Temperature record of the past 1000 years

From Wikipedia, the free encyclopedia Jump to: navigation, search Main article: Temperature record

The temperature record of the 2nd millennium describes the reconstruction of temperatures since 1000 CE on the Northern Hemisphere, later extended back to 1 CE and also to cover the southern 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 able to better detect and attribute anthropogenic global warming. Note, however, that 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 of which the proxy reconstructions are only a small part.

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 Michael E. 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 by Stephen McIntyre and Ross McKitrick, but the graph is overall acknowledged by the scientific community.

1 General techniques and accuracy
- 1.1 Quantitative methods using proxy data
- 1.2 Qualitative reconstruction using historical records
- 1.3 Limitations
2 The hockey stick controversy
3 See also
4 References
5 External links

General techniques and accuracy

By far the best observed period is from 1850 to the present day, with coverage improving over time. Over this period the recent instrumental record, mainly based on direct thermometer readings, has approximately global coverage. It shows a general warming in global temperatures.

Before this time various proxies must be used. These proxies are less accurate than direct thermometer measurements, have lower temporal resolution, and have less spatial coverage. Their only advantage is that they enable a longer record to be reconstructed. Since the direct temperature record is more accurate than the proxies (indeed, it is needed to calibrate them) it is used when available: i.e, from 1850 onwards.

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 dendroclimatology, 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 region. 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. 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 20th century (see, for example, ). More recently, Mann and Jones have extended their reconstructions to cover the 1st and 2nd millennia (GRL, 2003 ). The work was reproduced by Wahl and Ammann in 2005 according to a press release published computer code and a paper in press.

The Mann, Bradley and Hughes (1998) version of the temperature record is known as the "Hockey Stick" graph, first coined by Jerry Mahlman, director of the Geophysical Fluid Dynamics Laboratory.

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 centu

Nothing was found!