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

The climate-carbon cycle feedback is one of the most important climate-amplifying feedbacks of the Earth system, and is quantified as a function of carbon-concentration feedback parameter ( β ) and carbon-climate feedback parameter ( γ ). However, the global climate-amplifying effect from this feedback loop (determined by the gain factor, g ) has not been quantified from observations. Here we apply a Fourier analysis-based carbon cycle feedback framework to the reconstructed records from 1850 to 2017 and 1000 to 1850 to estimate β and γ . We show that the β -feedback varies by less than 10% with an average of 3.22 ± 0.32 GtC ppm −1 for 1880–2017, whereas the γ -feedback increases from −33 ± 14 GtC K −1 on a decadal scale to −122 ± 60 GtC K −1 on a centennial scale for 1000–1850. Feedback analysis further reveals that the current amplification effect from the carbon cycle feedback is small ( g is 0.01 ± 0.05), which is much lower than the estimates by the advanced Earth system models ( g is 0.09 ± 0.04 for the historical period and is 0.15 ± 0.08 for the RCP8.5 scenario), implying that the future allowable CO 2 emissions could be 9 ± 7% more. Therefore, our findings provide new insights about the strength of climate-carbon cycle feedback and about observational constraints on models for projecting future climate. How to curb climate change is uncertain, in part because determination of allowable emissions depends on models with low accuracy. Here the authors re-analyze climate-carbon feedbacks and find that CO 2 emissions could be 9 ± 7% higher and still meet Paris Agreement goals.