Temperature record of the past 1000 years

For people who check facts

The last 1000 years of the northern hemisphere historical temperature record has been quantitatively reconstructed from tree ring proxy data by scientists, principally Mann, Jones and Briffa; and used by the IPCC. More recently, the record has been extended to the last 2000 years (Mann and Jones, GRL, 2003 [1]).

Quantitative reconstructions

The graphs of these reconstructions show a separation into two trends. From 1000 A.D. to 1880 the temperature graphs show a slow, irregular steady decline. From 1880 to present temperatures increase about 0.6 °C.

This temperature record has an unofficial name, the "Hockey Stick graph, first coined by Jerry Malhman, a colleague of Mann's.

Northern Hemisphere temperature variations.

The work of Mann et al. and others [1] [1] 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" [1].

The reconstructions mentioned above are quantitative: numerical temperature time series, either from observations or a variety of proxies, are merged and averaged to produce an average for the northern hemisphere. In the process, it is possible to produce error estimates that generally get larger further back in time.

Qualitative reconstruction

It is also possible to use historical data - 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 sparesely 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. Scientists such as astrophysicist Sallie Balunias note that these ups and downs 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.

The apparent differences between the quantitative and qualitative approaches are not fully reconciled. One possibility is that the fluctuations recorded in the historical records are regional rather than hemispheric in scale.

Skepticism and rebuttals thereof

Skeptics complain that the IPCC had previously accepted a temperature record which showed large natural variations such as the medieval climate optimum and the Little Ice Age, but unaccountably selected a different set of data that fit its preordained conclusions. Such skeptics have presumably failed to notice that the graph used in the earliest (1990) IPCC report was a schematic (non-quantitative; as discussed above): the 1990 report further noted that it was not clear "whether all the fluctuations indicated were truly global". The graph disappeared from the 1992 supplementary report, and was replaced in the 1995 report by a northern hemisphere summer temperature reconstruction from 1400 to 1979 by Bradley and Jones (1993); this in turn was updated in the 2001 report to northern hemisphere warm-season and annual reconstructions from 1000 AD to present by Mann et al (1999), Jones et al (1999) and Briffa (2000) [1].

All quantitative hemispheric temperature reconstructions show the same pattern of slow cooling followed by more rapid warming. Skeptics, however, dislike the "Hockey Stick": for example John Daly in The ÃÂÃÂHockey StickÃÂÃÂ: A New Low in Climate Science. Note that that article incorrectly states the graph displayed therein is from the 1995 IPCC SAR when it is from the 1990 report.

Stephen McIntyre and Ross McKitrick attempted an "audit" of MBH98 [1]; obtained what was available from Mann et. al. and published the results of examining the data in Corrections to the Mann et. al. (1998) Proxy Data Base and Northern Hemispheric Average Temperature Series. This publication claimed there were collation errors; unjustifiable truncation or extrapolation errors; obsolete data; geographical location errors; incorrect calculations; other quality control defects. The result is a claim that the 'hockey stick' shape is an artefact of the methods used. M&M offer no explanation as to why their analysis also differs from other reconstructions [1].

In turn, Mann has disputed the claims made by McIntyre and McKitrick [1], saying "...MM have made critical errors in their analysis that have the effect of grossly distorting the reconstruction of MBH98. Key indicators of the original MBH98 network appear to have been omitted for the early period 1400-1600, with major consequences for the character of the MM reconstruction of Northern Hemisphere temperatures over that interval."

External links

http://www.evsc.virginia.edu/faculty/people/mann.shtml