What the earth’s temperature and CO2 levels were can be determined, surprisingly enough, as far back as 650,000 years ago. For the last millennium, accurate records of temperatures recorded with thermometers can be found. Before that, there are ancient documents telling of extreme weather events, the dates of spring planting, or occurrence of plagues from which some idea of the weather can be gleaned. For prehistoric eras, there were no direct observations, but data can be found indirectly from what are called proxies. Tree rings, ice cores, and cores of layered sediments in soil or sea bottoms give annual records that can be counted ring by ring. Trapped air bubbles in ice cores give the CO2 concentration hundreds of millennia ago. The frac­tional abundance of oxygen or hydrogen isotopes in ice cores and coral yields the temperature, as do other ratios, such as Mg to Ca. These proxies can be correlated with one another to give higher accuracy in recent times for which there are more data. The result from Antarctica ice is shown in Fig. 1.4. As the earth undergoes long glacial ages and short interglacial warm periods, the CO2 and CH4 abundances follow the temperature quite closely. Of course, we cannot tell which is the cause and which is the effect here. The present warm period, which allows life to exist, looks no dif­ferent from previous interglacial periods, except for the spike seen at the far right. For that, we now know that CO2 is the cause, and the temperature rise the effect.

When considering the climate tens of thousands of years back, we have to take into account changes in the earth’s orbit. The earth’s spin axis is not perpendicular to the plane of its orbit but is tilted at 23.5°, thus causing winter in the northern hemisphere while it is summer in the south. This tilt can change from 22° to 24.5° over a period of 20,000 years or so. This does not change the total sunlight on the earth, but it distributes differently between the northern and southern hemispheres. Since there is more land in the north and more water in the south, this re-distribu­tion of sunlight can affect the climate. A bigger effect comes from the precession of the equinoxes, when the earth’s axis spins around like a gyroscope. The effects come from an interaction with the ellipticity of the earth’s orbit, which means that solar radiation is stronger when the earth is near the sun (perihelion) than when it is far away (aphelion). Thus, in one orientation, the northern hemisphere has sum­mer during perihelion; and, 10,000 years later, the southern hemisphere gets the hotter summers. The shape of the earth’s orbit can also change between more cir­cular and more elliptical due to the pull of other planets, mainly Jupiter. This hap­pens every 100,000 years or more. The ice ages may have started at a coincidence of these orbital forcings, triggering runaway feedback, as we described before. The recovery into warm periods is equally remarkable. There is an intriguing theory that the most recent recovery (the last shaded bar in Fig. 1.4) may have been caused by humans when they started farming about 11,000 years ago [7]. Methane is pro­duced by decaying vegetative and animal matter produced in agriculture, and deforestation decreases CO2 absorption by trees.

The paleoclimate data for the last 20,000 years on how CO2 and CH4 abundances changed with time are so good that observations from different proxies agree amaz­ingly well. This is shown in Fig. 1.5. The CO2 level increased slowly from 190 ppm to the preindustrial level of 280 ppm, followed by the recent rapid increase to 379 ppm in 2005. The present level is much higher than any level that existed over the past 650,000 years (indicated by the gray bar at the left). The current spike is also seen in the methane data.