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)). Although temperature reconstructions from proxy data help us understand the character of natural climate variability, attribution of recent climate change relies on a broad range of methodologies in addition to the proxy reconstructions[1] [2].
According to all major temperature reconstructions published in peer-reviewed journals (see graph), the increase in temperature in the 20th century and the temperature in the late 20th century is the highest in the record. Attention has tended to focus on the early work of Mann, Bradley and Hughes (1998), whose "hockey stick" graph was featured in the 2001 United Nations Intergovernmental Panel on Climate Change report. The methodology and data sets used in creating the Mann et al. (1998) version of the hockey stick graph are disputed, most notably by Stephen McIntyre and Ross McKitrick.
For general information about temperature records see the main article: Temperature record. For information on the description of the Medieval Warm Period and Little Ice Age in various reports see the main article: MWP and LIA in IPCC reports
Contents |
General techniques
By far the best observed period is from 1850 to the present day. Over this period the recent instrumental records, mainly based on direct thermometer readings, has approximately global coverage. It shows a general warming in global temperatures.
Quantitative methods using proxy data
As there are few instrumental records before 1850 temperatures before then must be reconstructed based on proxy methods. One such method, based on principles of dendrochronology, uses the width and other characteristics of tree rings to infer temperature. The isotopic composition of snow, corals, and stalactites can also be used to infer temperature. Other techniques which have been used include examining records of the time of crop harvests, the treeline in various locations, and other historical records to make inferences about the temperature. These proxy reconstructions are indirect inferences of temperature and thus tend to have greater uncertainty than instrumental data.
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. Proxy records must be averaged in some fashion if a global or hemispheric record is desired. Considerable care must be taken in the averaging process; for example, if a certain region has a large number of tree ring records, a simple average of all the data would strongly over-weight that regions. Hence data-reduction techniques such as principal components analysis 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 various proxy records 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 [3]. This produces a result in general agreement with the conventional multi-proxy studies.
Several reconstructions suggest there was minimal variability in temperatures prior to the past century (see, for example, [4]). More recently, Mann and Jones have extended their reconstructions to cover the last 2000 years (GRL, 2003 [5]). The work was reproduced by Wahl and Ammann in 2005 [6] [7] [8].
The Mann, Bradley and Hughes (1998) version of the temperature record is known as 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 [9] [10] 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" [11].
Qualitative reconstruction using historical records
It is also possible to use historical data such as 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 sparesly 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.
Astrophysicist Sallie Balunias notes that these temperature variations 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.
Limitations of models
The apparent differences between the quantitative and qualitative approaches are not fully reconciled. The reconstructions mentioned above rely on various assumptions to generate their results. If these assumptions do not hold, the reconstructions would be unreliable. For quantitative reconstructions, the most fundamental assumptions are that proxy records vary linearly with temperature and that non-temperature factors do not confound the results. In the historical records temperature fluctuations may be regional rather than hemispheric in scale.
Ongoing debate about temperature reconstructions
There is an ongoing debate about the details of the temperature record and the means of its reconstruction. The debate centers arround several discussion points:
- Was the late 20th century the warmest period during the last 1,000 years?
- Did the Medieval Warm Period exist?
Attention has centered on the Mann, Bradley and Hughes (1998), "hockey stick" graph, due to its central use in the UN's 2001 IPCC report. The basic conclusion of Mann et al. (1998, 1999) was that the late 20th century warmth in the Northern Hemisphere was unprecedented during at least the last 1,000 years.
Hans von Storch and colleagues claimed that the method used by Mann et al. probably underestimates the temperature fluctuations in the past by a factor of two or more;[12] however, this conclusion rests at least in part on the reasonableness of the GCM simulation used, which has been questioned.
Anders Moberg and his Swedish and Russian collaborators have also generated reconstructions with significantly more variability than the reconstructions of Mann et al.[13] [14]
On February 12, 2005 the Geophysical Research Letters paper by Stephen McIntyre and Ross McKitrick [15] claimed various errors in the methodology of Mann et al. (1998) claiming that the "Hockey Stick" shape was a result of collation errors, unjustifiable truncation or extrapolation of source data, obsolete data,geographical location errors, incorrect calculation of principal components and other quality control defects. [16]. They claimed that using the same steps as 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. [17]
McIntyre and McKitrick only audited the work of Mann et al. (1998), and made no assessment of the other studies that are broadly consistent with the results of Mann (1998). However, the findings from the Wegman report suggest that authors in the area of paleoclimate studies are closely connected and thus ‘independent studies’ may not be as independent as they might appear on the surface.
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 [18] [19], 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 1998 article, correcting a number of mistakes in the online supplementary information that accompanied their article but leaving the actual results unchanged.
In a letter to Nature (August 10, 2006) Bradley, Hughes and Mann pointed at the original title of their 1998 article: Northern Hemisphere temperatures during the past millennium: inferences, uncertainties, and limitations (Geophys. Res. Lett. 26, 759–762; 1999) and pointed out more widespread high-resolution data are needed before more confident conclusions can be reached and that the uncertainties were the point of the article.
