Observing changes in the Climate

Observed Changes in the Climate System

Observations of the climate system are based on direct measurements and remote sensing from satellites and other platforms. Global-scale observations from the instrumental era began in the mid-19th century for temperature and other variables, with more comprehensive and diverse sets of observations available for the period 1950 onwards. Paleoclimate reconstructions extend some records back hundreds to millions of years. Together, they provide a comprehensive view of the variability and long-term changes in the atmosphere, the ocean, the cryosphere, and the land surface.

Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, sea level has risen, and the concentrations of greenhouse gases have increased

Each of the last three decades has been successively warmer at the Earth’s surface than any preceding decade since 1850 . In the Northern Hemisphere, 1983–2012 
was likely the warmest 30-year period of the last 1400 years (medium confidence). {2.4, 5.3}
B.1 Atmosphere
• The globally averaged combined land and ocean surface temperature data as calculated by a linear trend, show a warming of 0.85 [0.65 to 1.06] °C3 , over the period 1880 to 2012, when multiple independently produced data sets exist. 
The total increase between the average of the 1850–1900 period and the 2003–2012 period is 0.78 [0.72 to 0.85] °C, based on the single longest data set available.

• For the longest period when calculation of regional trends is sufficiently complete (1901 to 2012), almost the entire globe  has experienced surface warming
• In addition to robust multi-decadal warming, global mean surface temperature exhibits substantial decadal and  inter annual variability .  Due to 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. As one example, the rate of warming over the past 15 years (1998–2012; 0.05 [–0.05 to 0.15] °C per decade), which begins with a strong El Niño, is smaller  than the rate calculated since 1951 (1951–2012; 0.12 [0.08 to 0.14] °C per decade)


• Continental-scale surface temperature reconstructions show, with high confidence, multi-decadal periods during  the Medieval Climate Anomaly (year 950 to 1250) that were in some regions as warm as in the late 20th century. 
These regional warm periods did not occur as coherently across regions as the warming in the late 20th century (high confidence). 
• It is virtually certain that globally the troposphere has warmed since the mid-20th century. More complete observations  allow greater confidence in estimates of tropospheric temperature changes in the extratropical Northern Hemisphere  than elsewhere. There is medium confidence in the rate of warming and its vertical structure in the Northern Hemisphere  extra-tropical troposphere and low confidence elsewhere. 

• Confidence in precipitation change averaged over global land areas since 1901 is low prior to 1951 and medium afterwards. Averaged over the mid-latitude land areas of the Northern Hemisphere, precipitation has increased since 1901 (medium confidence before and high confidence after 1951). For other latitudes area-averaged long-term positive or negative trends have low confidence.

• Changes in many extreme weather and climate events have been observed since about 1950 (see Table SPM.1 for details). It is very likely that the number of cold days and nights has decreased and the number of warm days and nights  has increased on the global scale 6

 * It is likely that the frequency of heat waves has increased in large parts of Europe, 
Asia and Australia. There are likely more land regions where the number of heavy precipitation events has increased than  where it has decreased. The frequency or intensity of heavy precipitation events has likely increased in North America and Europe. In other continents, confidence in changes in heavy precipitation events is at most medium.