Climate Effects on Atmospheric Carbon Dioxide Over the Last Century

Since 1958, the buildup of atmospheric CO2 can be quite accurately explained by the simple premise that 56% of the industrial emissions (fossil fuel burning and cement manufacture) has remained airborne. The constancy of this airborne fraction is in contrast to the large variations in the airborne fraction seen in the 19th and early 20th centuries. The constancy since 1958 is also surprising because the airborne fraction was expected to decline after ~1980, when the growth rate of fossil fuel emissions decreased, allowing the oceans to absorb a greater fraction. We show that these features can be reconciled assuming that a portion of the CO2 variability is driven by variations in global land temperature, with a proportionality of around 10 ppm/degrees C. Using a simple land biospheric model, we show that the global land temperature can drive this CO2 variability through temperature-dependent respiration using a Q10 of approximately 2, as expected. Our analysis suggests that the constancy of the airborne fraction over the past 50 years is coincidental: a warming trend started around the same time as the decrease in the growth rate of fossil fuel emissions, and compensated for the expected decrease in the airborne fraction. If the land biosphere behaves similarly over the next century, this will contribute to a small but significant positive feedback on future global warming.