There are six primary causes for climate change on Earth: 1) changes in the sun’s output 2) changes in the position of the Earth’s landmasses 3) changes in the Earth’s orbit 4) changes in Earth’s atmospheric composition 5) changes in ocean circulation 6) changes in the reflective properties of the Earth. The first three of these occur independently of changes in the Earth’s climate (that is, they affect the Earth’s climate, but are not themselves affected it). The factors numbered 4-6 are have an interdependent relationship with the Earth’s climate (that is, they affect the Earth’s climate, but are also affected by it). For example, ocean circulation is strongly influenced by Earth’s surface winds (a component of climate), and water-temperature differences (which are related closely to atmospheric temperature). An explanation of the six causes of climate change follows below.
1) Changes in the sun’s output occur over both long and short periods of time. The sun now emits roughly 30% more energy than it did when the Earth formed approximately 4.6 billion years ago. However, the Earth’s temperature has remained remarkably stable over this time period because the Earth’s atmosphere has also changed. Sometime within the next 4 to 5 billion years, the sun will grow into a red giant star and will either engulf the Earth, or heat it beyond the range of life. Shorter cycles in radiation intensity also occur, including an 11-year cycle, during which higher solar output is associated with higher numbers of sunspots. Longer periods of variation in sunspots also exist, although more lengthy periodic cycles have not been fully identified.
2) The theory of plate tectonics explains how continental landmasses move around the globe at a rate of a few inches or less per year. Since land areas heat up and cool down more quickly than oceans, changes in the positions of continents can therefore affect climate over time periods of millions of years. For example, 260 million years ago, large seasonal variations in temperature existed over the super-continent called Pangea, which was made up of all land on Earth. Furthermore, the position of continents greatly influences the pattern of ocean currents, and the Earth’s surface cover and reflectivity, which will be discussed below.
3) Changes in the Earth’s orbit, known as Milankovitch Cycles, occur over time scales of 100-, 41-, and 22-thousand years. Over a period of 100,000 years, the eccentricity of the Earth’s orbit ranges between being nearly circular, when the total solar radiation received by the Earth is roughly constant throughout the year, to a more elongated elliptical path, when radiation varies much more on an annual basis. Over a 41,000 year cycle, the tilt of the Earth on its axis ranges from 22 to 24.5 degrees, causing seasonal temperature variations to be respectively less or more extreme. Finally, over the course of 22,000 years, the direction of tilt of the Earth with respect to the sun varies along its elliptical orbit. For example, at present, the Earth is at the point in its orbit closest to the sun when the Northern Hemisphere is tilted away from it, but 11,000 years ago, the opposite was true. These cycles interact with each other in a complicated manner, but they seem to be the most likely cause of Earth’s glacial episodes (sometimes called Ice Ages).
4) Changes in the Earth’s atmospheric composition determine how much of the sun’s energy is absorbed, stored, and reradiated near the Earth’s surface. Volcanic ash, dust, and other particulate matter block solar radiation and cool the Earth’s atmosphere, as has been observed following major volcanic eruptions. This would also occur following meteorite impacts, which may have even caused mass extinctions in the past. Greenhouse gases, such as carbon dioxide, methane, and water vapor, trap heat in the Earth’s atmosphere, causing global temperature to rise. The concentration of greenhouse gases in the atmosphere change naturally over time as their components are stored in varying amounts in Earth’s different subsystems (for example, rocks, oceans, plants, the atmosphere). Human activity has also sharply increased the concentration of atmospheric greenhouse gas over a very short time period.
5) The ocean and the atmosphere are each responsible for approximately half of global heat circulation, so the Earth’s climate is strongly affected by ocean currents. Significant changes in ocean circulation, such as El Nino, occur over periods of a few years, and much longer cycles are undoubtedly significant.
6) Changes in the reflective properties of the Earth occur as a result of changes in land cover and cloud cover. Ice, snow, and low clouds tend to reflect radiation back to space and cause the Earth to cool. Vegetation and dark surfaces tend to absorb solar radiation and re-radiate it into the atmosphere as heat, warming the Earth. Clearly, the amount of ice, snow, cloud, and vegetation cover is dependent upon the Earth’s initial climate, so there are complicated feedback mechanisms that occur between the climate system and the Earth’s reflective properties.
The Earth’s climate system is extremely complex, and although the six main factors that cause the Earth’s climate to change have been discussed here, none is fully understood. Furthermore, these factors interact in extremely complicated ways, making accurate prediction of future climate change extremely difficult. The combination of this uncertainty with the fact that the Earth’s climate changes naturally has fueled much of the debate surrounding issues of the role of humans global warming.