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10 June 2010

Climate Change: Data, facts, background

Research Climate Change

2009 was the fifth-warmest year since 1850, despite the notable absence of record temperatures since 1998. Have the critics been right all along – is climate change a thing of the past?

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In many parts of the world, 2009 tem peratures were signifi cantly above the 1971–2000 average (red dots). Lower temperatures (blue dots) were recorded in only a few regions, primarily in southern latitudes. The larger the dot, the greater the deviation from the mean temperature.
Source: National Climatic Data Center/NESDIS/NOAA

Global mean annual temperatures
In 2009, global surface temperatures, according to provisional World Meteorological Organization (WMO) figures, were 0.44°C above the 1961–1990 average of 14°C. If confirmed, 2009 will exceed the previous three years and go down as the fith-warmest year of the data series that began in 1850. In any case, 2000–2009 is the warmest decade since 1850.

Although 1998 – the warmest year on record – was followed by comparatively cooler years with a gradual reduction in mean annual temperatures following a relative maximum in 2005, this in no way proves that climate change has come to an end, as claimed by the climate-science contrarians in the run-up to the climate summit in Copenhagen. It is rather due to the fact that any long-term upward trend includes a number of phases when global mean annual temperatures stagnate or even fall.

Such linear trends-within-trends occurred, for instance, in the years subsequent to 1944 and in the periods 1981–1986 and 1997–2000. Thus, in 1997/98, an El Niño event prevented cold deep water from rising to the surface of the tropical East Pacifi c, so that 1998 was an exceptionally warm year. In 2007/2008, a large expanse of cold sea-surface water in the Pacific caused by a La Niña event brought comparatively low temperatures. Despite these natural fluctuations in the time series, the warming trend will continue in the medium term, there having been no fundamental change in the physical causes such as increasing greenhouse gas concentrations.

The year began with a waning La Niña regime in the Equatorial Pacific giving way to an El Niño regime from mid-2009 onwards. This accounted for a rise in global mean temperatures relative to 2007 and 2008. Central Africa, much of South Asia and China, Australia, the southern part of North America and northern high latitudes in particular experienced exceptionally warm temperatures in 2009.

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