|image credit WWF|
Radiant energy output of sun. This has increased by around 25% since the sun was formed, i.e. over more than 4 billion years. Very small variations occur over decades and centuries and bear some relationship to the number of sunspots (which is related to solar activity). The Little Ice Age included a period of low sunspot activity, with no sunspots observed over 1650-1700, when the mean temperature was lowest.
Milankovitch cycle. The amount of sunlight reaching the earth varies according to the changing shape of the earth’s orbit (eccentricity), the tilt of the N-S axis and the precession of this axis. The crucial factor appears to be the amount of sunlight falling on the northern hemisphere in any one year. Milankovitch (1970s, Serbia) showed that the coming and going of ice ages over the last 600,000 years was due to these factors. If there were no other factors we would expect to be entering another ice age now, instead of a warm period.
Heat from below the crust. The biosphere has more heat going into it than can be accounted for by the sun. The difference is believed to be due to radioactive decay in the earth’s core.
Atmospheric composition. The importance of this arises from the way it affects the absorption and reflection of radiation coming in from the sun or being reflected back upwards from the earth’s surface. Carbon is the main gas responsible fo the greenhouse effect – it acts like the glass in a greenhouse to trap in heat. Methane is also a greenhouse gas, much more powerful than CO2 but also much less prevalent.
Plants. While alive these take in carbon dioxide from the air, thereby cooling it through the reduced greenhouse effect. But as they decay they give it out again, but over a longer period. Large areas of trees affect the climate not only in this way but also by their moistening effect on the air.The Amazon rain forest appears to have a pivetol role in determining the global climate.
Animals . The main effect of these is due to the methane from their defecated waste and rotting carcasses. Methane produced in this way is more of a problem than automobile greenhouse emissions as livestock herds grow in response to the westernization of diets in China, India etc.
Microscopic life. Bacteria and spores living in land, sea and air sometimes affect rainfall. E.g. spores in the ocean can be whisked up into the atmosphere by strong winds and dispersed. Here they act as condensation nuclei for the formation of the water drops and thence clouds. Insects can reduce the balance of combustible debris in a forest and this in turn means fewer and smaller forest fires emitting CO2.
Clouds. The type of cloud and its coverage affect the amount of sunlight striking the earth’s surface and the amount of radiation reflected back to its surface instead of radiated away into space. They have been and still are a problem in creating global warming models.
Atmospheric convection. Heat from the ground boils up the air and the convection currents (i.e. wind) produced distribute warm air over the planet and also affect the cloud type, amount and global distribution. This in turn affects temperature, rain and snow. Hurricanes, tornadoes, gales and the jet stream are all driven by atmospheric convection.
Ocean currents. The bulk of the heat in the biosphere which we inhabit is stored in the ocean and it the global currents in the sea (e.g. the Gulf Stream) which determine the global patterns of temperature in the air over the sea.
Methane from seabed and tundra deposits. As the climate warms it releases large bubbles of methane trapped in fozen deposits under the ocean or in tundra. This causes further warming.
Polar ice caps. Both Arctic and Antarctic ice sheets reflect large amounts of heat from the sun back into space. Small reductions in area cause significant increases in the amount of heat absorbed from the solar heat reaching the earth’s surface. Similarly, the greater the ice coverage the more incident radiant heat will be reflected.
Snow cover. As with snow in the polar regions the snow settled on large mountain ranges like the Himalayas and the Alps affects the percentage of sun radiation reflected or absorbed by the earth’s surface.
Melting glaciers. When a glacier melts it not only causes flooding but reduces the area of earth which reflects incident sunlight away from the surface, i.e. the ground retains incident solar energy instead of reflecting it back into space. When melt water flows into the sea it dilutes the concentration of salt in the seawater and this has a major effect on ocean currents which in turn affects the climate. (The more salt the denser the water.)
Volcanoes. Eruptions from these inject huge amounts of sulphur dioxide into the air and, like carbon dioxide and methane, this produces a greenhouse effect. The ash and dust from eruptions also affects cloud formation and directly blots out sunlight.
Meteor impacts. Hits by large meteors can have global repercussions including climate change. If a large enough object hit the earth it would of course cause a mass extinction event, like the one which wiped out the dinosaurs 66 million years ago.
Cosmic rays. These can also affect cloud formation , since the particles which make up cosmic rays can cause nucleation of water drops. There does seem to be some link between them and average temperature/rainfall.
Human activity. Carbon dioxide from fossil fuel burning and methane from livestock farming are the main human source of global warming gases which appear to be tipping the global balance towards warming rather than the cooling we would expect according to the Milankovitch cycle (see above).
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