 Although radiocarbon dating has been applied as far back as 50,000 years, during the deglaciations at the start of the Holocene the biosphere and its carbon uptake changed dramatically making estimation before this impractical; after about 1900 the Suess effect and nuclear bomb-tests makes interpretation difficult.10Be concentrations in stratified polar ice cores provide an independent measure of activity.Both measures agree reasonably with each other and with the Zurich sunspot number of the last two centuries.