Atmospheric nuclear weapons testing in the mid-20th century (particularly the 1950s-early 1960s) produced a large excess of radiocarbon (14C, so-called 'bomb carbon') in the atmosphere via neutron activation of nitrogen. Since the Partial Test Ban Treaty of 1963 halted atmospheric testing, this excess bomb-produced 14C has been steadily declining, not because of radioactive decay (14C's half-life of ~5730 years is far too long to explain the observed decline over decades) but because of uptake and redistribution of atmospheric carbon into the terrestrial biosphere (via photosynthesis) and the oceans (via air-sea gas exchange and dissolution).