Changes to Carbon Isotopes in Atmospheric CO(2) Over the Industrial Era and Into the Future

• Published in: Global Biogeochemical Cycles
• Main Authors: Graven, Heather, Keeling, Ralph F., Rogelj, Joeri
• Format: Text
• Language: English
• Published: John Wiley and Sons Inc. 2020
• Subjects: Commissioned Manuscript; The Spike; ice core
• Online Access: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7757245/; http://www.ncbi.nlm.nih.gov/pubmed/33380771; https://doi.org/10.1029/2019GB006170

In this “Grand Challenges” paper, we review how the carbon isotopic composition of atmospheric CO(2) has changed since the Industrial Revolution due to human activities and their influence on the natural carbon cycle, and we provide new estimates of possible future changes for a range of scenarios. Emissions of CO(2) from fossil fuel combustion and land use change reduce the ratio of (13)C/(12)C in atmospheric CO(2) (δ(13)CO(2)). This is because (12)C is preferentially assimilated during photosynthesis and δ(13)C in plant‐derived carbon in terrestrial ecosystems and fossil fuels is lower than atmospheric δ(13)CO(2). Emissions of CO(2) from fossil fuel combustion also reduce the ratio of (14)C/C in atmospheric CO(2) (Δ(14)CO(2)) because (14)C is absent in million‐year‐old fossil fuels, which have been stored for much longer than the radioactive decay time of (14)C. Atmospheric Δ(14)CO(2) rapidly increased in the 1950s to 1960s because of (14)C produced during nuclear bomb testing. The resulting trends in δ(13)C and Δ(14)C in atmospheric CO(2) are influenced not only by these human emissions but also by natural carbon exchanges that mix carbon between the atmosphere and ocean and terrestrial ecosystems. This mixing caused Δ(14)CO(2) to return toward preindustrial levels in the first few decades after the spike from nuclear testing. More recently, as the bomb (14)C excess is now mostly well mixed with the decadally overturning carbon reservoirs, fossil fuel emissions have become the main factor driving further decreases in atmospheric Δ(14)CO(2). For δ(13)CO(2), in addition to exchanges between reservoirs, the extent to which (12)C is preferentially assimilated during photosynthesis appears to have increased, slowing down the recent δ(13)CO(2) trend slightly. A new compilation of ice core and flask δ(13)CO(2) observations indicates that the decline in δ(13)CO(2) since the preindustrial period is less than some prior estimates, which may have incorporated artifacts owing to offsets from different laboratories' ...