The mainland United States, which was largely recovering Monday from a near-nationwide heat wave, has experienced the warmest 12 months since record-keeping began in 1895, a top government science and weather agency announced Monday. The report from the National Climatic Data Center, which is part of the National Oceanic and Atmospheric Administration, does not take into account blistering heat from this month, with 2,116 high temperature marks either broken or tied between July 2 and 8 in communities nationwide. But it does incorporate the warmest March ever recorded as well as extreme heat in June, which also helped make the first six months of 2012 the warmest recorded of any January-June stretch.