Mann has stated that the criticisms directed at his statistical methodology are purely political and add nothing new to the debate [20].
Political controversy
The controversy has entered the political arena, primarily in the U.S. Two governmentally-commissioned reports have been issued on the matter, one assembled under the auspices of the Senate Committee on Energy and Commerce and the other a National Academy of Sciences panel ordered by the U.S. Congress.
Committee on Energy and Commerce Report (Wegman report)
McIntyre and McKitrick's paper has been reinforced by a team of statisticians led by Edward J. Wegman, George Mason University and chair of the National Academy of Sciences’ (NAS) Committee on Applied and Theoretical Statistics. The Wegman team was assembled at the request of Sen. Joe Barton, an outspoken global warming skeptic. [21] Findings presented in this report (commonly known as the "Wegman Report") include the following:
- MBH98 and MBH99 were found to be somewhat obscure and incomplete and the criticisms by McIntyre and McKitrick were found to be valid and compelling.
- It is noted that there is no evidence that Mann or any of the other authors in paleoclimatology studies have had significant interactions with mainstream statisticians.
- A social network of authorships in temperature reconstruction of at least 43 authors having direct ties to Mann by virtue of coauthored papers with him is described. The findings from this analysis suggest that authors in the area of paleoclimate studies are closely connected and thus ‘independent studies’ may not be as independent as they might appear on the surface.
- It is important to note the isolation of the paleoclimate community; even though they rely heavily on statistical methods they do not seem to interact with the statistical community. Additionally, the Wegman team judged that the sharing of research materials, data and results was haphazardly and grudgingly done.
- Overall, the committee believes that Mann’s assessments that the decade of the 1990s was the hottest decade of the millennium and that 1998 was the hottest year of the millennium cannot be supported by his analysis.
The Wegman report has itself been criticized for a number of things:
- The report was subject to scientific peer review.
- The result of fixing the alleged errors in the overall reconstruction does not change the general shape of the reconstruction. Similarly, studies that use completely different methodologies also yield very similar reconstructions [22].
- The social network analysis has no value without comparative studies in other tightly defined areas of science. The so-called network of co-authorship is not unusual at all.
National Research Council Report
At the request of the U.S. Congress, a special "Committee on Surface Temperature Reconstructions for the Past 2,000 Years" was assembled by the National Research Council's Board on Atmospheric Sciences and Climate. The Committee published its report in 2006. [23] The report summarizes its main findings as follows:[24]
- The instrumentally measured warming of about 0.6°C during the 20th century is also reflected in borehole temperature measurements, the retreat of glaciers, and other observational evidence, and can be simulated with climate models.
- Large-scale surface temperature reconstructions yield a generally consistent picture of temperature trends during the preceding millennium, including relatively warm conditions centered around A.D. 1000 (identified by some as the “Medieval Warm Period”) and a relatively cold period (or “Little Ice Age”) centered around 1700. The existence and extent of a Little Ice Age from roughly 1500 to 1850 is supported by a wide variety of evidence including ice cores, tree rings, borehole temperatures, glacier length records, and historical documents. Evidence for regional warmth during medieval times can be found in a diverse but more limited set of records including ice cores, tree rings, marine sediments, and historical sources from Europe and Asia, but the exact timing and duration of warm periods may have varied from region to region, and the magnitude and geographic extent of the warmth are uncertain.
- It can be said with a high level of confidence that global mean surface temperature was higher during the last few decades of the 20th century than during any comparable period during the preceding four centuries. This statement is justified by the consistency of the evidence from a wide variety of geographically diverse proxies.
- Less confidence can be placed in large-scale surface temperature reconstructions for the period from A.D. 900 to 1600. Presently available proxy evidence indicates that temperatures at many, but not all, individual locations were higher during the past 25 years than during any period of comparable length since A.D. 900. The uncertainties associated with reconstructing hemispheric mean or global mean temperatures from these data increase substantially backward in time through this period and are not yet fully quantified.
- Very little confidence can be assigned to statements concerning the hemispheric mean or global mean surface temperature prior to about A.D. 900 because of sparse data coverage and because the uncertainties associated with proxy data and the methods used to analyze and combine them are larger than during more recent time periods.
Updates
Ongoing updates and future events related to the MBH work are accessible in two weblogs:
- RealClimate - Climate scientists blog, including Mann
- ClimateAudit - McIntyre blog
References
External links
- 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
- "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.
- Myth vs. Fact Regarding the "Hockey Stick" — Real Climate
- Was the climate of the 20th century unusual?
- Surface Temperature Reconstructions for the Last 2,000 Years
- Ad hoc committee report on the ‘Hockey Stick’ Global Climate Reconstruction
- Discussion article about the relevance of the debate to climate policy and climate science policy with contributions from both 'sides'
Categories
Climate change | History of climate