Historical Perspectives of the Global Carbon Cycle

Quantifying atmospheric carbon dioxide (CO2) concentration and carbon (C) cycling during Earth’s ancient greenhouse episodes is essential for accurately interpreting current global climate and predicting the future climate due to elevated CO2 concentrations associated with increased anthropogenic CO2 concentration. While the trends in atmospheric CO2 concentration and global C cycling in recent decades are clear, its significance is only revealed when viewed in the context of geological timescales. Beyond the direct instrumental record, air bubbles trapped in ice cores has provided concentrations of greenhouse gases (GHGs) and reveal that the atmospheric CO2 concentration was 278 ± 2 ppmv at the onset of the Industrial Revolution in 1750. Ice core covering a period of the past 800,000 years, which incorporates the past eight glacial/interglacial cycles have been extracted and characterized. During the glacial/interglacial period, the atmospheric CO2 concentration oscillated between 170 and 200 ppmv during glacial periods and 240–290 ppmv during interglacial periods, revealing coupling of the global temperature and atmospheric CO2 concentration. It is broadly accepted that changes in atmospheric CO2 concentration constitutes a feedback rather than the primary cause of climate variation observed during the glacial-interglacial cycles, however. The drivers and mechanisms controlling the onset of and variations in atmospheric CO2 concentration during glacial/interglacial are highly debated, but it is broadly accepted that the succession of glacial/interglacial cycles are driven by the shape of Earth’s orbit and tilt of its spin axis termed as Milankovitch cycles . However, the exact mechanisms on how these cycles initiate or terminate glacial cycle is still not known. The C cycling processes and the associated changes in climatic factor acts as feedback mechanisms. During the interval of global warming from the last glacial maximum to early Holocene, climate system underwent large-scale changes, including decay of ice sheets which caused the sea level rise, estimated at 80–120 m and net release of CO2 to the atmosphere, which increased the atmospheric concentration to 265 ppmv at early Holocene. An increase of 20 ppm is observed during Holocene, which is generally attributed to decomposition of deep-sea organic matter (OM). The C cycling and atmospheric CO2 concentration for geologic timescale beyond the ice core record is normally reconstructed from geological proxies and geochemical models. On a multimillion-year timescale the long-term or geochemical C cycle involves slow exchange of C between the rocks (i.e., lithosphere) and the surface reservoirs consisting of the atmosphere, the ocean, the biota and soils. The processes affecting the atmospheric CO2 concentration are the uptake of atmospheric CO2 during silicate minerals, transport, precipitation and burial of carbonates as limestone as well as burial of organic matter (OM), thereby removing CO2 from the atmosphere. Degassing of CO2 from rocks and buried OM on the other hand return CO2 back to the atmosphere. Geologic records show evidence of coupling of climate and C cycling during Phanerozoic. The atmospheric CO2 concentration was low ( 1000 ppm). These records, are highly correlated with the atmospheric CO2 predicted from geochemical models.