Causes of Climate Change
  • Long-term (millions of years) climate changes are linked to plate tectonic processes
  • Short-term (thousands of years) changes are associated with changes in the earth’s orbit

T.gif (106 bytes)he cause of such long-term global climate changes has to be processes that operate on a global scale over very long time intervals. The most likely causes are associated with the changing locations of continents and oceans (plate tectonics) that would in turn affect atmospheric and oceanic circulation patterns.

For example, ice ages have occurred at four widely separated times in the earth’s history from the Precambrian (700 million years ago) to the recent past (~10,000 years ago). The causes of such events are thought to be linked to global scale processes. Some potential requirements for such long-term cooling events may be:

  1. Continents near poles: Continental ice sheets that are characteristic of ice ages can only form on land. Glaciation on the ancient supercontinent, Pangaea, occurred when several continents were grouped around the South Pole;
  1. Uplift associated with continental collisions: One tectonic element that is thought to have contributed to global cooling beginning ~40 million years ago is the formation of the Himalaya Mountains. Some scientists have suggested that the mountains changed the regional (maybe even global) atmospheric circulation patterns that contributed to a cooler climate;
  1. Reduction in greenhouse gas concentrations: Increased rainfall following uplift may have stripped carbon dioxide from the atmosphere to be used in chemical weathering. The presence of less carbon dioxide resulted in a reduction in global warming (i.e. a cooling event).

Times in the geologic past when temperatures where much higher than today are related to periods of more rapid plate movements and greater volcanic activity. Both processes produced greater volumes of greenhouse gases that caused long-term warming of the atmosphere.

Short-term climate fluctuations that occur on cycles lasting thousands of years are related to variations in the earth’s orbit around the sun. These variations (called Milankovitch cycles after the astronomer who identified them) cause the amount of insolation (incoming solar radiation) to vary with time.

  1. The eccentricity of the earth’s orbit. The exact path of the orbit around the sun changes with time and may become anim_ecc.gif (7096 bytes)less eccentric (more circular) or more eccentric (more elliptical). These changes occur on a 100,000 year cycle.
  1. Changes in the tilt of the earth’s axis. The tilt of Earth's axis is currentlytilt_new.gif (2413 bytes) tilted at 23.5 but axial tilt ranges from approximately 22-25 degrees over a 41,000-year cycle. Decreasing tilt reduces the contrast of insolation associated with the seasons, increasing tilt exaggerates seasonal differences. Lesser tilt promotes the buildup of ice at the poles, greater tilts allow for more insolation during polar summers, causing more snow melt.
  1. The precession of the earth on its axis: The earth "wobbles" on its axis (precession), changing the direction of axial tilt. Precession precession_new.gif (2627 bytes)occurs on a 26,000 year cycle - the length of time taken for the axis to trace a complete loop. Half-way through the precession cycle, Earth would be tilted away from the Sun during the "Summer" solstice (Northern Hemisphere) and the Sun would be overhead at the Tropic of Capricorn.  Maximum precession therefore results in a switch between Summer and Winter seasons, with the warmest months occurring in what we now call Winter and cooler months during our the middle of the year.
precess2.gif (1091 bytes)
Precession results in the seasons alternating position as the Sun will be overhead at the Tropic of Capricorn (Southern Hemisphere summer) during the middle of the year (Northern Hemisphere summer)

All of these factors contribute to changes in the amount of solar radiation that reaches the earth and the amount of heat that is transferred from the sun. During ice ages these cycles correlate well with jumps from cold intervals (glacials) to warmer intervals (interglacials). However, the variations in solar radiation are not considered sufficient to account for the magnitude of observed temperature variations.

Cooler temperatures during the glacials were associated with lower concentrations of carbon dioxide (a heat trapping gas) and higher levels of atmospheric dust (blocked incoming sunlight). What is less clear is how these factors are linked to variations in the earth’s orbit. Climate fluctuations that occur on an even shorter time scales (decades to centuries) may be linked to variations in sun spot activity or catastrophic volcanic eruptions.