Solar and CO2 Forcings for the Last 420 Million Years



One of the most interesting papers[1] I’ve seen in the last few years looks at future of climate forcing compared to the last 420 million years of geologic history. The study compiled CO2 proxy data for the last 420 million years from “1,241 independent estimates coming from five proxy methods and 112 published studies.” The resulting CO2 timeline is expressed both in terms of absolute concentrations and radiative forcing, accounting for both CO2 and the steady increase of solar output over time. The resulting timeline was then compared to the “representative concentration pathways” for future CO2 emissions. Under the “business as usual” RCP 8.5, “fossil fuel emissions suggest that atmospheric CO2 could peak in 2,250 AD at ∼2,000 p.p.m.” This would produce CO2 levels similar to the latter half of the Triassic (220-200 million years ago). However, given the increase in solar output, the radiative forcing this would produce would be similar to the early Eocene, likely exceeding the geologic record for 99.9% of the last 420 Myrs. Even under RCP 4.5, though, we would still expect to see an increase in radiative forcing of +5 W/m2.

The graph uses a log time scale, and the bottom half expresses the same data of the top half in terms of radiative forcing accounting for the influence of both CO2 and the sun. Over this time frame, there has been a long-term decrease in atmospheric CO2 concentrations, but this trend has been compensated for by the steady increase in solar output.

References:

[1] Gavin L. Foster, Dana L. Royer & Daniel J. Lunt, “Future climate forcing potentially without precedent in the last 420 million years,” Nature Communications 8.14845 (2017).
https://www.nature.com/articles/ncomms14845

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