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Temperature Record Of The Past 1000 YearsThe 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 variablility 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 can generally be divided into two classes: - 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.
An important aspect to the difference between the two classes of reconstructions is whether the warmth during the past century is within the bounds of previous natural variability or not. Instrumental temperature record The recent instrumental temperature record extends to approximately 1850. These records of thermometer readings show a general warming in global temperatures. For general information about temperature records see the main article: Temperature record General techniques Reconstructions of temperature rely on so-caled 'proxy' records. For example, the width of tree rings is related to temperature as is the thickness of annual snow falls which are preserved in ice-cores. 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 temerature 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. http://www.grida.no/climate/ipcc_tar/wg1/fig2-19.htm http://www.cru.uea.ac.uk/cru/info/milltemp/.) More recently, they have extended their reconstructions to cover the last 2000 years (Mann and Jones, GRL, 2003 http://www.ngdc.noaa.gov/paleo/pubs/mann2003b/mann2003b.html). The Mann, Bradley and Hughes (1998) version of the 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., Jones et al., Briffa and others http://www.grida.no/climate/ipcc_tar/wg1/fig2-20.htm 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" http://www.grida.no/climate/ipcc_tar/wg1/071.htm. For a comparison of the common temperature plots, see http://www.ngdc.noaa.gov/paleo/globalwarming/images/revgraph3.gif. Reconstructions with greater variability 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. Anders Moberg and his Swedish and Russian collaborators who published their results in Nature on February 10, 2005 http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v433/n7026/abs/nature03265_fs.html http://www.realclimate.org/index.php?p=122 have also generated reconstructions with significantly more variablity than the reconstructions of Mann et al. Scientists such as astrophysicist Sallie Balunias 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. Regional reconstructions from various historical records have also tended to suggest there was significant temperature variability in the past. (Links?) 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. Criticism 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 http://www.agu.org/pubs/crossref/2005/2004GL021750.shtml by Steven McIntyre and Ross McKitrick. In that paper McIntyre and McKitrick attack the recent warming reflected in an iconic paleoclimate record as an artifact of a programming error. 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 http://www.uoguelph.ca/~rmckitri/research/trc.html 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 http://www.grida.no/climate/ipcc_tar/wg1/fig2-21.htm. McKitrick has been accused of making errors in his own analyses 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 http://www.cru.uea.ac.uk/~timo/paleo/ 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...". 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 http://www.agu.org/pubs/crossref/2005/2004GL021750.shtml on February 12th, 2005, claiming that the "Hockey Stick" shape was a result of a programming error, 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 the blog RealClimate. There is an ongoing debate about the details of the temperature record and the means of its reconstruction. Updates There are ongoing updates and future events related to the MBH work. External links
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