The global average temperature calculated from the Hadley Centre and Climate Research Unit's (HadCRUT3, Brohan et al., 2006) data set for 2011 was 0.35 ± 0.09 degC above the 1961–1990 average (Figure 1(a)), making 2011 nominally the 12th warmest year in the 162-year record. Global average temperature anomalies produced by the National Aeronautics and Space Adminis-tration's Goddard Institute for Space Studies (NASA GISS, Hansen et al., 2010) and the National Oceanic and Atmospheric Administrations's National Climate Data Center (NOAA NCDC, Smith et al., 2008) were +0.45 degC (9th warmest) and +0.42 degC (11th warmest) respectively. The datasets are based on a network of around 1200 land stations that are updated monthly, together with around a million observations per month from ships and drifting buoys.
Uncertainties on global annual average temperature are reckoned to be around 0.1 degC, mostly reflecting poor coverage at high latitudes but with contributions from instrumental error. Some of the differences between the global temperature data sets, particularly the warmth of GISS relative to HadCRUT3, can be explained by the different coverage of the data sets over the Arctic. Allowing for the uncertainty, there is a very high probability that 2011 was one of the 15 warmest years on record and was most likely between the 9th and 12th warmest.
In HadCRUT3, 2011 was also nominally the 12th warmest year on record in both the northern and southern hemispheres separately (Figures 1(b) and (c)). Once again the uncertainties mean that it is not possible to assign a definitive rank, particularly in the southern hemisphere where data coverage is thinner and the warming has been slower. The tropical average temperature anomaly in 2011 (Figure 2) was similar to that of 2008 and lower than in El Niño years such as 2010 and 1998.
2011 was cooler than the average for the past decade, reflecting the transition from El Niño to La Niña during 2010. La Niña was well established at the beginning of 2011. Sea-surface temperatures in the tropical Pacific returned to near normal levels during the summer and then subsequently dropped back to La Niña thresholds in October (Figures 3 and 4). The 2010/2011 La Niña was a relatively strong event, particularly in respect of the atmospheric response to it; the late 2011 La Niña in contrast was much weaker.
The pattern of La Niña can be clearly seen in the map showing annual average temperatures for the world (Figure 5). A cold ‘tongue’, with sea-surface temperatures below the 10th percentile of occurrence (Horton et al., 2001), was observed in the tropical Pacific. Around the cold tongue, in an equally distinctive horseshoe pattern, temperatures were significantly (above the 90th percentile of occurrence) higher than average. The negative anomalies extended along the western edge of the Americas and the pattern in the north Pacific is characteristic of the current negative phase of the Pacific Decadal Oscillation (Mantua et al., 1997).
Mean annual temperatures (Figures 5 and 6) were significantly above the average over large areas of north Africa, the Mediterranean and the Indian Ocean. Areas of northeast Russia and the southern US states experienced their warmest years on record. Areas of significant cold (temperatures below the 10th percentile) included northern Australia, and parts of western Canada and the south Atlantic. North Atlantic temperatures in 2011 were above the long-term average. This reflects, in part, the continuing positive phase of the Atlantic Multi-decadal Oscillation (AMO, Knight et al., 2005), which began in the mid-1990s. The AMO influences a range of phenomena (Knight et al., 2006), including rainfall in the African Sahel and Atlantic hurricane formation. 2011 was an active hurricane season in the Atlantic (Strachan, 2012) with 19 named tropical storms (average 9.6), and 7 hurricanes (average 5.9) of which 4 were major hurricanes (average 2.3). Hurricane Irene made landfall on the eastern seaboard of the US in late August leading to heavy rain that saw some states receiving record precipitation totals for August.
The Arctic (based on an average of land temperature north of 65°N) was 2.3 ± 0.7 degC warmer than average in 2011, continuing a run of years that have seen temperatures equalling or exceeding those in the previous Arctic warm phase in the 1930s and 1940s. Figure 4 shows that large areas of the Arctic were warmer than average throughout the year, with the most widespread warmth in the summer and autumn. September is the month of the climatological minimum in sea-ice extent; in 2011, the Arctic sea-ice extent this month was the second lowest (after 2007) in the satellite record (1979-present) and probably the second lowest since the late 1950s when regular monitoring of Arctic sea ice began (Figure 7). There was rapid melting in June and early July when the atmospheric circulation was similar to that seen in the period of strong melting during the summer of 2007. Although December sea-ice extent has declined at a slower rate than that in September, sea-ice concentrations in an area to the west of Svalbard remained low during the month; the temperature station at Svalbard reported a monthly mean of –0.1°C, 15.7 degC above the 1961–1990 average.
