The growing concentrations of human-generated GHGs have resulted in an increased absorption, largely in the lower atmosphere, of the heat radiated from Earth’s surface, causing an increase in the global (land and ocean) mean surface temperature of 0.85 ± 0.20 °C from 1880 to 2012 (Stocker et al. 2013a)—a long-term average increase of 0.06–0.07 °C per decade. However, this rise did not occur evenly across the century—for example, average global temperatures did not increase between 1880 and 1910, or between 1940 and 1970.
From 1998 to 2012, global temperatures increased by 0.05 °C per decade, which is similar to the long-term trend, but less than the rate of increase from 1951 to 2012 (0.12 °C per decade). Because of natural variability, trends based on short records are very sensitive to the beginning and end dates, and do not, in general, reflect long-term climate trends (Stocker et al. 2013a). The year 1998 was a strong El Niño year with, at that time, record high global average temperatures (0.1–0.2 °C above 1990–97 temperatures). Thus, decadal or longer temperature trends commencing in 1998 are seemingly suppressed.
The so-called hiatus in global mean surface warming since 1998 was discussed extensively in AR5. Hiatus periods of 10–15 years can arise as a manifestation of internal variability. The relative heat accumulations by the atmosphere and the oceans across short timeframes are likely to be strongly influenced by internal variability of the atmosphere–ocean system. During 1998–2012, the overall climate system, including the ocean below a depth of 700 metres, has continued to accumulate heat and the sea level has continued to rise, consistent with the observed ocean heating and glacial melting.
Each of 2005, 2010, 2013, 2014 and 2015 (a strong El Niño year) set record high annual average global temperatures, showing that the long-term global temperature increase is continuing. The decade 2000 to 2010 has been the warmest decade in the instrumental record.