There is strong evidence that the Earth has experienced long periods during which average global temperatures were much colder and much warmer than today. Changes in the Earth’s climate system throughout geologic time can be linked to changes in the components of the climate system, including changes in the Earth itself, the composition of the atmosphere, and the seasonal distribution and total amount of incoming solar energy.
There have been enormous changes in the surface of the Earth − with continents moving, mountain ranges growing and eroding away, and the area covered by oceans and by ice growing or shrinking. The composition of the atmosphere has also changed as a result of biological and geophysical processes, including storage of carbon in the ocean and its subsequent release, volcanic eruptions, and the occasional sudden release of methane from sediments on the ocean floor. In addition, there have been changes in solar output, in the Earth’s orbit, and Earth-Sun geometry. All of these changes affect climate at both the global and regional scale.
Consider, for example, the effects of slow changes in the Earth’s orbit around the Sun. Over the course of approximately 100,000 years, the Earth’s orbit around the Sun changes shape from a thin oval to a circle, and back again. At present, the shape of the Earth’s orbit is almost a perfect circle. There is only a small difference in our distance from the Sun at the time when we are closest to it (the perihelion, currently in January), and when we are farthest away (the aphelion, currently in July). The fact that the Earth is now closest to the Sun during the northern hemisphere winter is just a coincidence, because the date of the perihelion slowly moves to come later in the year, following a 21,000-year cycle. In other words, 10,000 years from now, the perihelion will occur in the northern hemisphere summer, causing northern hemisphere seasonal contrasts to be somewhat more pronounced than at present (Figure 1).
Figure 1. Graphic illustration of the Earth's orbit and average solar radiation comparing present conditions with those 9 thousand years ago (9ka). Source: Copyright, 2000 University Corporation for Atmospheric Research.
Even such subtle differences can have profound impacts on regional climates. When the perihelion last occurred in the northern hemisphere summer, the Sahara was much wetter than it is now and was covered with savanna-like vegetation. As the seasonal distribution of solar radiation gradually changed to modern conditions, the Sahara dried out. Its transformation to the present-day desert accelerated dramatically about 5,500 years ago. The abruptness of the change suggests that the climate system crossed a threshold, triggering a series of biophysical feedbacks that amplified the trend toward regional drying (IGBP 2001).
Seasonal contrasts would also tend to be more extreme when the shape of the Earth’s orbit is more elliptical than it is at present. In addition, the Earth wobbles slightly on its axis, so that the angle of the tilt varies over a 41,000-year cycle. Recall that the Earth’s tilt causes seasons in the first place. So, the greater the angle of tilt, the stronger the seasonal contrasts. These astronomical Milankovich cycles appear to have played a significant role in the timing of ice ages and interglacial periods in the recent past, but they clearly cannot explain all of the Earth’s climate history.
Changes in the seasonal distribution of incoming solar energy may have triggered the beginning and end of previous ice ages. However, the solar impacts were greatly amplified by positive feedbacks within the climate system, including changes in the reflection of sunlight back into space by ice-covered areas, changes in ocean circulation, and dramatic changes in atmospheric concentrations of greenhouse gases, especially carbon dioxide and methane. Over the past 400,000 years, the record of temperatures in the world’s high-latitude regions followed a saw-toothed pattern. Global concentrations of carbon dioxide and methane followed a nearly identical pattern (Figure 2). There were four long but erratic periods of cooling, each followed by a dramatic warm-up. Scientists do not fully understand the reasons for this pattern, but changes in the ocean’s thermohaline circulation and changes in the release of carbon dioxide from the oceans, and the release of methane from wetlands, appear to have played important roles. In Figure 2, one can see that rapid warming and increases in atmospheric carbon dioxide and methane occurred nearly simultaneously. This suggests a positive feedback loop, with initial warming causing the greenhouse gas concentrations to rise, and rising concentrations promoting further warming. Figure 2 also shows a correspondence between the temperature record and long periods of wet or dry conditions in Central and East-Asia. Wind-borne dust deposits, both in Antarctica (Vostok) and on the Chinese Loess Plateau tended to peak during glacial periods, indicating expansion of Asian deserts.
Based on pattern of thermohaline circulation in the World’s oceans − that is, the connection between the movement of cold, salty water in the oceans’ depths and the movement of warm, less saline water at the surface (Broecker 1997). Warm, low-salinity water from the tropical Pacific and Indian Ocean flows around the tip of South Africa and ultimately joins the Gulf Stream to transport heat from the Caribbean to Western Europe. As the water moves northward, evaporative heat loss cools the water and leaves it saltier and more dense. The cold, salty water sinks in the North Atlantic and flows back toward Antarctica, thus pushing the conveyor along. One hypothesis is that the inflow of fresh water into the North Atlantic during warm periods can cause this conveyor to dramatically slow down or even collapse. Such a mechanism could explain the sudden reversals of warming that appear in the geologic record.
Figure 2. Four glacial cycles are recorded in Vostok ice cores. The graphic represents thousands of years before the present. The top three lines from the Vostok ice core record show Deuterium – a proxy for local temperature (blue); CO2 (black); methane (red); and dust (purple). The green line is a measure of Chinese loess deposition. Source: Alverson, K.D., R.S. Bradley and T.F. Pedersen (eds.). 2003. Paleoclimate, Global Change and the Future."Late Quaternary History of Trace Glass." Figure 2.2, p. 19., Springer-Verlag, Berlin, Germany. With kind permission of Springer Science+Business Media.