7.2 The Changing Land Climate System
7.2.1 Introduction to Land Climate
The land surface relevant to climate consists of the terrestrial biosphere, that is, the fabric of soils, vegetation and other biological components, the processes that connect them and the carbon, water and energy they store. This section addresses from a climate perspective the current state of understanding of the land surface, setting the stage for consideration of carbon and other biogenic processes linked to climate. The land climate consists of ‘internal’ variables and ‘external’ drivers, including the various surface energy, carbon and moisture stores, and their response to precipitation, incoming radiation and near-surface atmospheric variables. The drivers and response variables change over various temporal and spatial scales. This variation in time and space can be at least as important as averaged quantities. The response variables and drivers for the terrestrial system can be divided into biophysical, biological, biogeochemical and human processes. The present biophysical viewpoint emphasizes the response variables that involve the stores of energy and water and the mechanisms coupling these terms to the atmosphere. The exchanges of energy and moisture between the atmosphere and land surface (Boxes 7.1 and 7.2) are driven by radiation, precipitation and the temperature, humidity and winds of the overlying atmosphere. Determining how much detail to include to achieve an understanding of the system is not easy: many choices can be made and more detail becomes necessary when more processes are to be addressed.