The Greenland Ice Sheet Project II (GISP2) completed drilling to bedrock at Summit, Greenland, in 1993 with a 3053 meter ice core. The core contains a record of atmospheric chemistry accumulated by the annual precipitation at Summit for over 200,000 years. The top 2960 meters provides a well-dated climate history for the past 110,000 years and it is this record which has led to the climate symphony.
The chemical record from the GISP2 core consists of 8 time series recording the concentrations of the 8 major ions found in the soluble components of the atmosphere, These are: calcium, potassium, magnesium (primarily from continental dust), sodium, chloride (primarily from seasalt aerosols), sulfate (from marine biological production, burning of fossil fuels, and volcanoes), nitrate (from lightning, biological production, and many other sources), and ammonium (primarily from terrestrial biological production in high latitudes). Together these ion records provide a history of changing drought and flood, storms and calm, and biological growth and decay as the great continental ice sheets have advanced and retreated over the past 110,000 years.
The first step in the effort to explore and understand the history contained in the GISP2 time series involved a mathematical technique called principle component analysis which combined the eight time series into just 3 series with only a minimal loss of information. The second and third of these series clearly described the biological response of changing climate (recorded in the nitrate and ammonium series). The first, and most important of these summary series, directly records the atmospheric circulation response to the growth and decay of the continental ice sheets. It has come to be known as the "polar circulation index" (PCI) and is the series on which the climate symphony is based.
In their effort to understand the processes which underlie the environmental changes they study, scientists often hope to find evidence for periodic behavior. Such evidence can be important in identifying those factors which influence the changes the scientists observe. If a possible forcing agent is known to vary periodically (for example, like the solar intensity underlying the sunspot cycle) and the observed record has a periodic component with a similar period, then this may be evidence for a physical link between the agent and the response under study. In the case of the PCI, such a harmonic analysis was a natural step to take and proved to be very important.
The current scientific understanding of the mechanism behind the Earth's series of ice ages occurring over the past million years rests on the Milankovitch theory of changes in the orbit of the Earth relative to the Sun. As a consequence of the gravitational effects of the larger planets the Earth's orbit changes periodically in such a way that the amount and distribution of energy it receives from the Sun varies significantly over time. These changes have periodic components with periods of around 400,000, 100,000, 40,000, and 22,000 years which combine to create a very complex history of climate forcing by variation in insolation over the Earth. While the PCI record is too short (110,000 years) to contain a cycle of the longest Milankovitch cycles, it does show strong periodic behavior with the other two periods. In addition, other periodic features were shown to be important components of the PCI. Most of the additional periodic features had been previously identified in other climate records or were likely subharmonics of the Milankovitch cycles acting through the various components of the climate system (the atmosphere, cryosphere, biosphere, and lithosphere).
Ultimately, it was found that the major features of the 110,000 years of the PCI record could be summarized by the sum of 10 components oscillating periodically but with an amplitude which varied as the ice sheets came and went. Thus, this important climate record from the Greenland ice, like an orchestral score, can be synthesized by a relatively few "pure tones". It was this observation which has, ultimately, led to the climate symphony.