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The last 1000 years of the Northern Hemisphere historical temperature record has variously been described as:
- qualitatively, going up and down significantly during the Medieval Warm Period and the 19th-century Little Ice Age; and,
- quantitatively, slightly and irregularly descending until rising sharply from just before the 20th century.
Several records agreeing with the latter view has been quantitatively reconstructed from tree ring and other proxy data by scientists, principally Mann, Jones and Briffa, Pollack et al. [1] (http://www.grida.no/climate/ipcc_tar/wg1/fig2-19.htm) [2] (http://www.cru.uea.ac.uk/cru/info/milltemp/) and used by the Intergovernmental Panel on Climate Change (IPCC). More recently, the record has been extended to the last 2000 years (Mann and Jones, GRL, 2003 [3] (http://www.ngdc.noaa.gov/paleo/pubs/mann2003b/mann2003b.html)).
In the September 30th, 2004 issue of Science, Hans von Storch and his colleagues claimed that the particular method of Mann et al. probably underestimates the temperature fluctuations in the past by a factor of two or more.
Statistical reconstructions
The graphs of these reconstructions show a separation into two trends. From 1000 C.E. 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.
 One view of Northern Hemisphere temperature for the last 1000 years. 50 year averaged data are shown for 1000-1950, in blue, 10 year averaged data for 1950-2000 are shown, in red The work of Mann et al., Jones et al., Briffa and others [4] (http://www.grida.no/climate/ipcc_tar/wg1/fig2-20.htm) [5] (http://www.grida.no/climate/ipcc_tar/wg1/fig2-21.htm) 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" [6] (http://www.grida.no/climate/ipcc_tar/wg1/071.htm). However, it forms a small part of the conclusion that recent climate change can be attributed to human activity. For a comparison of the common temperature plots, see [7] (http://www.ngdc.noaa.gov/paleo/globalwarming/images/revgraph3.gif).
The reconstructions mentioned above are quantitative and tend to have many data points for each source: 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.
Historical description reconstruction
It is also possible to use historical data - times of grape harvests; ice-free periods in harbours; diary entries of frost or heat waves - to produce indications of when it was warm or cold in particular regions. These records are harder to calibrate, are often only available sparsely through time, may only be available from regions with written records, and are unlikely to come with good error estimates. Calibration of individual items is often needed, such as temperature tolerance of plants being compared to records of types of plants which were grown, or comparing recent temperature and freezing records of a specific canal to historical descriptions and paintings of the canal.
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. Similarly, 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.
Reconciliation of the two approaches
The apparent differences between the statistical and historical approaches are not fully reconciled. One possibility is that the fluctuations recorded in the historical records are regional rather than hemispheric in scale.
The difference between the interpretations of the historical record affects how the most recent warming trend is viewed: the quantitative records show the recent warming trend, and the current warmth, as unusual; from the qualitative record, many "skeptics" believe that the recent trend is not unusual, and reject calls for actions such as the Kyoto Protocol (see global warming controversy, historical temperature record).
Criticism
Historical temperature estimates
See MWP and LIA in IPCC reports.
Mann, Bradley and Hughes temperature reconstructions
Quantitative hemispheric temperature reconstructions (http://www.ngdc.noaa.gov/paleo/recons.html#airtemp) show the same pattern of slow cooling followed by more rapid warming. Skeptics, however, dislike theThe "Hockey Stick": for example John Lawrence Daly in The ‘Hockey Stick’: A New Low in Climate Science (http://www.greeningearthsociety.org/Articles/2000/hockey.htm). Note that that article incorrectly states the graph displayed therein is from the 1995 IPCC SAR when it is from the 1990 report.
Stephen McIntyre and Ross McKitrick attempted an "audit" of MBH98 [8] (http://www.uoguelph.ca/~rmckitri/research/trc.html); obtained what was available from Mann et. al. and published the results of examining the data in Corrections to the Mann et. al. (1998) Proxy Data Base and Northern Hemispheric Average Temperature Series (http://www.multi-science.co.uk/ee_openaccess.htm). This publication claimed there were collation errors; unjustifiable truncation or extrapolation errors; obsolete data; geographical location errors; incorrect calculations; other quality control defects. The result is a claim that the 'hockey stick' shape is an artefact of the methods used. M&M offer no explanation as to why their analysis also differs from other reconstructions [9] (http://www.grida.no/climate/ipcc_tar/wg1/fig2-21.htm). McKitrick has been accused of making errors in his own analyses
[10] (http://cgi.cse.unsw.edu.au/~lambert/cgi-bin/blog/2004/08#mckitrick6).
In turn, Mann (supported by Tim Osborn, Keith Briffa and Phil Jones of the Climatic Research Unit) has disputed the claims made by McIntyre and McKitrick [11] (http://www.cru.uea.ac.uk/~timo/paleo/) [12] (http://holocene.evsc.virginia.edu/Mann/EandEPaperProblem.pdf), saying "...MM have made critical errors in their analysis that have the effect of grossly distorting the reconstruction of MBH98. Key indicators of the original MBH98 network appear to have been omitted for the early period 1400-1600, with major consequences for the character of the MM reconstruction of Northern Hemisphere temperatures over that interval."
In 2004 Mann, Bradley, and Hughes published a corrigendum to their Nature 392, 779-787 (1998) article, correcting a number of mistakes in the online supplementary information that accompanied their article but leaving the actual results unchanged.
External links
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![[7]](http://www.ngdc.noaa.gov/paleo/globalwarming/images/revgraph3.gif){kind=link}