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Little Ice Age (“LIA”)

Filed under: — group @ 28 November 2004 - (Français)

Term originally introduced in the late 1930s by Matthes (1939) to describe a broad interval of the late Holocene during which significant glacial advances were observed. In the climatological literature the LIA has now come to be used to characterize a more recent, shorter recent interval from around A.D. 1300 to 1450 until A.D. 1850 to 1900 during which regional evidence in Europe and elsewhere suggest generally cold conditions. Variations in the literature abound with regard to the precise definition, and the term is often used by paleoclimatologists and glaciologists without formal dates attached. The attribution of the term at regional scales is complicated by significant regional variations in temperature changes due to the the influence of modes of climate variability such as the North Atlantic Oscillation and the El Nino/Southern Oscillation. Indeed, the utility of the term in describing past climate changes at regional More »

Medieval Warm Period (“MWP”)

Filed under: — group @ 28 November 2004

Period of relative warmth in some regions of the Northern Hemisphere in comparison with the subsequent several centuries. Also referred to as the Medieval Warm Epoch (MWE). As with the ‘Little Ice Age’ (LIA), no well-defined precise date range exists. The dates A.D. 900–1300 cover most ranges generally used in the literature. Origin is difficult to track down, but it is believed to have been first used in the 1960s (probably by Lamb in 1965). As with the LIA, the attribution of the term at regional scales is complicated by significant regional variations in temperature changes, and the utility of the term in describing regional climate changes in past centuries has been questioned in the literature. As with the LIA, numerous myths can still be found in the literature with regard to the details of this climate period. These include the citation of the cultivation of vines in Medieval England, and the More »

Microwave Sounding Unit (“MSU”)

Filed under: — group @ 28 November 2004

A Microwave Sounding Unit (“MSU”) is a device that has been installed on polar orbiting satellites to measure, from space, the intensity of microwave radiation emitted by earth’s atmosphere. Different “channels” of the MSU measure different frequencies of radiation which can, in turn, be related to temperature averages of the atmosphere over different vertical regions. Channel 2 measurements provide a vertically-weighted temperature estimate that emphasizes the mid-troposphere (with small contributions from the stratosphere), while Channel 4 largely measures temperatures in the lower stratosphere. Information from MSUs have been used to generate the “MSU Temperature Record“. More information on MSU can be found here.

Mid-Holocene Climatic Optimum

Filed under: — group @ 28 November 2004

This is a somewhat outdated term used to refer to a sub-interval of the Holocene period from 5000-7000 years ago during which it was once thought that the earth was warmer than today. We now know that conditions at this time were probably warmer than today, but only in summer and only in the extratropics of the Northern Hemisphere. This summer warming appears to have been due to astronomical factors that favoured warmer Northern summers, but colder Northern winters and colder tropics, than today (see Hewitt and Mitchell, 1998; Ganopolski et al, 1998). The best available evidence from recent peer-reviewed studies suggests that annual, global mean warmth was probably similar to pre-20th century warmth, but less than late 20th century warmth, at this time (see Kitoh and Murakami, 2002).

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MSU Temperature Record

Filed under: — group @ 28 November 2004

Combinations of different channels of individual Microwave Sounding Unit (“MSU”) measurements have been used to generate a record of estimated atmospheric temperature change back to 1979, the “MSU Temperature Record”. The complex vertical weighting functions relating the the various channels of the MSU to atmospheric temperatures complicate the interpretation of the MSU data. Moreover, while MSU measurements are available back to 1979, a single, continuous long record does not exist. Rather, measurements from different satellites have been combined to yield a single long record, further complicating the interpretation of the MSU record. Direct comparisons of the MSU Temperature Record with the surface temperature record are therefore difficult. More information on the MSU Temperature Record can be found here.