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Politicians and the media assume Earth's climate is warming as the result of human activity. Various types of evidence of previous climate changes were investigated as a means of testing the validity of assigning anthropogenic causes to this change.

Data with broad geographic coverage indicative of temperature and climate were evaluated. It included records of glacial advance and retreat, sedimentologic evidence of sea level change and glacial activity, palynologic indications of species succession, dendrochronologic evidence of tree-growth response to environment, and continental-ice core parameters indicating accumulation rates as well as other climate surrogates. Also reviewed were historical sources such as explorers' journals, which document significant climate effects over time. Each type of evidence has particular value. Among these are preservation of regional versus local conditions, transport in or out of the system, age–date reliability, correlation between data types, and data (as well as human) bias. All the data indicate that the Holocene has been characterized by ten or more global “little ice ages” irregularly spaced. Each lasted a few centuries separated by sometimes sudden and dramatic global warming events.

It is difficult to develop precise paleothermometry. Qualitative evaluations indicate frequent, sudden, and dramatic climate changes. Changes can be rapid, swinging from warmer than today to full glacial conditions within 100 years. The converse can be true. All available data indicate that current climate change is no greater in rate or magnitude, and probably less in both, than many changes that have occurred in the past.