Coinciding with significantly above-average temperatures, southern US states were drier than average with some areas receiving less than 40% of the annual average rainfall (Figure 8); Texas had its driest year on record. In contrast, it was the wettest year on record for several states in the north and east of the USA.
In La Niña years, higher sea-surface temperatures combined with enhanced tropical convection often lead to higher than average precipitation totals in Australia. Figures from the Australian Bureau of Meteorology (http://www.bom.gov.au/climate/annual_sum/2011/index.shtml) indicate that 2011 was the second wettest year on record and that 2010 and 2011 were the wettest consecutive years on record.
Figure 9 shows annual rainfall anomalies for the north African Sahel, on the southern fringes of the Sahara. Rainfall in 2011 was patchy and below the long-term average: see also Figure 8 and Cornforth (2012). 2011 was drier than 2010 but comparable with other years in the past decade when, nevertheless, rainfall totals were generally higher than during the droughts in the 1970s and 1980s. The rainfall series is not representative of mountainous, poorly-observed Ethiopia and other semi-arid areas of north Africa (Conway et al., 2004). However, the overall reliability of the long-term evolution of the series has been confirmed by Dai et al. (2004).
Global climate: seasonal
Seasonal average temperature and precipitation anomalies are shown in Figures 4 and 10. During the northern hemisphere winter of 2010/2011, temperatures over central and north Africa were significantly above average, and warmer-than-average conditions extended through the Middle East and across southern Asia. In contrast, the north of Eurasia was colder than average. The west of South America was predominantly colder than average and the east warmer than average. In Australia, temperatures were below the 10th percentile in the north of the country and much of the country was wetter than average.
In the northern spring, large areas of Eurasia were significantly warmer than average; only in southeast Asia were there areas of significant cold. The spring was unusually dry across much of Europe, with only the more southerly and northerly areas being wetter than average. Areas of Africa from which we have observations were warmer than average. Australia was significantly cold. In North America there was a distinct north-south split, with the north being significantly cold and wet and the south significantly warm and dry.
The northern summer again saw significant warmth across much of Eurasia, the Middle East and north Africa. North America was significantly warmer than average in the south and east, but there were areas of below average temperature in the west. South America was warmer than average in the north and cooler than average in the south. The north of Australia continued to be colder than average, but significant positive anomalies were recorded in the south. Above-average summer and autumn rainfall in parts of Indochina led to severe flooding in Thailand.
The northern autumn brought widespread significant warmth over land areas, only parts of the Middle East, northern Australia, the southeastern US and Alaska being cooler than average. Over Europe, the pattern of temperature and rainfall during the autumn was similar to that of the spring. Drier-than-average conditions extended eastwards from the southern UK, with wetter-than-average conditions in the far north and south. Temperatures were generally above average.
Temperatures in the troposphere and stratosphere are measured by weather balloons (Thorne et al., 2005) and can also be estimated from microwave emissions measured by satellites in polar orbits (Christy et al., 2003; Mears and Wentz, 2009a; 2009b). Figure 11 shows global average temperature changes in thick layers of the troposphere and stratosphere to which the microwave instruments are sensitive; the weather-balloon data have been processed to provide a comparable measure of temperature change.
Recent temperature changes in the troposphere have mirrored those at the surface with La Niña-influenced 2011 being much cooler than 2010 (Figure 11). The amplitude of variations in the troposphere is generally larger, at shorter timescales, than at the surface. This effect is expected to be most pronounced in the tropics and it is also expected to hold for longer periods, with trends in the tropical troposphere exceeding those at the surface. That the tropospheric amplification has not been clearly observed is a persistent conundrum and suggests that either the theories are incorrect or that observational uncertainties are too large to resolve it (Thorne et al., 2011). In the stratosphere, a general downward trend in temperature associated with reduced stratospheric ozone concentrations and increased greenhouse gas concentrations has been punctuated by abrupt peaks associated with the three major volcanic eruptions marked on the diagram. Recently, temperatures in the stratosphere have been relatively steady with competing effects from increases in stratospheric ozone and changes in greenhouse gas concentrations.
