Physical Phenomena Affecting Climate

The following is a summary of physical phenomena which effect climate and climate change on earth to varying degrees.

• Milankovitch Cycles (period changes in the earth's orbit):
  
• Obliquity (a measure of the tilt of the earth's axis): varies 21^o to 24.5^o over a 40, 000 year period.
• Eccentricity (a measure of how elliptic the earth's orbit is): varies from near zero to 0.06 over a 96,600 year period.
• Precession (a wobble of the earth's axis of rotation): Varies over a 23,000 or 18,000 year period.
• Geomagnetism: the earth's magnetic field reverses on an irregular period of just under a million years, possibly resulting in a larger influx of solar radiation, leading to heating of the atmosphere and oceans.
• Volcanism: Volcanoes add large amounts of particulate and greenhouse gases to the atmosphere over short periods of time. This tends to reflect solar radiation leading to cooling of the climate. Volcanism, along with variations in the solar constant are thought to be the reason for the 'little ice age' in Europe; aproximately 1550 to 1850 (exact dates are not well established) when temperatures were below normal by about -0.8C. The lower temperatuers caused many social problems during this period of time, particularly crop failiers.
• Meteor impact: Meteors colliding with the earth are thought to have effects similar to volcanoes, resulting in a temporary cooling of the climate. Several major mass extinction's of flora and fauna found in the fossil record are now thought to be a result of meteor impacts which resulted in global cooling trends. The best established event was the Cretaceous extinction of both land and sea species, including the dinosaurs which was the result of a meteor striking the earth just off the coast of the Yucatan peninsulaaround 65 million years ago.
• Sun spots: The sun's magnetic field reverses every 22 years resulting in an 11 year sunspot cycle. This periodicity has been linked to draught cycles on the earth from tree ring data. This is about a 1 to 5 W/m² change in the solar constant.
  
• Other changes in the solar constant: There are other pseudo periodic changes in the radiation output of the sun which have been detected. There is also a long term trend- the solar constant 4.7 billion years ago (when the earth formed) was about 70% of todays value. At the time of the dinasaurs (300 million years ago) it was about 2.5% less than today.
  
• Ocean circulation: Current flow in the ocean moves warm water towards the poles at the surface of the ocean and cooler, saltier water towards the equator. The climate of northern Europe is significantly warmer as a result of this circulation. Climate models show that the world climate would be significantly different if this circulation were to change.
• Feedback as a result of albedo changes: Glacial periods where the earth is partially covered with ice are reinforced partly as a result of positive feed back mechanisms. The more ice, the higher the albedo, the more solar radiation is reflected and the cooler the climate. Increased cloud coverage as the result of greater moisture content due to atmospheric warming would lead to a negative feedback, cooling the climate.
  
• El Nino/ La Nina Southern Oscillation (ENSO): A periodic (3 to 7 year) sloshing of warm water in the pacific ocean causing periodic weather changes in asia and South America.
  
• North Atlantic Oscillation (NOA). A periodic weather pattern similar to ENSO existing between Iceland and the Azores (off the coast of Portugal) which affects the weather in Europe on a short time scale (2 to 5 years).
• The greenhouse effect: The atmosphere is transparent in the visible part of the spectrum, allowing solar radiation (the sun is a blackbody with an electromagnetic radiation peak in the visible spectrum) to enter the atmosphere. The earth re-radiates (as a blackbody) this energy as electromagnetic radiation which peaks in the infra red spectrum. Because of various gases present, the atmosphere is not transparent in the infrared so this energy heats the earth's atmosphere, on average, 33K higher than would otherwise be the case (average surface temperatures would be -18C if there were no atmosphere but 15C with the current atmosphere). Without this amount of atmospheric warming the earth would be uninhabitable.
• Greenhouse gases: The exact mixture and quantities of greenhouse gases (CO₂, N₂O, CH₄, O₃, H₂O, CFC) affect the amount of global warming of the atmosphere due to the greenhouse effect. These change as a result of natural and man made processes; some of these gases may be the result of increasing temperatures (caused by other sources) while other gases may be involved in positive feedback, causing temperature increases which then cause an increase in the presence of that gas.