Using measurements taken worldwide, scientists estimated that 2015’s global average carbon dioxide concentration was 399.4 parts per million (ppm), a new record high. At Mauna Loa Observatory in Hawai’i, where atmospheric carbon dioxide has been recorded longer than anywhere else in the world, the annual average carbon dioxide concentration was 400.8—also a new record, and a new milestone.
Ocean heat storage has increased substantially since 1993, hitting a record high in 2015, according to the State of the Climate in 2015 report. Ocean warming accounts for over 90% of the warming in Earth’s climate system.
The global mean sea level in 2015 was approximately 7 centimeters (2.7 inches) above the 1993 average, making it the highest observed since the satellite altimeter record began in 1993. Regional variations highlighted the short-term influence of climate phenomena like the Pacific Decadal Oscillation and the largest El Niño event since 1997/98.