Causes of climate change are broadly categorized as external and internal. These terms, however, are defined relative to the focus of study; stating which components are external or internal to the climatic system depends on the time period and spatial scale being examined, as well as on the phenomena being considered. External causes of climate change do not have to be physically external to the Earth (such as the sun), but do occur outside of the climate system under examination. If our focus is on atmospheric change on a one-week time scale (i.e., the weather), the oceans, land surfaces, biota, and human activities that produce CO2 are all external (i.e., they are not influenced much by the atmosphere in such a short time). If our focus is on 100,000-year ice age interglacial cycles, however, the oceans, ice sheets, and biota are all part of the internal climatic system and vary as an integral part of the Earth's environmental systems. On this longer scale we must also include as part of our internal system the "solid" Earth, which really is not solid but viscous and elastic.
Fluctuations in heat radiated by the sun—perhaps related to varying sunspots—are external to the climate system. Influences of the gravitational tugs of other planets on the Earth's orbit are also external. Human-caused changes in the Earth's climate could not perceptibly alter either one of these cycles. Carbon dioxide and methane levels rise and fall with ice age cycles, meaning they are certainly internal on a 10,000-year time scale. But on a 20-year scale these greenhouse gases become largely an external cause of climate change because small changes in climate have little feedback effect on, for example, humans burning fossil fuels or clearing land.
Changes in the character of the land surface, such as those caused by human activities, are largely external. If vegetation cover changes because of climate change, however, land surface change then becomes internal because changes in plant cover can influence the climate by changing greenhouse gas concentrations, albedo (reflectivity to sunlight), evapotranspiration, surface roughness, and relative humidity (Henderson-Sellers et al. 1993).
Snow and ice are important factors in climate change because they have higher albedo than warmer surfaces and, in the instance of sea ice, can inhibit transfer of heat and moisture between air and wet surfaces. Salinity, which affects changes in both sea ice and the density of seawater (which helps control where ocean waters sink), may also be an internal cause of climatic variation. The sinking and upwelling of ocean waters are biologically significant because the upwelling waters are often nutrient-rich.
Unusual patterns of ocean surface temperature—such as the El Niño—demonstrate the importance of internally caused climatic fluctuations because the atmospheric circulation can change simultaneously with ocean surface temperatures. When the atmosphere rubs on the ocean, the ocean responds by modifying its motions and temperature pattern, which forces the atmosphere to adjust, which changes the winds, which changes the way the atmosphere rubs on the ocean, and so forth (Trenberth 1993). As a result, air and water interact internally in this coupled system like blobs of gelatin of different size and stiffness, connected by elastic bands or springs, all interacting with one another while also being pushed from the outside (by solar, volcanic, or human-caused change).
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