A ten‐year decrease in the atmospheric helium isotope ratio possibly caused by human activity

The isotope ratio of terrestrial air helium is believed to be constant on a global scale since the mixing time for the atmosphere is significantly shorter than the residence time for helium. Recently, the ³He/ 4 He ratio at Ueno Park, central Tokyo, Japan, was found to be significantly lower than previously measured values. We report here a change in the atmospheric helium isotope ratio from 1.362 × 10 −6 in December 1977 to 1.339 × 10 −6 in September 1988 or a decrease of ∼1 × 10 −9 /year. Although this change could be due to local/nonglobal effects, minor changes in the atmospheric flux balance for helium or experimental artifacts, the observations are also consistent with a significant influx of a low ³He/ 4 He ratio source to the atmosphere. The magnitude of the flux, 0.48 ‐ 2.9 × 10 16 cm³STP He/year, is compatible with estimates of the anthropogenic release of crustal helium from gas and oil production from the solid Earth. Because of the inert chemistry of helium, a quantification of this change may provide a marker against which to calibrate the absolute flux and retention of anthropogenic CO 2 in the atmosphere.