| Temperature Record Of The Past 1000 Years |
Article Index for Temperature |
Shopping 1000 |
Website Links For Temperature |
Information AboutTemperature Record Of The Past 1000 Years |
| CATEGORIES ABOUT TEMPERATURE RECORD OF THE PAST 1000 YEARS | |
| climate change | |
| history of climate | |
|
The reconstructions of temperature of the last 1000 years vary between:
temperatures for the last 1000 years according to various older articles (bluish lines), newer articles (reddish lines), and instrumental record (black line).]] In all cases, the increase in temperature in the 20th century is the largest of any century during the record. INSTRUMENTAL TEMPERATURE RECORD of the last 150 years]] The recent Instrumental Temperature Record dates from 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 ' 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. These records must then be averaged in some fashion if a "global" or "hemispheric" record is desired. Since certain regions contain, for example, a large number of tree ring records, a simple average of all the data would strongly over-weight some small regions - hence data-reduction techniques (PCA) are used to combine some of these regional records before they are globally combined. 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. Usually, the data are combined arithmetically, in some weighted average. More recently, Osborn and Briffa used a simpler technique, counting the proportion of records that are positive, negative or neutral in any time period {Link without Title} . This produces a result in general agreement with the conventional multi-proxy studies. 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.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 The work has recently been reproduced by Wahl and Ammann [http://www.cgd.ucar.edu/ccr/ammann/millennium/CODES_MBH.html [http://www.cgd.ucar.edu/ccr/ammann/millennium/refs/WahlAmmann_ClimaticChange_inPress.pdf]. 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 [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 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; however, this conclusion rests at least in part on the reasonableness of the GCM simulation used, which has been questionned. Anders Moberg and his Swedish and Russian collaborators who published their results in Nature on February 10, 2005 [http://www.realclimate.org/index.php?p=122 have also generated reconstructions with significantly more variability 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 Sunspot s 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 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. 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 {Link without Title} by Steven McIntyre and Ross McKitrick. Historical temperature estimates For information on the description of the '' 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 [http://www.multi-science.co.uk/ee_openaccess.htm 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]. In turn, 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 [http://www.realclimate.org/index.php?p=11 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
|
|
|