Ron Smith continues his Yale course on the atmosphere and ocean with a lecture on the climate record of the Holocene epoch. He traces the Medieval Warm Period, the Little Ice Age, and the 1816 Year Without a Summer before turning to the twentieth century, where temperature data show strong warming from about 1970 onward. Smith walks through the forcings behind that record, including greenhouse gases, aerosols, and volcanic eruptions such as Pinatubo in 1991, explaining how eruption aerosols raise atmospheric albedo and produce short term cooling blips in the data. He then describes how general circulation models reproduce twentieth century climate using both natural and human forcings, and why those models point to anthropogenic causes for the recent temperature rise. The lecture is structured around dated chapters moving from paleoclimate context into the specific factors shaping twentieth century observations, part of Yale's Atmosphere, Ocean and Environmental Change course recorded in fall 2011.