Hockey Stick Controversy - Origins: The First Paleoclimate Reconstructions - IPCC First Assessment Report, 1990, Supplement, 1992

IPCC First Assessment Report, 1990, Supplement, 1992

Publicity over the concerns of scientists about the implications of global warming led to increasing public and political interest, and the Reagan administration, concerned in part about the political impact of scientific findings, successfully lobbied for the 1988 formation of the Intergovernmental Panel on Climate Change to produce reports subject to detailed approval by government delegates.

The IPCC First Assessment Report in 1990 noted evidence that Holocene climatic optimum around 5,000-6,000 years ago had been warmer than the present (at least in summer) and that in some areas there had been exceptional warmth during "a shorter Medieval Warm Period (which may not have been global)" about AD 950-1250, followed by a cooler period of the Little Ice Age which ended only in the middle to late nineteenth century. The report discussed the difficulties with proxy data, "mainly pollen remains, lake varves and ocean sediments, insect and animal remains, glacier termini" but considered tree ring data was "not yet sufficiently easy to assess nor sufficiently integrated with indications from other data to be used in this report." A "schematic diagram" of global temperature variations over the last thousand years has been traced to a graph based loosely on Lamb's 1965 paper, nominally representing central England, modified by Lamb in 1982. Mike Hulme describes this schematic diagram as "Lamb's sketch on the back of an envelope", a "rather dodgy bit of hand-waving".

In Bradley et al. 1991, a working group of climatologists including Raymond S. Bradley, Malcolm K. Hughes, Jean Jouzel, Wibjörn Karlén, Jonathan Overpeck and Tom Wigley proposed a project to improve understanding of natural climatic variations over the last two thousand years so that their effect could be allowed for when evaluating human contributions to climate change. Climate proxy temperature data was needed at seasonal or annual resolution covering a wide geographical area to provide a framework for testing the part climate forcings had played in past variations, look for cycles in climate, and find if debated climatic events such as the Little Ice Age and Medieval Warm Period were global. Reconstructions were to be made of key climate systems, starting with three climatically sensitive regions: the Asian monsoon region, the El Niño–Southern Oscillation region and the Atlantic region. Areas where more data was needed were to be identified, and there was a need for improved data exchange with computer=based archiving and translation to give researchers access to worldwide paleoclimate information.

The IPCC supplementary report, 1992, reviewed progress on various proxies. These included a study of 1,000 years of tree ring data from Tasmania which, like similar studies, did not allow for possible overestimate of warming due to increased CO₂ levels having a fertilisation effect on tree growth. It noted the suggestion of Bradley et al. 1991 that instrumental records in specific areas could be combined with paleoclimate data for increased detail back to the 18th century.