Predictions of global climate change are based on large computer-simulation models that are “history-matched” to weather records compiled from the early nineteenth century onward. Climate-change model forecasts would be more convincing if they were based on the natural records of the Holocene (≈10,000 years) and were capable of simulating climate characteristics of this epoch. Temperature records estimated from δ¹⁸O measurements on ice cores from the Greenland ice cap and the Antarctic could be used to develop models based on geochronological data rather than historically brief weather records.

The 20-year average record of δ¹⁸O values from the Greenland Ice Sheet Project 2 (GISP2) ice core exhibits a long-term trend of declining temperatures over most of the Holocene, except during the last 100 years, when temperatures have increased—a change widely blamed on carbon-dioxide (CO₂) emissions from fossil fuels. However, the range in temperatures since the start of the industrial age is typical for the Holocene, and the current rate of increase in temperatures is unusual but not unprecedented. Past periods of consistently increasing (or decreasing) temperatures have not persisted much longer than the current interval, so temperature trends may well reverse in the near future. There are distinct cyclic patterns in temperatures recorded in the GISP2 ice core, including a pronounced sawtoothed 560-year sequence of relatively abrupt change followed by a gradual reversal. The present trend may be the initial phase of such a pattern. In summary, the present climate does not appear significantly different from the past climate at times prior to industrialization.