7.2.2 Dependence of Land Processes and Climate on Scale
7.2.2.1 Multiple Scales are Important
Temporal variability ranges from the daily and weather time scales to annual, interannual, and decadal or longer scales: the amplitudes of shorter time scales change with long-term changes from global warming. The land climate system has controls on amplitudes of variables on all these time scales, varying with season and geography. For example, Trenberth and Shea (2005) evaluate from climatic observations the correlation between surface air temperature and precipitation, and find a strong r > 0.3) positive correlation over most winter land areas (i.e., poleward of 40°N) but a strong (|r| > 0.3) negative correlation over much of summer and tropical land. These differences result from competing feedbacks with the water cycle. On scales large enough that surface temperatures control atmospheric temperatures, the atmosphere will hold more water vapour and may provide more precipitation with warmer temperatures. Low clouds strongly control surface temperatures, especially in cold regions where they make the surface warmer. In warm regions without precipitation, the land surface can become warmer because of lack of evaporation, or lack of clouds. Although a drier surface will become warmer from lack of evaporative cooling, more water can evaporate from a moist surface if the temperature is warmer (see Box 7.1).
Box 7.1: Surface Energy and Water Balance
The land surface on average is heated by net radiation balanced by exchanges with the atmosphere of sensible and latent heat, known as the ‘surface energy balance’. Sensible heat is the energy carried by the atmosphere in its temperature and latent heat is the energy lost from the surface by evaporation of surface water. The latent heat of the water vapour is converted to sensible heat in the atmosphere through vapour condensation and this condensed water is returned to the surface through precipitation.
The surface also has a ‘surface water balance’. Water coming to the surface from precipitation is eventually lost either through water vapour flux or by runoff. The latent heat flux (or equivalently water vapour flux) under some conditions can be determined from the energy balance. For a fixed amount of net surface radiation, if the sensible heat flux goes up, the latent flux will go down by the same amount. Thus, if the ratio of sensible to latent heat flux depends only on air temperature, relative humidity and other known factors, the flux of water vapour from the surface can be found from the net radiative energy at the surface. Such a relationship is most readily obtained when water removal (evaporation from soil or transpiration by plants) is not limited by availability of water. Under these conditions, the increase of water vapour concentration with temperature increases the relative amount of the water flux as does low relative humidity. Vegetation can prolong the availability of soil water through the extent of its roots and so increase the latent heat flux but also can resist movement through its leaves, and so shift the surface energy fluxes to a larger fraction carried by the sensible heat flux. Fluxes to the atmosphere modify atmospheric temperatures and humidity and such changes feed back to the fluxes. Storage and the surface can also be important at short time scales, and horizontal transports can be important at smaller spatial scales.
If a surface is too dry to exchange much water with the atmosphere, the water returned to the atmosphere should be on average not far below the incident precipitation, and radiative energy beyond that needed for evaporating this water will heat the surface. Under these circumstances, less precipitation and hence less water vapour flux will make the surface warmer. Reduction of cloudiness from the consequently warmer and drier atmosphere may act as a positive feedback to provide more solar radiation. A locally moist area (such as an oasis or pond), however, would still evaporate according to energy balance with no water limitation and thus should increase its evaporation under such warmer and drier conditions.
Various feedbacks coupling the surface to the atmosphere may work in opposite directions and their relative importance may depend on season and location as well as on temporal and spatial scales. A moister atmosphere will commonly be cloudier making the surface warmer in a cold climate and cooler in a warm climate. The warming of the atmosphere by the surface may reduce its relative humidity and reduce precipitation as happens over deserts. However, it can also increase the total water held by the atmosphere, which may lead to increased precipitation as happens over the tropical oceans.