Geoscience Reference
In-Depth Information
neutrons produced by cosmic rays (Zreda
et al.
, 2008; Shuttleworth
et al.
, 2010), and
at regional scales using satellite systems (Kerr
et al.
, 2001; Entekhabi
et al.
, 2010).
Future research that exploits these new measurement techniques is, therefore,
a priority, and will likely give improved understanding and modeling of this
land-atmosphere feedback mechanism, leading to an assessment of 'good.
2.
Effect of transient changes in vegetation cover
Because water often leaves soil and enters the atmosphere via transpiration from
plant leaves, the status of the vegetation in terms of leaf cover and plant vigor can
influence the surface energy balance and, through this, weather and climate.
Moreover, living plants can intervene in subterranean flow processes by modifying
soil characteristics or by redistributing water vertically in the soil through the
body of the plant, thus changing the ease and extent to which soil water is
accessible to the atmosphere. The influence of transient changes in vegetation
vigor on weather and climate is therefore physically plausible.
As discussed in detail in Chapter 23, there is long-established evidence of
seasonal changes in evaporation (and therefore in the surface energy balance)
through the growth cycle of annual crops. A common approximate representation
(Allen
et al.
, 1998) has been to assume a seasonal pattern in the crop factor used in
Equation (23.12), see Fig. 23.2. However, most of the evidence for there being
an effect of vegetation cover on climate and weather is derived from modeling
studies, and it takes the form of a simulated difference in model-calculated climate
with and without representation of seasonal changes in leaf area index of the
vegetation covering the ground. The early one-dimensional 'micrometeorological'
land surface models (e.g., Dickinson
et al.
, 1986; Sellers
et al.
, 1986) used in GCMs
(see Chapter 24 for description) took it as self-evident that transient changes in
vegetation cover will influence climate, and they had seasonal variations in the leaf
area index of the vegetation prescribed using look-up tables to accommodate
this. More recently (again see Chapter 24) SVATS have emerged with improved
representation of carbon dioxide exchange (e.g., Sellers
et al.
, 1996; Dickinson
et al.
, 1998; Cox
et al.
, 1998) and this allows simulation of seasonal changes in
simulated leaf area index of vegetation through the year. Such SVATS are some-
times referred to as having
interactive vegetation
. Incorporating interactive
vegetation can result in significant changes in the modeled surface evaporation
and precipitation, see Fig. 25.5. Because remote sensing can be used to provide an
indirect estimate of the amount and vigor of vegetation (see Fig. 5.6 and associated
text), some modeling studies have also considered the effect of introducing
remotely sensed estimates of changing vegetation on modeled climate and
have found that doing so can give significant effects (e.g., Matsui
et al.
, 2005).
It is widely accepted among the scientific community that transient (generally
seasonal) changes in vegetation cover will affect weather and climate and,
consistent with this, this influential mechanism is assessed as being 'extremely
likely' in Table 25.1. Motivated by a desire to include the feedback effects of
vegetation change in response to atmospheric carbon dioxide concentration, over
