Climate Cycles

There are identifiable cycles in the weather patterns of the Earth. There are four seasons every year in the temperate zones. In the polar regions, there are seasons of light and dark; and in the tropical regions, there are seasons of wet monsoons and dry periods. These identifiable annual cycles are like the cycles of the climate of the Earth over vast eons of the geologic eras.

Geologists estimate the age of the Earth at about 4 billion years old. For much of that time, it was a ball of gas, then the scene of enormous volcanic activity, and then weather activity dominated the whole of the Archaic or Pre-Cambrian eras of Earth’s history. During that time, repetitions of climate patterns may have occurred, but there were also dramatic changes. There was a time when oxygen came to be a major part of the atmosphere, when it was not as abundant as now. These point to cycles in the climate, or the longterm average of the weather on Earth.

The last ice age ended around 12,000 years ago. However, it was not the first ice age on Earth, nor the last. It was only the end of the most recent ice age. There were at least three others in the recent history of the Earth. These ice ages, between interglacial periods, are identifiable cycles of climate in the history of the Earth.

One possible cause of cyclical changes in Earth’s climate is its orbiting of the sun (the Milankovich cycle). The solar calendar is keyed to the annual journey of the Earth around the sun. It is a journey that occurs every year and causes seasonal change. Although there are the same seasons every year, they are never exactly alike. Astronomers and others have observed that there is a cycle that some set at roughly 24,000, or 48,000, or even 72,000 years. The cycle is believed to be responsible for climate changes because the Earth wobbles a bit as it orbits the sun because of the variations in the gravitational pull of other planets and of the sun itself. The slight changes in orbit and the tilt of the Earth in relationship to the sun’s rays means more or less sunlight hitting the Earth, increasing or reducing the amount of energy available to warm the Earth and its atmosphere.

Some scientists believe that ice ages may be caused by the variations in sunlight hitting the Earth during its solar journeys. However, the amount of sunlight during the solar journey has to be combined with solar variations. The sun goes through cycles of activity in which varying levels of energy are emitted. The solar energy variations are related to the presence and absence of sunspots. Increases in these are likely to also bring about an increase in aurora lights, in both the Northern Hemisphere and Southern Hemisphere.

The aurora lights are connected to the magnetic field of the Earth. Researchers studying the sun’s magnetic activity over 100,000-year cycles have proposed the theory that the climate of the Earth is affected by this solar activity cycle. Waldo S. Glock attempted to show a relationship between the weather as a part of a climate pattern and variations in periodic solar activity.

The pattern that paleoclimatologists have discovered is one that describes significant climatic changes with relatively small changes in the Earth’s solar orbit. The variations in sunlight have been compared to variations in weather on Earth. Instruments have been developed to measure the amount of solar energy striking the Earth. Cycles have been detected using data from weather records. Weather records have also been compared statistically with wheat prices. The higher the price of wheat in historically available data, the poorer the weather likely was. Other plant information that has been available to study climate cycles is found in the rings of trees. Dendrochronology is the study of tree rings over time. Trees grow faster in wet, warm years than in dry years or in cooler times.

The discovery of climate cycles that has been verified by a variety of sources points to the current phenomenon of global warming. However, it was preceded by an ice age in which there was global cooling. Consequently, the Earth has experienced both global cooling and global warming in its 4 billion-year-old history. Opposed to theories of climatic cycles is the fact that the climate of the Earth also changes in noncyclical ways when charted on a variety of timescales.

There are shorter cycles, such as the El Niño Southern Oscillation, the Pacific Decadal Oscillation, the Arctic Oscillation, and the North Atlantic Oscillation, the 1,500 year cycle from ice core samples, and the sunspot cycle (the Hale cycle). All of these cycles can be hypothesized from climate records. They are being used in debates over global warming to argue that either the cause is anthropogenic, or that global warming is part of a natural cycle that is only apparently anthropogenic.