This study explores origins for record setting 2006 temperatures over the conterminous United States. The efficacy of two mechanisms is quantified; one associated with sea surface temperatures (SSTs) in the equatorial Pacific related to El Niño, and the other associated with increased greenhouse gas concentrations. We use historical records of US temperatures observed during past El Niños, model simulations subjected to El Niño SST conditions and to projections of the 2006 greenhouse gas concentrations. We use ensemble methods to yield probabilistic estimates of temperature anomalies related to each forcing. Neither historical data nor model simulations reveal a US warm response to El Niño indicating it was not a factor in elevating US temperatures. Instead, over half of the anomalous warmth in 2006 is attributed to greenhouse gas forcing, whose strength now exceeds the standard deviation of natural fluctuations. We conclude that the record warmth was primarily due to human influences.