Observed temperature changes
Main article: Instrumental temperature record
The increase in ocean heat contentis much larger than any other store ofenergy in the Earth’s heat balance over the two periods 1961 to 2003 and 1993 to 2003, and accounts for more than 90% of the possible increase in heat content of the Earth system during these periods.[26]
Two millennia of mean surface temperatures according to different reconstructions from climate proxies, each smoothed on a decadal scale, with the instrumental temperature recordoverlaid in black.
The Earth's average surface temperature rose by
0.74
±
0.18 °C over the period 1906–2005. The rate of warming over the last half of that period was almost double that for the period as a whole (
0.13
±
0.03 °Cper decade, versus
0.07
±
0.02 °C per decade). Theurban heat island effect is very small, estimated to account for less than
0.002 °C
of warming per decade since 1900.[27] Temperatures in the lowertroposphere have increased between 0.13 and
0.22 °C
(0.22 and
0.4 °F
) per decade since 1979, according tosatellite temperature measurements. Climate proxiesshow the temperature to have been relatively stable over the one or two thousand years before 1850, with regionally varying fluctuations such as the Medieval Warm Period and the Little Ice Age.[28]
The warming that is evident in the instrumental temperature record is consistent with a wide range of observations, as documented by many independent scientific groups.[29] Examples include sea level rise(water expands as it warms),[30] widespread melting of snow and ice,[31] increased heat content of theoceans,[29] increased humidity,[29] and the earliertiming of spring events,[32] e.g., the flowering ofplants.[33] The probability that these changes could have occurred by chance is virtually zero.[29]
Recent estimates by NASA's Goddard Institute for Space Studies (GISS) and the National Climatic Data Center show that 2005 and 2010 tied for the planet's warmest year since reliable, widespread instrumental measurements became available in the late 19th century, exceeding 1998 by a few hundredths of a degree.[34][35][36] Estimates by the Climatic Research Unit (CRU) show 2005 as the second warmest year, behind 1998 with 2003 and 2010 tied for third warmest year, however, "the error estimate for individual years ... is at least ten times larger than the differences between these three years."[37] The World Meteorological Organization (WMO) statement on the status of the global climate in 2010 explains that, "The 2010 nominal value of
+0.53 °C
ranks just ahead of those of 2005 (
+0.52 °C
) and 1998 (
+0.51 °C
), although the differences between the three years are not statistically significant..."[38]
NOAA graph of Global Annual Temperature Anomalies 1950–2012, showing the El Niño-Southern Oscillation
Temperatures in 1998 were unusually warm because global temperatures are affected by the El Niño-Southern Oscillation (ENSO), and the strongest El Niño in the past century occurred during that year.[39] Global temperature is subject to short-term fluctuations that overlay long term trends and can temporarily mask them. The relative stability in temperature from 2002 to 2009 is consistent with such an episode.[40][41] 2010 was also an El Niño year. On the low swing of the oscillation, 2011 as an La Niña year was cooler but it was still the 11th warmest year since records began in 1880. Of the 13 warmest years since 1880, 11 were the years from 2001 to 2011. Over the more recent record, 2011 was the warmest La Niña year in the period from 1950 to 2011, and was close to 1997 which was not at the lowest point of the cycle.[42]
