Temperature record of the past 1000 years

The last 1000 years of the northern hemisphere historical temperature record has been quantitatively reconstructed from tree ring proxy data by scientists, principally Mann, Jones and Briffa; and used by the IPCC. More recently, the record has been extended to the last 2000 years (Mann and Jones, GRL, 2003 [1]).

Table of contents
1 Quantitative reconstructions
2 Qualitative reconstruction
3 Skepticism and rebuttals thereof
4 Links

Quantitative reconstructions

The graphs of these reconstructions show a separation into two trends. From 1000 A.D. to 1880 the temperature graphs show a slow, irregular steady decline. From 1880 to present temperatures increase about 0.6 °C.

This temperature record has an unofficial name, the "Hockey Stick graph, first coined by Jerry Malhman, a colleague of Mann's.

The work of Mann et al. and others [1] forms a major part of the IPCC's conclusion that "the rate and magnitude of global or hemispheric surface 20th century warming is likely to have been the largest of the millennium, with the 1990s and 1998 likely to have been the warmest decade and year" [1].

The reconstructions mentioned above are quantitative: numerical temperature time series, either from observations or a variety of proxies, are merged and averaged to produce an average for the northern hemisphere. In the process, it is possible to produce error estimates that generally get larger further back in time.

Qualitative reconstruction

It is also possible to use historical data - times of grape harvests; seaice-free periods in harbours; diary entires of frost or heatwaves - to produce indications of when it was warm or cold in particular regions. These records are harder to calibrate, are often only available sparesely through time, and may only be available from "civilised" regions, and are unlikely to come with good error estimates.

These historical observations of the same time period show periods of both warming and cooling. Scientists such as astrophysicist Sallie Balunias note that these ups and downs correlate with solar activity and assert that the number of observed sunspots give us a rough measure of how bright the sun is.

Balunias and others believe that periods of decreased solar radiation are responsible for historically recorded periods of cooling such as the Maunder Minimum and the Little Ice Age. Similary, they say, periods of increase solar radiation contributed to the Medieval Warm Period, when the Greenland's icy coastal areas thawed enough to permit farming and colonization.

The apparent differences between the quantitative and qualitative approaches are not fully reconciled. One possibility is that the fluctuations recorded in the historical records are regional rather than hemispheric in scale.

Skepticism and rebuttals thereof

Skeptics complain that the IPCC had previously accepted a temperature record which showed large natural variations such as the medieval climate optimum and the Little Ice Age, but unaccountably selected a different set of data that fit its preordained conclusions. Such skeptics have presumably failed to notice that the graph used in the earliest (1990) IPCC report was a schematic (non-quantitative; as discussed above): the 1990 report further noted that it was not clear "whether all the fluctuations indicated were truly global". The graph disappeared from the 1992 supplementary report, and was replaced in the 1995 report by a northern hemisphere summer temperature reconstruction from 1400 to 1979 by Bradley and Jones (1993); this in turn was updated in the 2001 report to northern hemisphere warm-season and annual reconstructions from 1000 AD to present by Mann et al (1999), Jones et al (1999) and Briffa (2000) [1]. All the quantitative reconstructions (as opposed to schematic) show the same pattern of slow cooling followed by more rapid warming. For skeptics, however, the "Hockey Stick" belies environmentalist claims of the objectivity of the UN's climate agency. See John Daly's article here. Note that that article gets its history wrong and incorrectly states the graph displayed therein is from the 1995 IPCC SAR: it is not: it is from the 1990 report (and reproduced, as far as can be seen, with no respect for copyright).


Ref: http://www.evsc.virginia.edu/faculty/people/mann.shtml

http://www.ngdc.noaa.gov/paleo/recons.html - A collection of various reconstructions of global and local temperature from centuries on up

http://www.ngdc.noaa.gov/paleo/data.html - A collection of individual data records

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