The relationship between near-surface air temperature over land and the annual amplitude of the atmosphere's seasonal CO₂ cycle

The seasonal growth and decay of Northern Hemispheric terrestrial vegetation creates a seasonal oscillation in the atmospheric CO₂ concentration in the Northern Hemisphere. Over the past several decades, the amplitude of this seasonal cycle has risen substantially. Many potential causes of this phenomenon have been suggested; but the one that seems currently to be of most concern is global warming. This study thus seeks to determine if the rise in the near-surface air temperature of the past 35 years has been the major factor in driving the contemporaneous increase in the amplitude of the Northern Hemisphere's seasonal CO₂ cycle. Annual CO₂ amplitude data from ten Northern Hemispheric stations of the NOAA/CMDL continuous monitoring network were regressed against mean near-surface air temperatures over land areas located within each 5°latitude band stretching from the equator to 85°N. Temperature effects were non-existent when the CO₂ amplitude and temperature data were contemporaneous, whereas they were maximal when the CO₂ amplitude data lagged the temperature data by 2 years, declining slowly thereafter to become non-existent again between a lag time of 6-7 years. For the peak-impact 2-year lag situation, low-latitude temperatures were much more strongly correlated with the CO₂ amplitude data than were mid- or high-latitude temperatures. The 2-year-lag results for Mauna Loa and Point Barrow indicate that Northern Hemispheric warming may have been responsible for about a fifth of the annual CO₂ amplitude increase observed at Mauna Loa from 1960 to 1995 and approximately a tenth of the amplitude increase recorded at Point Barrow over this period. Consequently, the majority of the Northern Hemispheric CO₂ amplitude increase appears to be due to the influence of some other factor or combination of factors, a number of which have been suggested in the literature.