Inter-decadal trends in the annual cycles of atmospheric CO₂ at Mauna Loa

Abstract

Cyclical rise and fall of monthly mean CO₂ concentration occurred rather regularly at Mauna Loa, Hawaii, USA, during 1958-2008, with peaks occurring in April or May followed by troughs in September or October. However, the frequency of troughs in September increased from 32% in 1958-1974 to 82% in 1992-2008 and the depletion season correspondingly decreased from 4.8 months on average to 4.2 months. Does this seemingly small change in the seasonal rhythm point to changes on an inter-decadal scale in the CO₂ accumulation rates? This question is studied here by examining linear and exponential trends of seasonal and annual CO₂ accumulation rates in three equal phases (1958-1974, 1975-1991 and 1992-2008). We also discuss trends of average accumulation rates in the accretion and depletion seasons, and of the amplitude and standard deviation of the annual CO₂ cycles at Mauna Loa, which can be viewed as representative of the North Central Tropical Pacific.