A small climate-amplifying effect of climate-carbon cycle feedback.

Area of Science:

• Earth System Science
• Climate Science
• Carbon Cycle Research

Background:

• The climate-carbon cycle feedback is a critical Earth system process amplifying climate change.
• Quantifying this feedback's global amplifying effect (gain factor, g) from observational data has been a challenge.
• Key parameters include the carbon-concentration feedback (β) and carbon-climate feedback (γ).

Purpose of the Study:

• To quantify the global climate-amplifying effect of the climate-carbon cycle feedback using observational data.
• To estimate the carbon-concentration feedback parameter (β) and carbon-climate feedback parameter (γ) over different time scales.
• To compare observational estimates of the feedback strength with those from Earth system models.

Main Methods:

• Application of a Fourier analysis-based carbon cycle feedback framework.
• Reconstruction of climate and carbon records from 1850 to 2017 and 1000 to 1850.
• Estimation of feedback parameters β and γ, and the gain factor g.

Main Results:

• The carbon-concentration feedback (β) remained relatively stable (3.22 ± 0.32 GtC ppm⁻¹ for 1880-2017).
• The carbon-climate feedback (γ) showed significant strengthening with time scale, from -33 ± 14 GtC K⁻¹ (decadal) to -122 ± 60 GtC K⁻¹ (centennial) for 1000-1850.
• The observed amplification effect (g = 0.01 ± 0.05) is substantially lower than model estimates (0.09 ± 0.04 historical, 0.15 ± 0.08 RCP8.5).

Conclusions:

• The current observed amplification of climate change by the carbon cycle is small.
• Earth system models may overestimate the strength of the climate-carbon cycle feedback.
• Findings suggest potentially higher future allowable CO₂ emissions (9 ± 7%) than previously estimated by models.