Proxies can be combined to produce temperature reconstructions longer than the instrumental temperature record and can inform discussions of global warming.

The skill of algorithms used to combine proxy records into an overall hemispheric temperature reconstruction may be tested using a technique known as "pseudoproxies".

His research has been part of the Hockey stick controversy with his temperature reconstructions, that have been cited as being evidence for both sides of the controversy.

For instance, in [1], it is stated that: "The observed twentieth century temperature increase appears unprecedented compared to our mean North American temperature reconstruction of the past 14,000 years."

Similarly, in [2], it was stated that: "A review of temperature reconstructions is made and similar results are obtained."

This conclusion has been subsequently supported by an array of evidence that includes both additional large-scale surface temperature reconstructions and pronounced changes in local proxy indicators.

Before around AD 1600, the uncertainty of temperature reconstructions rises due to the relative paucity of data sets and their limited geographic distribution.

Chinese temperature reconstructions aren't hockey-stick-shaped.

"Validity of the temperature reconstruction from water isotopes in ice cores"

The spatial extent of the change is unclear; it does not show up as a major, or even identifial, event in hemispheric temperature reconstructions.