Temperature record of the last 2,000 years

Large-scale reconstructions covering part or all of the 1st millennium and 2nd millennium have shown that recent temperatures are exceptional: the Intergovernmental Panel on Climate Change Fourth Assessment Report of 2007 concluded that "Average Northern Hemisphere temperatures during the second half of the 20th century were very likely higher than during any other 50-year period in the last 500 years and likely the highest in at least the past 1,300 years."

As of 2010[update] this broad pattern was supported by more than two dozen reconstructions, using various statistical methods and combinations of proxy records, with variations in how flat the pre-20th-century "shaft" appears.

Networks of such records are used to reconstruct past temperatures for regions: tree ring proxies have been used to reconstruct Northern Hemisphere extratropical temperatures (within the tropics trees do not form rings) but are confined to land areas and are scarce in the Southern Hemisphere which is largely ocean.

Over this period the recent instrumental record, mainly based on direct thermometer readings, has approximately global coverage.

The "Composite Plus Scaling" (CPS) method is widely used for large-scale multiproxy reconstructions of hemispheric or global average temperatures.

More recently, Osborn and Briffa used a simpler technique, counting the proportion of records that are positive, negative or neutral in any time period.

[7] It is also possible to use historical data such as times of grape harvests, sea-ice-free periods in harbours and diary entries of frost or heatwaves to produce indications of when it was warm or cold in particular regions.

For quantitative reconstructions, the most fundamental assumptions are that proxy records vary with temperature and that non-temperature factors do not confound the results.

In the 1960s, Hubert Lamb generalised from historical documents and temperature records of central England to propose a Medieval Warm Period in the North Atlantic region, followed by Little Ice Age.

Using proxy indicators for quantitative estimates of past temperature record had developed sporadically from the 1930s onwards, and Bradley & Jones 1993 harvnb error: no target: CITEREFBradleyJones1993 (help) introduced the "Composite Plus Scaling" (CPS) method.

The graph was featured in publicity, and became a focus of dispute for those opposed to the strengthening scientific consensus that late 20th century warmth was exceptional.

[16] In 2003, as lobbying over the 1997 Kyoto Protocol intensified, efforts by the Bush administration to remove climate reconstructions from the first Environmental Protection Agency and Jim Inhofe's Senate speech claiming that man-made global warming is a hoax both drew on the Soon and Baliunas controversy.

Arguments against the MBH studies were reintroduced as part of the Climatic Research Unit email controversy, but dismissed by eight independent investigations.

More than two dozen reconstructions, using various statistical methods and combinations of proxy records, have supported the broad consensus shown in the original 1998 hockey-stick graph, with variations in how flat the pre-20th century "shaft" appears.