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The assessment is based on satellite measurements of the average global temperature of the lower atmosphere made since 16 November 1978 and analysed by a team from UAH. This compares with a UK Meteorological Office estimate announced in November 2010 of an increase of 0.16C per decade between 1970 and 2000 and between 0.05C and 0.13C in the last decade (see below).
Christy and Spencer report that the globe continues to warm unevenly, with warming increasing as you go north. The Northern Hemisphere has been warming at 0.21C/decade while the Southern Hemisphere has been warming by 0.08C/decade. The Arctic Ocean has warmed an average of 1.66 C in the past 32 years and, by comparison, the Antarctic continent has cooled about 0.29 C during the same time, Christy and Spencer report. The continental, contiguous U.S. has warmed by about 0.67C since 1979.
Christy and Spencer's Global Temperature Report for December 2010 published today (6 January) also shows that 2010 finished in a photo finish with 1998 for the warmest year in the 32-year satellite temperature record. 2010 was only 0.013 C cooler than 1998, an amount that is not statistically significant according to Christy in the report and “is nowhere near statistically significant” according to Spencer on his blog earlier this week.
Christy points out that both 1998 and 2010 were years in which an El Nino Pacific Ocean warming event raised temperatures around the globe. In recent months a La Nina Pacific Ocean cooling event has been building; temperatures in the tropics were cooler than seasonal norms for both November and December.
With respect to the preliminary analysis of December temperatures the report states:
Global composite temperature was +0.18 C above 30-year average for December.
Northern Hemisphere was +0.21 C above 30-year average for December.
Southern Hemisphere was +0.15 C above 30-year average for December.
And the Tropics were -0.22 C below 30-year average for December.
The commentary on the revised November temperatures is as follows:
Global Composite temperature was +0.27 C above 30-year average
Northern Hemisphere was +0.37 C above 30-year average
Southern Hemisphere was +0.17 C above 30-year average
And the Tropics were -0.12 C below 30-year average
(All temperature anomalies are based on a 30-year average (1981-2010) for the month reported.)
Global land and ocean: +0.14C/decade
Global land: +0.18C/decade
Global ocean: +0.12C/decade
Northern Hemisphere land and ocean: +0.21C/decade
Southern Hemisphere land and ocean: +0.08C/decade
In a technical note Christy reports that beginning with the December 2010 Global Temperature Report, the baseline period used to determine seasonal norms changes. It has been the 20-year (1979 to 1998) period at the beginning of the satellite record. Starting this month the report will use a new 30-year (1981 to 2010) reference average to match the climatological period normally used with climate data by the U.N.'s World Meteorological Organization.
"This will not affect the long term trend, which is the most important of the numbers we produce, but will 'reshuffle' anomalies to reflect the new base period," said Christy.
Christy is professor of atmospheric science and director of the Earth System Science Center at UAH and Spencer is a principal research scientist in the UAH Earth System Science Center.
On 25 November 2010 the UK Meteorological Office in a pre-Cancun press conference stated that it believed the increase in average global surface temperatures has been around 0.16C per decade between 1970 and 2000 but has ranged between 0.05C and 0.13C in the last ten years. The Met Office said that the trend figures for the period between 2000-2009 were based on data from three different source; one from NASA (0.13C), one from the US National Oceanic and Atmospheric Administration (0.07C) and one from HadCRUT3 (0.05C) - the dataset managed by the Met Office and by the University of East Anglia's Climate Research Unit. See "Met Office to revise global warming data upwards" here.
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