Geoscience Reference
In-Depth Information
importance of what they do has an increasing influence on climate. The indirect
impact of human activity that results from changing the chemical composition of
the atmosphere is well-recognized and increasingly well-predicted (IPCC, 2007).
However, it is the direct impact of human-induced changes on land surface-
atmosphere exchange processes that concerns us in this chapter. Land surface
exchange processes depend on the nature of the vegetation present, but humans
increasingly change the type of vegetation cover and heterogeneity of land
surfaces, create large urban complexes that replace natural surfaces with artificial
surfaces, and make water more readily available to the atmosphere by irrigating
crops and building dams. As humans do this, the extent to which water, energy,
and momentum is transferred between the atmosphere and the ground
necessarily alters.
In the past, when human population was low, the proportion of the Earth's land
surfaces that we altered was not large, so its consequence when averaged to the
regional or global scales was less important. But even when conditions higher
in the ABL are little-changed, meteorological variables measured (say) 2 m above
the ground are altered if there is a change in local surface roughness and/or local
surface energy balance (e.g., Bastable
et al.
, 1993). The proportion of land subject
to human influence is already extensive (Fig. 25.1a), and for many of the world's
important biomes it will continue to increase (Fig. 25.1b) as human population,
which is currently around 6.5 billion, increases by about 50% by the year 2050.
Influence of land surfaces on weather and climate
The available scientific literature on studies of the influence of land surfaces on
weather and climate is large and diverse. To give structure this overview chapter
is divided into three main sub-sections that consider research into, and evidence
for, the influence on weather and climate of (a) existing land-atmosphere interac-
tions; (b) transient changes in land surfaces; and (c) imposed persistent changes
in land cover. When considering influential mechanisms within these three
classifications, in each case we consider first whether the mechanism has a
plausible physical basis. Then we review the evidence that it actually exists by
discussing relevant analyses of weather and climate data (including reanalysis
data), computer model experiments and any other related experimental and
observational studies. This leads to an assessment of the credibility of the
evidence that the mechanism considered is indeed a way in which land surfaces
influence climate and weather using 'IPCC-like' categories (IPCC, 2007) of
credibility. Finally we make an assessment of how well this influential mechanism
is currently quantified and represented in models. For convenience these
judgments on the evidence that a particular influential mechanism exists and
the adequacy of our current understanding and modeling of each influence are
tabulated at the end of the chapter. The goal of this table is to give guidance on
priorities by identifying where future research is needed.
