Temperature record of the past 1000 years
The temperature record of the past 1000 years describes the reconstruction of temperature for the last 1000 years on the Northern Hemisphere. A reconstruction is needed because a reliable surface temperature record exists only since about 1850. Studying past climate is of interest for scientists in order to improve the understanding of current climate variability and, relatedly, providing a better basis for future climate projections. In particular, if the nature and magnitude of natural climate variability can be established, scientists will be better positioned to identify and quantify human generated climate variability (commonly referred to as 'anthropogenic global warming' (AGW)).
The reconstructions of temperature of the last 1000 years vary between:
- ones with significant variability prior to the current century with particular warmth during the Medieval Warm Period and particular coolness during the 19th-century Little Ice Age; and,
- ones with minimal variability prior to the current century, generally involving a slight cooling until the 20th century.
In all cases, the increase in temperature in the 20th century is the largest of any century during the record.
- 1 Instrumental temperature record
- 2 General techniques
- 3 Reconstructions with minimal variability
- 4 Reconstructions with greater variability
- 5 Reconciliation of the two approaches
- 6 Criticism
- 7 External links
Instrumental temperature record
For general information about temperature records see the main article: Temperature record
Reconstructions of temperature rely on 'proxy' records. For example, the width of tree rings is related to temperature as is the amount of snowfall over many glacial sites. Further, the isotopic composition of snow, corals, and stalactites can also record temperature changes. Other techniques which have been used include examining the time of crop harvests, the treeline in various locations and other historical records to make inferences about the temperature.
In general, the recent history of the proxy records is calibrated against local temperature records to estimate the relationship between temperature and the proxy. The longer history of the proxy is then used to reconstruct temperature from earlier periods.
An important distinction is between so-called 'multi-proxy' reconstructions, which attempt to obtain a global temperature reconstructions by using multiple proxy records distributed over the globe and more regional reconstructions.
Reconstructions with minimal variability
Several reconstructions that suggested there was minimal variability in temperatures prior to the past century were generated by Mann and his co-authors. (See, for example, Mann, Jones and Briffa, Pollack et al.  .) More recently, they have extended their reconstructions to cover the last 2000 years (Mann and Jones, GRL, 2003 ).
The Mann, Bradley and Hughes (1998) version of the temperature record has an unofficial name, the "Hockey Stick" graph, first coined by Jerry Mahlman, a colleague of Mann's.
The work of Mann et al., Jones et al., Briffa and others   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" . For a comparison of the common temperature plots, see .
Reconstructions with greater variability
In the 22 October 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. Anders Moberg and his Swedish and Russian collaborators who published their results in Nature on February 10, 2005   have also generated reconstructions with significantly more variablity than the reconstructions of Mann et al.
Scientists such as astrophysicist Sallie Baliunas have argued that these ups and downs correlate with solar activity and 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 reconstructions mentioned above rely on various assumptions to generate their results. The most fundamental are that the proxy records vary linearly with temperature and that there are no non-temperature factors that confound the results. If these assumptions do not hold, the reconstructions would be unreliable.
The methodology of papers by Mann et al (MBH98 and MBH99) have been criticised by McIntyre and McKitrick on various grounds. In the February 11th, 2005 issue of Science, Richard A. Kerr describes Geophysical Research Letters paper that appeared on February 12th, 2005  by Steven McIntyre and Ross McKitrick.
Historical temperature estimates
For information on the description of the Medieval Warm Period and Little Ice Age in various reports of the United Nations Intergovernmental Panel on Climate Change, see the main article: MWP and LIA in IPCC reports
Mann, Bradley and Hughes temperature reconstructions
Quantitative hemispheric temperature reconstructions were showing the pattern of slow cooling followed by more rapid warming.
Stephen McIntyre and Ross McKitrick attempted an "audit" of MBH98  in Corrections to the Mann et. al. (1998) Proxy Data Base and Northern Hemispheric Average Temperature Series. This publication claimed various errors, but M&M offered no explanation as to why their analysis also differs from other reconstructions .
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  , saying "...MM have made critical errors in their analysis that have the effect of grossly distorting the reconstruction of MBH98...". 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.
M&M have published another Geophysical Research Letters article  on February 12th, 2005, claiming that the "Hockey Stick" shape was a result of a flawed principal component analysis, and that using the same steps like Mann et al., they were able to obtain the Hockey Stick graph in 99 percent of cases even if red noise was used as input. Mann and his collaborators have responded to the M&M articles via various means, including posts at the blog RealClimate. There is an ongoing debate about the details of the temperature record and the means of its reconstruction.
There are ongoing updates and future events related to the MBH work.
- RealClimate - Climate scientists blog, including Mann
- ClimateAudit - McIntyre blog
- A processing aliasing artefact in the early Quelccaya ice core record has been found. Original data was rounded to the nearest centimeter and this can cause analysis problems. A 2003 ice core will provide new data when processed. 
- Mann's home page
- A collection of various reconstructions of global and local temperature from centuries on up:
- An NOAA collection of individual data records
- Supplementary information for Mann, M. E. et al. corrigendum: Global-scale temperature patterns and climate forcing over the past six centuries Nature 430, 105(2004) Letters to Nature. http://www.nature.com/nature/journal/v430/n6995/suppinfo/nature02478.html
- Corrections to the Mann et al (1998) Proxy Data Base and Northern Hemisphere Average Temperature Series. Steven McIntyre, Ross McKitrick. Energy and Environment 14(6) 751-772. http://www.uoguelph.ca/~rmckitri/research/trc.html
- http://www.climate2003.com/ Webpage of Stephen McIntyre
- http://www.realclimate.org/index.php?p=98 - comment by Mann on M&M on "RealClimate" blog
- "A Global Warming Bombshell" by Richard A. Muller, Technology Review , Oct. 2004; calls into question famous graph by Michael Mann
- What is the ‘Hockey Stick’ Debate About? McKitrick reviews the criticisms of the Mann hockey stick for a general scientific audience.
- Rebuttal of the McIntyre and McKitrick dispute of the "Hockey Stick" extrapolation — Real Climate