Tropospheric ¹⁴CO₂ at Wellington, New Zealand: the world's longest record

Measurements of near-sea-level tropospheric ∆¹⁴CO₂ have been made at Wellington, New Zealand since December 1954; these measurements comprise the longest such record available. The ∆¹⁴C rose from –10‰ in 1955 peaking at 695‰ in 1965 as a result of "bomb ¹⁴C" production, before falling thereafter to the present day (2005) value of 73‰. The peak occurred about 1 year later in the southern hemisphere than in the northern hemisphere. The post-1965 fall is due to the transfer of ¹⁴C-enriched CO₂ to the biospheric and oceanic pools together with ongoing release of ¹⁴C-free CO₂ from fossil fuel combustion, during an era without major atmospheric nuclear-weapon tests. Time series analysis of the data using Loess decomposition and filtering indicates an approximately exponential decline in excess ∆¹⁴C₂ over 1967-2005 with an e-folding time of 18 years. The seasonal cycle from 1954 until 1980 had a maximum in the late (austral) summer, a minimum in winter, with peak-to-trough amplitude that peaked at 20‰ in 1966. For the period 1980-1989, a new seasonal cycle emerged, with a maximum in winter and a minimum in late summer/early autumn and peak-to-trough amplitude of 3.5‰, transitioning to a new seasonal structure after about 1990.