This annual summary of the climate, by NOAA's National Climatic Data Center, provides an overview of 2011 climate conditions across the United States and around the planet. As the scientific community works to better understand our changing climate, continual monitoring provides vital information about climate variability and trends. It also helps assess the incidence, impact and behavior of short-term and weather-scale events, such as droughts, tornado activity and tropical storms. Our primary focus is to give the recent climate record a historical perspective based on 132 years of atmospheric measurements as well as paleoclimate records that extend understanding of the Earth's climate to the more distant past. A discussion of century-scale variability and trends in temperature and precipitation is included, as is discussion of 2011 extreme events.