Anthropogenic and solar components of hemispheric ¹⁴C

A variable solar (helio‐magnetic) modulation of the cosmic ray flux causes atmospheric 14 C to change on a decadal to century timescale. Ocean‐atmosphere exchange rates and atmospheric circulation patterns play a role in Northern‐Southern Hemispheric 14 C offsets and regional 14 C differences that are significant for radiocarbon dating. Time dependent radiocarbon age differences, relative to Washington, were determined for Alaska (A.D. 1884–1932), South Chile (A.D. 1850–1952 and A.D. 1670–1722), Tasmania (A.D. 1895–1950) and Siberia (A.D. 1545–1715). Twentieth century fossil fuel CO 2 , lacking 14 C and mostly released in the Northern Hemisphere (N), has entered the Southern Hemisphere (S) by atmospheric N‐S exchange. Regional 14 C time series show that initial (19th century) positive N‐S 14 C offsets switch to negative values by AD 1940. First order carbon reservoir modeling predicts such crossovers.