Historical temperature record

The instrumental temperature record shows the fluctuations of the temperature of the atmosphere and the oceans since the invention of thermomenters. A quasi-global record exists since about 1850. See also temperature record.

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Instrumental global surface temperature measurements
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Expansion of the last 25 years of temperature variation

The Quasi-global Period: from 1850

The period for which reasonably reliable near-surface temperature records exist from actual observations from thermometers with quasi-global coverage is generally considered to start in about 1850 - earlier records exist, but coverage and instrument standardisation are less.

In the present day most meteorological observations are taken for use in weather forecasts. Centres such as ECMWF show instantaneous map of their coverage (http://www.ecmwf.int/products/forecasts/d/overview/monitoring/coverage/dcover!ssmi!00!pop!od!oper!w_coverage!latest!obs/); or the Hadley Centre show the coverage for the average of the year 2000 (http://www.meto.gov.uk/research/hadleycentre/CR_data/Annual/HadCRUTanm_2000.gif). Coverage for earlier in the 20th and 19th centuries would be significantly less. While temperature changes vary both in size and direction from one location to another, the numbers from different locations are combined to produce an estimate of a global average change.

Most of the warming occurred during two periods: 1910 to 1945 and 1976 to 2000. See [1] (http://www.grida.no/climate/ipcc_tar/wg1/figspm-1.htm) for a picture of the temperature record. Attribution of the temperature change to natural or anthropogenic factors is an important question: see global warming and attribution of recent climate change.

The data for the record come from thermometer measurements from land stations and ships [2] (http://www.met-office.gov.uk/research/hadleycentre/pubs/talks/sld011.html), which independently show much the same warming since 1860. The data from these stations show an average surface temperature increase of 0.6 +/- 0.2 °C during the 20th century. The Intergovernmental Panel on Climate Change (IPCC) concludes:

"The best estimate of global surface temperature change is a 0.6°C increase since the late 19th century with a 95% confidence interval of 0.4 to 0.8°C" [3] (http://www.grida.no/climate/ipcc_tar/wg1/049.htm).

The US National Academy of Science, both in its 2002 report to President George W. Bush, and in its latest publications, has strongly endorsed evidence of an average global temperature increase in the 20th century.

There are important concerns about possible uncertainties in the instrumental temperature record, including the fraction of the globe covered, the effects of changing thermometer designs and observing practice, and the effects of changing land-use around the observing stations.

There exists a parallel record of marine observations from surface ships. These too suffer from changing practices (such as the switch from collecting water in canvas buckets to measuring the temperature from engine intakes) but they are at least immune to the urban heat island effect. The land and marine records can be compared. [4] (http://www.grida.no/climate/ipcc_tar/wg1/fig2-6.htm)

For information about the effects or otherwise of urbanization on the temperature record, see the main article: Urban heat island effect

Secondary data

Secondary evidence for temperature changes can be obtained by observing things that are predicted to be affected by temperature changes, such as variations in the snow cover and ice extent [5] (http://www.grida.no/climate/ipcc_tar/wg1/062.htm), sea level rise, precipitation [6] (http://www.grida.no/climate/ipcc_tar/wg1/077.htm), cloud cover [7] (http://www.grida.no/climate/ipcc_tar/wg1/082.htm), El Niño and extreme weather events [8] (http://www.grida.no/climate/ipcc_tar/wg1/089.htm). For example, satellite data shows a 10% decrease of snow cover since the late 1960s [9] (http://www.grida.no/climate/ipcc_tar/wg1/061.htm#fig213), and the Northern Hemisphere spring and summer sea-ice extent has decreased by about 10% to 15% since the 1950s and there has been a widespread retreat of mountain glaciers in non-polar regions throughout the 20th century.[10] (http://www.grida.no/climate/ipcc_tar/wg1/064.htm).

Spatial variability

The global temperature changes are not uniform over the globe, nor would they be expected to be, whether the changes were naturally or human forced. Certain places, such as the north shore of Alaska, showing dramatic rises temperature far above the average for the globe as a whole [11] (http://www.nytimes.com/2003/09/13/opinion/13KRIS.html?th). The Antarctic peninsula has warmed by 2.5 °C in the past 5 decades in certain places [12] (http://www.antarctica.ac.uk/met/gjma/) meanwhile East Antarctic has not significantly warmed [13] (http://huey.colorado.edu/LTER/news/NaturePaperReport_011302.html).

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