B1: Atmosphere (Observations)

Each of the last three decades has been successively warmer at the Earth’s surface than any preceding decade since 1850 (see Figure SPM.1). In the Northern Hemisphere, 1983–2012 was likely the warmest 30-year period of the last 1400 years (medium confidence). {2.4, 5.3}

 

(a) Observed global mean combined land and ocean surface temperature anomalies, from 1850 to 2012 from three data sets. Top panel: annual mean values, bottom panel: decadal mean values including  the  estimate  of  uncertainty  for  one  dataset  (black).  Anomalies  are  relative  to  the  mean  of 1961−1990.

 

(b) Map of the observed surface temperature change from 1901 to 2012 derived from temperature trends determined by linear regression from one dataset (orange line in panel a). Trends have been calculated where data availability permits a robust estimate (i.e., only for grid boxes with greater than 70% complete records and more than 20% data availability in the first and last 10% of the time period). Other areas are white. Grid boxes where the trend is significant at the 10% level are indicated by a + sign. For  a  listing  of  the  datasets  and  further  technical  details see the  Technical  Summary  Supplementary Material. {Figures 2.19–2.21; Figure TS.2}

 

  • 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] °C 3, over the period 1880–2012, when multiple independently produced datasets 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 dataset available4. (Figure SPM.1a) {2.4}

 

  • For the longest period when calculation of regional trends is sufficiently complete (1901–2012), almost the entire globe has experienced surface warming. (Figure SPM.1b) {2.4}

 

  • In  addition  to  robust  multi-decadal  warming,  global  mean  surface  temperature  exhibits substantial decadal and interannual variability (see Figure SPM.1). 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)5.{2.4}

 

  • 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). {5.5}

 

  • 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. {2.4}

 

  • 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. {Figure SPM.2, Figure TS.XX, 2.5}

 

Maps of observed precipitation change from 1901 to 2010 and from 1951 to 2010 (trends calculated using the same criteria as in Figure SPM.1b) from one data set. For further technical details see the Technical Summary Supplementary Material

 

  • 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 scale6. 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. To see a table of these changes, follow this link.

 

 

 

 

 

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