3. The world is warming

• There are three main records of global temperature at the Earth’s surface: the UK Met Office/UEA Climatic Research Unit (CRU) record, a record produced by NASA’s Goddard Institute for Space Studies, and a record produced by the US National Oceanic and Atmospheric Administration (NOAA). The records are in close agreement, showing that global average temperature has increased by approximately 0.75°C since 1900 (Figure 7, below).

Figure 7 (above): The three main records of global average surface temperature. Red line = NOAA record, blue line = NASA record, black line = Met Office/ UEA CRU record, with grey shading showing 95% confidence interval on Met Office/UEA CRU record. Source: Met Office Hadley Centre. Before 1850, instrumental time series measurements with global coverage are not available. [IPCC AR4 (2007) (Working Group 1; 1.3.2)]. (Larger version of Figure 7 (PNG, 402 Kb) )

• The global average temperature is currently calculated from measurements taken at about 5,000 land-based weather stations and over 1,200 free-floating buoys, as well as ships and moored buoys (Figure 8, below). A number of issues need to be managed to ensure the integrity of these raw data, including accounting for a lack of complete coverage in some areas, relocation of weather stations, and changes of instruments and measurement procedures. These issues are addressed using statistical techniques and the uncertainties they introduce are factored into the "error bars" associated with the records. The size of the error bars decreases towards the present day as the number and accuracy of measurements underpinning the records increases.
  

Figure 8 (above): Maps showing (top) land stations used to construct the UEA Climatic Research Unit land surface temperature dataset, and (bottom) sea surface temperature measurements made in May 2010 by ships (blue dots) drifting buoys (red dots) and moored buoys (grey dots) (note that over the space of a year, the gaps in sea surface measurements gradually get ‘filled in’ along the shipping routes by ships and in other places by drifting buoys). Statistical methods are used to calculate the global average in a way that takes account of the uneven distribution of observations. Source: Met Office. (Larger version of Figure 8 (PPT, 498 Kb) )

• Records of global average temperature change on land, at the sea surface and over the oceans at night all show a clear warming trend, with somewhat greater warming over land, as predicted by climate models [21].
• Satellites measure temperature change in the lower atmosphere (the ‘troposphere’). At the global scale, these records are in close agreement with the surface temperature records (Figure 9) [22]. In the tropics, warming in the lower atmosphere is expected to be slightly greater than at the surface due to changes in the amount of water vapour in the atmosphere as it warms, although estimates of the precise amount of additional warming from different climate models varies. This higher rate of warming is not seen in some satellite records, however the error bars on satellite records are large (because satellites measure temperature using indirect methods and the data from them need to be corrected for factors such as instrument temperature and drift of the satellite in orbit) [22].
  

Figure 9 (above): Three satellite records of global average temperature in the lower atmosphere (black, blue and red lines) compared to the Met Office/UEA CRU record of global average temperature at the surface (green line). Source: Met Office. (Larger version of Figure 9 (PNG, 137 Kb) )

• Although other factors might influence each of them individually, trends observed in a wide range of other physical variables over the past few decades are consistent with those expected from the warming seen in temperature records, including:
  
  
  • a steady rise in global sea level (Figure 10a) [21];
  • retreat of Arctic sea ice (the late summer minimum in Arctic sea-ice extent [23] has decreased by about 10% each decade since satellite records began in the 1970s – Figure 10b, below) [24];
  • widespread ice mass losses from glaciers and ice caps [21][25];
  • earlier retreat of snow cover in spring in the Northern Hemisphere [21];
  • shifts in rainfall patterns consistent with those expected in a warming world (including increases in the Northern Hemisphere mid-latitudes and drying in the Northern Hemisphere subtropics and tropics [26]);
  • increases in atmospheric humidity in the lower atmosphere [21];
  • increases in the number of heavy rainstorms and heatwaves over many land areas [21].
    

Figure 10 (a) (above, top): Annual averages of the global mean sea level based on reconstructed sea level fields since 1870 (red), tide gauge measurements since 1950 (blue) and satellite altimetry since 1992 (black). Units are in mm relative to the average for 1961 to 1990. Error bars are 90% confidence intervals. (Source: IPCC AR4) (Larger version of Figure 10 (a))

Figure 10 (b) (above, bottom): average Arctic sea ice extent in September from 1979 to 2009 (Source: NSIDC) (Larger version of Figure 10 (b)

• Changes in natural systems consistent with a warming trend have also been observed over recent decades, including:
  
  
  • shifts in the ranges of some terrestrial plant and animal species to higher latitudes and altitudes (for example, the ranges of a number of butterfly species have shifted poleward or uphill in Europe) [27];
  • warming of lakes and rivers [27];
  • changes in the distribution of some marine species (for example a northerly shift in the distribution of plankton in the North Atlantic ocean [27]);
  • earlier arrival of spring and an increase in the length of the growing season, on average, in many regions of the Northern Hemisphere [27].
• Natural variability, resulting from internal adjustments in the climate system, solar variability and volcanic activity, causes global average temperature to fluctuate on timescales of a few years to a decade or more. Consequently it is possible to find many periods of a few years when global average temperature has levelled off or declined. In order to detect whether the climate system is changing over and above these natural factors, it is helpful to consider the trend in global temperature after it has been averaged over each decade to remove some of the short-term natural variability (Figure 11, below). It is clear that the decadal-scale trend in temperature has been upward. Even allowing for uncertainties in the observations, the last three decades have each been significantly warmer than the previous one. The size and sustained nature of the warming since the 1950s is unprecedented over the instrumental record.

Figure 11 (above): Global average surface temperature record averaged over each decade since 1850 (expressed as temperature difference from the 1961-1990 average). The uncertainty in the observed estimates is shown in the error bars. Source: Met Office (Larger version of Figure 11 (PNG, 235 Kb) )

• The rate of the warming observed has varied regionally, highlighting how important it is to avoid making deductions about global climate from what happens in one part of the world (Figure 12). For example, over the past century the rate of warming in much of the United States has been lower than in many other parts of the world and temperatures have decreased in the northern North Atlantic, while the rate of warming in the Arctic has been almost twice the global average [21]. This regional variation occurs because local factors such as retreat of snow and ice and multi-decadal changes in ocean circulation affect the distribution of warming.

Figure 12 (above): Linear trend of annual temperatures 1901-2005. Grey areas indicate areas with insufficient data to calculate a robust trend. Trends significant at the 5% level are shown by white crosses. (Source: IPCC AR4) (Larger version of Figure 12 (JPG, 252 Kb) )

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Key references 

Footnotes

1. 21. IPCC AR4, Working Group I Ch. 3,4&5 (2007)
2. 22. Karl et al. (2006)
3. 23. The extent of the decline in sea ice cover is variable through the year, reaching its minimum area in September. The decline observed in some winter months is not statistically significant against the background of natural variability.
4. 24. National Snow and Ice Data Centre (NSIDC): www.nsidc.org
5. 25. Note that changes in glacier extent are affected by changes in precipitation as well as temperature.
6. 26. Zhang et al. (2007)
7. 27. IPCC AR4, Working Group II Report, Ch 1. (2007)

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