Carbon system state determines warming potential of emissions

Area of Science:

• Climate Science
• Earth System Science
• Carbon Cycle Research

Background:

• Current climate strategies rely on limiting cumulative carbon emissions based on a near-linear relationship between emissions and surface warming.
• This relationship is a cornerstone of IPCC assessments for setting emission budgets for targets like 1.5°C or 2°C warming.

Purpose of the Study:

• To test the proportionality assumption between cumulative carbon emissions and surface warming using advanced Earth System Models (ESMs).
• To investigate the influence of emission timing on climate system responses, specifically the Transient Climate Response to Cumulative Carbon Emissions (TCRE).

Main Methods:

• Utilized specially designed simulations with a latest-generation ESM featuring an interactive carbon cycle and updated terrestrial processes.
• Analyzed a suite of Coupled Model Intercomparison Project (CMIP) simulations, including ZecMIP and ScenarioMIP.

Main Results:

• Found that atmospheric CO2 concentrations and surface warming can vary significantly (up to ~100 ppmv and ~0.31°C, respectively) for the same cumulative carbon emissions.
• Demonstrated that TCRE is dependent not only on total emissions but also on the timing of those emissions, a factor previously overlooked.
• Identified lagged carbon sequestration processes in oceans and land as key drivers of this time dependency.

Conclusions:

• The established TCRE concept is not fundamentally invalidated but is subject to additional uncertainties related to the evolving state of the carbon system.
• Accurate future emission impact projections require improved understanding of carbon system state dependency through observations and refined ESMs.