IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Working Group I: The Physical Science Basis Spatial Dependence

Drivers of the land climate system have larger effects at regional and local scales than on global climate, which is controlled primarily by processes of global radiation balance. Myhre et al. (2005) point out that the albedo of agricultural systems may be only slightly higher than that of forests and estimate that the impact since pre-agricultural times of land use conversion to agriculture on global radiative forcing has been only –0.09 W m–2, that is, about 5% of the warming contributed by CO2 since pre-industrial times (see Chapter 2 for a more comprehensive review of recent estimates of land surface albedo change). Land comprises only about 30% of the Earth’s surface, but it can have the largest effects on the reflection of global solar radiation in conjunction with changes in ice and snow cover, and the shading of the latter by vegetation.

At a regional scale and at the surface, additional more localised and shorter time-scale processes besides radiative forcing can affect climate in other ways, and possibly be of comparable importance to the effects of the greenhouse gases. Changes over land that modify its evaporative cooling can cause large changes in surface temperature, both locally and regionally (see Boxes 7.1, 7.2). How this change feeds back to precipitation remains a major research question. Land has a strong control on the vertical distribution of atmospheric heating. It determines how much of the radiation delivered to land goes into warming the near-surface atmosphere compared with how much is released as latent heat fuelling precipitation at higher levels. Low clouds are normally closely coupled to the surface and over land can be significantly changed by modifications of surface temperature or moisture resulting from changes in land properties. For example, Chagnon et al. (2004) find a large increase in boundary layer clouds in the Amazon in areas of partial deforestation (also, e.g., Durieux et al., 2003; Ek and Holtslag, 2004). Details of surface properties at scales as small as a few kilometres can be important for larger scales. Over some fraction of moist soils, water tables can be high enough to be hydrologically connected to the rooting zone, or reach the surface as in wetlands (e.g., Koster et al., 2000; Marani et al., 2001; Milly and Shmakin, 2002; Liang et al., 2003; Gedney and Cox, 2003).

The consequences of changes in atmospheric heating from land changes at a regional scale are similar to those from ocean temperature changes such as from El Niño, potentially producing patterns of reduced or increased cloudiness and precipitation elsewhere to maintain global energy balance. Attempts have been made to find remote adjustments (e.g., Avissar and Werth, 2005). Such adjustments may occur in multiple ways, and are part of the dynamics of climate models. The locally warmer temperatures can lead to more rapid vertical decreases of atmospheric temperature so that at some level overlying temperature is lower and radiates less. The net effect of such compensations is that averages over larger areas or longer time scales commonly will give smaller estimates of change. Thus, such regional changes are better described by local and regional metrics or at larger scales by measures of change in spatial and temporal variability rather than simply in terms of a mean global quantity.

Box 7.2: Urban Effects on Climate

If the properties of the land surface are changed locally, the surface net radiation and the partitioning between latent and sensible fluxes (Box 7.1) may also change, with consequences for temperatures and moisture storage of the surface and near-surface air. Such changes commonly occur to meet human needs for agriculture, housing, or commerce and industry. The consequences of urban development may be especially significant for local climates. However, urban development may have different features in different parts of an urban area and between geographical regions.

Some common modifications are the replacement of vegetation by impervious surfaces such as roads or the converse development of dry surfaces into vegetated surfaces by irrigation, such as lawns and golf courses. Buildings cover a relatively small area but in urban cores may strongly modify local wind flow and surface energy balance (Box 7.1). Besides the near-surface effects, urban areas can provide high concentrations of aerosols with local or downwind impacts on clouds and precipitation. Change to dark dry surfaces such as roads will generally increase daytime temperatures and lower humidity while irrigation will do the opposite. Changes at night may depend on the retention of heat by buildings and can be exacerbated by the thinness of the layer of atmosphere connected to the surface by mixing of air. Chapter 3 further addresses urban effects.