Posted by: Tony Carson | 3 December, 2009

How are global-average tempertures made?

Tracking global temperatures is essential to understanding how the global surface temperature changes — from month to month, from to decade to decade, explains the UK Met Office.

From on-going temperature records we can see how warm specific months, years or decades are, and we can discern trends in our climate over longer periods of time. Global records go back about 160 years, giving a long period from which to draw conclusions about how our climate is changing.

There are three centres which calculate global-average temperature each month.

• Met Office, in collaboration with the Climatic Research Unit (CRU) at the University of East Anglia (UK)

• Goddard Institute for Space Studies (GISS), which is part of NASA (USA)

• National Climatic Data Center (NCDC), which is part of the National Oceanic and Atmospheric Administration (NOAA) (USA)

These work independently and use different methods in the way they collect and process data to calculate the global-average temperature. Despite this, the results of each are similar from month to month and year to year, and there is definite agreement on temperature trends from decade to decade (Figure 1). Most importantly, they all agree global-average temperature has increased over the past century and this warming has been particularly rapid since the 1970s.


A clear correlation can be seen between the three global-average temperature records, which were created independently. They all show a marked warming trend, particularly over the past three decades. Data provided courtesy of NCDC/NESDIS/NOAA and NASA GISS.

How are they made?

Tens of thousands of temperature observations are taken across the globe, on land and at sea, each day. Land stations use these daily readings to create a monthly average (see boxed text), which is then sent off for use by climate researchers. Individual ship and buoy observations are transmitted on the Global Telecommunication System. These figures are checked before they are used to calculate the global-average temperature.

The monthly updates are combined with archives of historical observations that have been gathered over the past 160 years. The historical data are adjusted to minimise the effects of changes in the way measurements were made.

Each of the three global temperature centres uses different observation sets, although there are large overlaps in the information used. Each centre also has its own methods for checking and processing data, as well as for making the final calculation.

The HadCRUT3 record, which is produced by the Met Office in collaboration with the Climatic Research Unit, takes in observations from about 2,000 land stations each month. The figures for each one are checked both by computer and manually to find and remove any problems. Sea-surface temperature observations come from about 1,200 drifting buoys deployed across the world’s oceans and around 4,000 ships in the Voluntary Observing Ship programme. There are also numerous moored buoys in the tropics and in coastal regions, principally around the US. Together they take around 1.5 million observations each month. These are checked by computer and any obviously inaccurate readings are excluded.

For all three global average temperature records, the 2009 value is based on data from January to October only.

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