 This is a schematic of the climate system. We can think of the climate as representing essentially four subsystems that are coupled together. Among those subsystems are the atmosphere, so that's one of the spheres, and that of course represents the chemistry and the dynamics of that component of the system, the atmosphere. Then we have the hydrosphere, which is all of the water that exists on the face of the earth in liquid form, so that would be the oceans and seas, rivers, lakes, etc. Water that exists in vapor form, water vapor, is part of the atmosphere. Then we have the cryosphere, which is all of the water that exists in the form of snow and ice. That would include mountain glaciers, that would include the major ice sheets, and that would include snow that falls in the extra tropics during the winter. Finally then we have the biosphere, and that represents all living things on the face of the earth. As we'll see in this course, the biosphere does indeed play a key role. It influences the composition of the atmosphere through the global carbon cycle. It influences the surface characteristics of the land, which has implications for climate. Ultimately, what we have are these four systems, the hydrosphere, the atmosphere, the cryosphere, and the biosphere, interacting with each other to form what we call the climate system.