William Happer, Professor of Physics at Princeton, on the history of CO2 and warming:

There have been many warmings and coolings in the past when the CO2 levels did not change. A well-known example is the medieval warming, about the year 1000, when the Vikings settled Greenland (when it was green) and wine was exported from England. This warm period was followed by the “little ice age” when the Thames would frequently freeze over during the winter. There is no evidence for significant increase of CO2 in the medieval warm period, nor for a significant decrease at the time of the subsequent little ice age. Documented famines with millions of deaths occurred during the little ice age because the cold weather killed the crops. Since the end of the little ice age, the earth has been warming in fits and starts, and humanity’s quality of life has improved accordingly.

A rare case of good correlation between CO2 levels and temperature is provided by ice-core records of the cycles of glacial and interglacial periods of the last million years of so. But these records show that changes in temperature preceded changes in CO2 levels, so that the levels were an effect of temperature changes. This was probably due to outgassing of CO2 from the warming oceans and the reverse effect when they cooled.

The most recent continental ice sheets began to melt some twenty thousand years ago. During the “Younger Dryas” some 12,000 years ago, the earth very dramatically cooled and warmed by as much as 10 degrees Celsius in fifty years.

The earth’s climate has always been changing. Our present global warming is not at all unusual by the standards of geological history, and it is probably benefiting the biosphere. Indeed, there is very little correlation between the estimates of CO2 and of the earth’s temperature over the past 550 million years (the “Phanerozoic” period). The message is clear that several factors must influence the earth’s temperature, and that while CO2 is one of these factors, it is seldom the dominant one. The other factors are not well understood. Plausible candidates are spontaneous variations of the complicated fluid flow patterns in the oceans and atmosphere of the earth—perhaps influenced by continental drift, volcanoes, variations of the earth’s orbital parameters (ellipticity, spin-axis orientation, etc.), asteroid and comet impacts, variations in the sun’s output (not only the visible radiation but the amount of ultraviolet light, and the solar wind with its magnetic field), variations in cosmic rays leading to variations in cloud cover, and other causes.

The existence of the little ice age and the medieval warm period were an embarrassment to the global-warming establishment, because they showed that the current warming is almost indistinguishable from previous warmings and coolings that had nothing to do with burning fossil fuel. The organization charged with producing scientific support for the climate change crusade, the Intergovernmental Panel on Climate Change (IPCC), finally found a solution. They rewrote the climate history of the past 1000 years with the celebrated “hockey stick” temperature record.

The first IPCC report, issued in 1990, showed both the medieval warm period and the little ice age very clearly. In the IPCC’s 2001 report was a graph that purported to show the earth’s mean temperature since the year 1000. A yet more extreme version of the hockey stick graph made the cover of the Fiftieth Anniversary Report of the United Nation’s World Meteorological Organization. To the surprise of everyone who knew about the strong evidence for the little ice age and the medieval climate optimum, the graph showed a nearly constant temperature from the year 1000 until about 150 years ago, when the temperature began to rise abruptly like the blade of a hockey stick. The inference was that this was due to the anthropogenic “pollutant” CO2.