External and Internal Influences
To better organize the factors that relate to climate, we can define internal influences and external influences of the climate system. External influences, which are illustrated in Figure 1 by arrows crossing the exterior boundary of the domain, are those that do not respond to changes occurring within the climate system we have defined. For instance, fluctuations in solar emission and variations in the earth's orbital parameters are external influences since they do not change if the earth's climate warms or cools. Anthropogenically produced greenhouse gases and dust and changes of albedo of the earth's surface due to human activity are generally considered external influences, although it could be argued that humans are a part of the larger definition of climate.
Internal conditions include such factors as variations in ocean surface temperature, changes in reflectivity of the surface due to seasonal changes in vegetation or snow cover. Ocean water salinity (saltiness) also is internal to the climate system because it depends on the local amount of rainfall (which dilutes the salt content of surface water) or flow of rivers into the ocean. We will see later that, because salt water is more dense than fresh water, salinity is a very important factor in driving ocean circulation. Fresh water from precipitation or melting sea ice tends to float on top of salty water and suppress the mixing to deeper layers.