Estimates of climatic influence on the carbon cycle

Abstract. The influence of climatic change on the carbon cycle is important as part of a CO₂-climate feedback loop. However the magnitude of the coupling depends on the timescales involved. We expand on previous analyses of the ice-core CO₂ data from the pre-industrial period 1000–1750, extending the analysis into the 20th century. Our results emphasise the limitations of characterising the climate-to-CO₂ influence by a single number γ. Even once a time-scale dependence is incorporated, the coldest part of the Little Ice Age seems to reflect different behaviour to that in earlier or later centuries. Different temperature reconstructions appear to capture distinct aspects of pre-industrial climate fluctuations that lacked global coherence. An exploratory study extends the analysis into the industrial period. In this study, most paleo-temperature data fail to fit the plateau (or plateaus) in 20th century ice-core CO₂, with one particular reconstruction as an exception. One interpretation of this fit is that although the reconstruction does not closely reflect hemispheric temperature changes, it samples a pattern of variation where the terrestrial carbon exchange is anomalously sensitive to regional climate variations. These various results suggest that this type of empirical study may have limited applicability to the 21st century.