Climate of the UK and Europe
Figure 12 shows annual average temperatures and rainfall for 2011 for the north Atlantic and Europe and Figure 13 shows annual average temperatures for the Central England Temperature (CET) series (Manley, 1974; Parker et al., 1992) and for Europe, as well as the daily CET, for 2011. Table 1 summarizes UK temperatures and the CET, as well as England and Wales rainfall (Alexander and Jones, 2000).
UK and European temperatures were above the long-term average in 2011. It was the second warmest year on record in a UK-wide series beginning in 1910, as well as in the much longer CET series; only 2006 was warmer. 2011 was also a very warm year over Europe, one of the five warmest in the CRUTEM3 record (Brohan et al., 2006).
The winter of 2010/2011 had started much colder than average in the UK and much of Europe, with the second coldest December in the UK in perhaps over 300 years; it was the second coldest in the CET series, which begins in 1659 (Prior and Kendon, 2011), and the coldest in the 102-year UK series (Perry and Hollis, 2005a, b). However, UK temperatures in January 2011 were near average and February was much warmer than average, with an anomaly of 2.4 degC (Table 1). In contrast to December 2010, December 2011 was mild with temperatures remaining mostly above average during the month.
Spring and autumn 2011 were both exceptionally warm over western and northern Europe; in both seasons there was an area of below-average temperatures further to the east. The UK mean temperature for spring was 9.1°C, 2.1 degC above average. It was the warmest spring across the UK in a series from 1910, though only slightly warmer than in 2007. April 2011 was also the warmest April in the UK series from 1910. In the CET series, the mean temperature of 10.2°C makes spring 2011 the equal-warmest (as warm as 1893 and warmer than 2007, when the mean was 10.1°C). The April CET at 11.8°C was the highest in the 353-year series, exceeding the previous record April CET of 2007 by 0.6 degC and the now third-warmest April of 1865 by more than 1 degC. The UK mean temperature for autumn was 11.2°C, 2.2 degC above average. It was the second warmest autumn in over 100 years, with only that of 2006 warmer (11.4°C). The number of air frosts was the lowest in at least the last 50 years. In central England, it was the second warmest autumn in over 350 years.
Spring and autumn 2011 were also both unusually dry across much of Europe; only the far north and south were wetter than average. Some western areas – particularly the southern UK and France – received less than 40% of the seasonal average rainfall in spring. In the autumn the driest areas (relative to climatology) were further to the east.
Summer 2011 was wetter than the long-term average over much of northern Europe. In the UK it was, overall, wetter than summer 2010 but drier than the summers of 2007, 2008 and 2009; these three were all among the ten wettest UK summers on record. Eastern Europe had a hot summer (Figure 7) but the UK had few warm spells, though a brief heatwave on 26–27 June brought the highest UK temperatures since 2006. Around a quarter of UK stations recorded their highest temperature of the year during a heatwave in late September and early October (Kendon, 2012).
More detail on the climate in 2011 can be found in the World Meteorological Organ-ization's (WMO) statement on the status of global climate in 2011 on http://www.wmo.int and in the State of the Climate report published in the July 2012 issue of the Bulletin of the American Meteorological Society and also online at http://www.ncdc.noaa.gov/bams-state-of-the-climate/. A more complete summary of European climate will be published by WMO in its annual bulletin on the climate in WMO region VI – Europe and the Middle East, 2011. Selected global and UK datasets can be accessed from http://www.metoffice.gov.uk/hadobs
The authors were supported by the Joint DECC/Defra Met Office Hadley Centre Climate Programme (GA01101). John Prior and Mike Kendon of the Met Office National Climate Information Centre assisted with the UK information. They would like to thank Phil Jones at the University of East Anglia who contributed to the land-surface temperature analysis and Andrew Colman who prepared the Sahel rainfall data.