Climates of the Past
Warm climates of the past may be useful models of what we can expect in future climates. The Pliocene Optimum occurred 3.3 million to 4.3 million years ago. At that time the CO2 level of the atmosphere may have been greater than 600 ppm. The Eemian Optimum occurred 125-130 thousand years ago and had a CO2 level somewhat less than current levels. The mid Holocene (5,000 to 6,000 years ago) is another period of temporarily and relatively warm conditions. Paleobotanic proxy data and oxygen isotopes are used to reconstruct evidence of climate conditions this far in the past. It should be cautioned that large uncertainties accompany any statements about climatic conditions from this far in the past. In some cases we have sufficient information about external conditions (External vs. Internal) to create computer model simulations of past climates for comparison of sparsely distributed proxy data.
The first graph in Figure 1 shows relative temperature changes over the last 1 million years compared to the present global mean temperature, given as the horizontal dashed line. Variations on these time scales are due to variations in the earth's orbital motion about the sun and are known as the Milankovitch effects. It is notable that for most of the record of the last million years the earth was considerably colder than today. Each minimum of the record corresponded to an ice age. During some of these ice ages, the hydrological balance tipped toward more H2O being resident in ice masses than today. This caused sea level to drop as much as 120 m below current levels.
The second graph of Figure 1 shows the last 11,000 years, a period that contains the Holocene maximum. This period evidently was not accompanied by a CO2 increase as was the case for warm periods in the more distant past. The Younger Dryas was an abrupt cooling that occurred about 12,700-11,500 years ago that returned the Northern Hemisphere to full glacial conditions during a general warming trend as it was emerging from the last full glacial period. Keigwin and Boyle relate such changes to changes in the thermohaline circulation. Graph (c) shows the period of the last 1100 years, which includes the so-called "Little Ice Age", which in fact may have been a more regional rather than global phenomenon. Some evidence suggests that the there was a minimum in solar activity during the period of the Little Ice Age, so it could be that this cool period resulted from reduced solar irradiation.
During the Eemian interglacial (125,000 to 130,000 years ago) temperatures in the North Polar region were estimated to be as much as 8°C warmer than today (Figure 2), with expected impacts on polar ice. For the Holocene climatic optimum, temperatures in polar regions were 3°C warmer than present (Figure 3). Some regions such as Africa may have had up to 300% of the current precipitation during this period. It is notable that during both the Eemian and Holocene periods the maximum departure from current temperatures seem to be in the polar regions.
The paleoclimate program of the National Oceanic and Atmospheric Administration (NOAA) has a wealth of information on paleoclimate research. Start with the NOAA paleoclimate primer and follow the many links imbedded therein.