1.5 The land surface

As discussed above, many characteristics of the climate are influenced by the distribution and topography of land surface. For instance, mountain chains such as the Andes or the Rocky mountains (Fig 1.21) are formidable barriers to the westerly winds that influence the climate on a continental scale. Mountains also have an important role at the hemispheric scale, by affecting planetary waves and the global atmospheric circulation (section 1.2.2). The distance to the coast influences the temperature and aridity of a region. The presence of land boundaries to the ocean (and more generally the ocean bathymetry) affects the location of the strong western boundary currents and of the straits that allow water exchanges between the different basins (section 1.3.2). The shape and even the existence of an ice sheet is strongly conditioned by the underlying bedrock (section 1.4.1).

In addition to the influence of the land geometry, the type of vegetation present on land also has a critical influence on climate at all spatial and temporal scales. One of the most important roles of terrestrial vegetation is related to its albedo (Fig 1.22), (see section 2.1.4). Vegetation usually has a lower albedo than soil (Table 1.3), in particular much smaller than that of deserts. This is why subtropical deserts such as the Sahara appear as regions of particularly high albedo on global maps (Fig 1.22). A maximum is also observed at high latitudes because of the presence of snow and ice. At these latitudes, the vegetation modulates the influence of the snow. In the absence of vegetation or in the presence of low-growing vegetation such as grass, the snow can cover the whole area, leading to highly reflective white areas with a high albedo. If snow falls on a forest, relatively dark trunks, branches and possibly needles or leaves will partially emerge from the snow, resulting in a much lower albedo than with an homogenous snow blanket.

The terrestrial biosphere also has a clear impact on the hydrological cycle (see section 2.2). Water storage is generally greater in soil covered by vegetation than on bare land where direct runoff often follows precipitation. Stored water can later be taken up by plant roots and transferred back to the atmosphere by evapotranspiration. A third effect of the vegetation cover is related to the surface roughness that influences the stress at the atmosphere-land interface and the turbulent exchanges at the surface (see section 2.1.6). Finally, the role of the terrestrial biosphere in the global carbon cycle will be discussed in section 2.3.3.

Because of this climatic role of vegetation, it is useful to describe the general distribution of the different biomes, which are regions with distinctive large-scale vegetation types (Fig 1.23). Their exact definition, as well as the number of important biomes that are considered, differ from one study to the other. Nevertheless, it is generally considered that the natural biomes can be classified, according to their typical percentage of grass and trees, into five groups: desert, grassland, shrubland, woodland and forest. Cropland and built-up areas can be added to take into account the role of land use associated with human activities.

Deserts are characterised by a very small amount of vegetation. Grassland, as indicated by its name, is mainly covered by grass and lichens. It can be found at various latitudes and includes tundra, steppe and savannah. In shrubland, low woody plants are present in addition to grass. The fraction of trees is higher in woodland, but there are still significant areas covered by grass and often relatively large distances between trees. Finally, in forests, a dense cover of trees is observed (as in tropical rainforest and boreal conifer forest, also called taiga).

We have discussed above how vegetation influences climate, but of course climate also influences vegetation. This leads to powerful feedbacks that will be described in more details in section 4.3.3. The dominant features of the climate are achieved through the distribution of incoming solar radiation, temperature and precipitation. If precipitation and/or temperature are too low, desert biomes dominate (as in the Sahara or Antarctica). At higher temperatures, forests can be maintained if a sufficient supply of water by rainfall is available. Between those two extremes, different combinations of grass and trees are found (see also Fig. 3.9).