower ocean by the solubility pump can be represented by w0(1 − wT T ), where w0 is the rate at which mixed layer ocean water is exchanged with the deep ocean and wT parameterises weakening of the overturning circulation that is expected to occur with future climate change (Collins et al, 2013)
Summary
The exchanges of carbon between the atmosphere and other components of the Earth system, collectively known as the carbon cycle, currently constitute important negative (dampening) feedbacks on the effect of anthropogenic carbon emissions on climate change. Carbon sinks in the land and the ocean each currently take up about one-quarter of anthropogenic carbon emissions each year (Le Quéré et al, 2016) These feedbacks are expected to weaken in the future, amplifying the effect of anthropogenic carbon emissions on climate change (Ciais et al, 2013). Our stylised and mechanistically based climate–carbon cycle model offers a workbench for investigating the influence of mechanisms that are at present too uncertain, poorly defined, or computationally intensive to include in current Earth system models. Such stylised models are valuable for exploring the uncertain but potentially highly impactful Earth system dynamics such as interactions between climatic and social tipping elements (Lenton et al, 2008; Kriegler et al, 2009; Schellnhuber et al, 2016) and the planetary boundaries (Rockström et al, 2009; Steffen et al, 2015). We conclude by speculating on how this stylised model could be used as a “workbench” for studying a range of complex Earth system processes, especially those related to the biosphere
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