The earth has only had its current atmosphere for about 400 million years. In that time, Earth has had climates ranging from being almost completely covered in ice to times when even the poles were subtropical. The current global average temperature is 59Â°F (15Â°C, 288K), but it is not staying constant. A few main factors and feedbacks are the driving forces for climate change.
When volcanoes erupt, they eject tons of sulfur dioxide and ash into the air. Both of these substances reflect solar energy back into space, leading to global cooling. This cooling normally peaks 1 to 3 years after the eruption occurs. Volcanoes also expel water vapor and carbon dioxide. These greenhouse gases actually cause warming long after the ash and sulfur dioxide have settled. Volcanic eruptions helped build up the earthâ€™s atmosphere and the current global average temperature to current levels. The strongest effects come from high-sulfur eruptions from tropical volcanoes like Krakatoa rather than sideways ash eruptions from more polar volcanoes like Mount St. Helens.
Three aspects of the earthâ€™s orbit that have an affect on climate are eccentricity, precession and tilt. Eccentricity is the shape of the earthâ€™s orbit around the sun. The orbit of the earth is not a perfect circle. The way it is now, the earth is farther from the sun in summer and closer to the sun in winter. If the shape of the orbit shifts, the earth could be closer to the sun in summer than winter, which would make the difference between the seasons even more extreme. The tilt of the earth causes the seasons. If the earth is tilted more, the seasons will be more pronounced and the difference between the two hemispheres would be greater. If it was tilted less, then the seasons would be less pronounced. The precession of the earth is how much the earth wobbles on its axis, like a top thatâ€™s starting to slow down. This can affect how much solar radiation reaches different areas of the earth at different times. Also see Milankovitch cycles.
The sun doesnâ€™t constantly give off the same amount of energy. You may have heard of the 11 year solar cycle. About every 11 years, the sun goes through a period where it emits more energy than normal through solar flares and electromagnetic storms. These solar storms affect our satellite and cell phone signals and can also coincide with slight variations in temperature. The Old Farmerâ€™s Almanac uses the solar cycles in their long-term weather forecasts. There are longer, more irregular solar cycles that have a larger effect on global climate. During the peak of the â€śLittle Ice Age (1350-1850)â€ť there was a large dip in solar activity from 1645-1715 called the Maunder Minimum. There were almost no sunspots, sources of very concentrated bursts of solar energy, which exacerbated the little ice age. The exact link between solar cycles and climate has not been well established. Also see Solar Cycles.
In this day and age, greenhouses gases are the most well known cause of climate change. Greenhouse gases keep solar energy trapped in our atmosphere, which keeps the earth warmer in the absence of other feedbacks. If there is a higher concentration of these gases, there will be a higher level of warming. Even before the Industrial Revolution, when we began releasing copious amounts of greenhouse gases through burning coal and fossil fuels, natural sources of greenhouse gases were shaping climate. As mentioned previously and in Greenhouse Gases, volcanic eruptions and various biological processes are the main natural producers of greenhouse gases. There is a theory that as one of the ice ages was ending a crack in an ice sheet released literally tons of natural gas (~87% methane), causing an abrupt rise in temperature and ice melt. There are also natural processes that remove greenhouse gases from the atmosphere. Carbon dioxide is removed from the atmosphere when plants use it during photosynthesis, when the chemical weathering of rocks breaks it down and when the oceans absorb it. When more greenhouse gas is removed from the atmosphere than added to it, the global climate cools.
Climate change is much more than just changing the atmosphere. The earth operates with a set of cycles with water, vegetation, atmosphere, rocks and landforms, and other properties, and all of these cycles are interconnected. These connections result in what we call "feedbacks."
A positive feedback is one that results in an increase in the original signal due to interactions between cycles. A simple example of a positive feedback occurs when you put a microphone in front of a speaker. The resulting loud squeal is an increase in the original sound due to running it through an amplifier and re-emitting it. On Earth, a positive feedback could occur when ice at high latitudes is melted, lowering the albedo and allowing more sunlight to heat the dark earth. This increases the speed of ice melt and subsequent heating.
A negative feedback is one that results in a dampening of the original signal. On Earth, if you warm up the oceans, more water will be evaporated from the surface water. This water vapor is likely to result in clouds, which will shade the surface of the ocean and act to cool it, reducing the original heating.
There are many feedbacks, both positive and negative, occurring at any time on the surface of the earth. That is what makes climate modeling so difficult, since many of these feedbacks are not well understood. Further feedbacks can occur , for example, when the circulation of the deep ocean is involved, since it interacts with the atmosphere in polar regions. Some recent science fiction movies have examined the climatic impacts of the deep ocean circulation slowing down or changing significantly. Our scientific understanding of these processes is not yet well developed and so it is difficult to determine what the effects of these more extreme occurrences might be and how likely they are to occur.