Instrumental data describing large-scale surface temperature changes are only available for roughly the past 150 years. Estimates of surface temperature changes further back in time must therefore make use of the few long available instrumental records and natural archives or ‘climate proxy’ indicators, such as tree rings, corals, ice cores and lake sediments, and historical documents, to reconstruct patterns of past surface temperature change. Due to the paucity of data in the Southern Hemisphere, recent studies have emphasized the reconstruction of Northern Hemisphere (NH) mean, rather than global mean temperatures over roughly the past 1000 years.
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Isotopes can be thought of as different ‘flavours’ of a particular element (such as oxygen or carbon), that are distinguished by the number of neutrons in their nucleus (and hence their atomic mass). Carbon for instance most commonly has a mass of 12 (written as 12C), but there are also a small fraction of carbon atoms with mass 13 and 14 (13C and 14C), similarly oxygen is normally 16O, but with small amounts of 17O and 18O. All of the isotopes of an element behave in similar way chemically. However, because the mass of each isotope is slightly different there are certain physical processes that will discriminate (or ‘fractionate’) between them. For instance, during evaporation of water, it is slightly easier for the lighter isotopes to escape from the liquid, and so water vapour generally has less 18O than the liquid water from which it came. Because of these physical effects, looking at the ratio of one isotope to another can often be very useful in tracing where these atoms came from.
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 »
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 »
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.