Read the following and tell me the key points and arguments and important info it’s trying to convey: "Climatic conditions, including rainfall, seasonal water
balance, the length of growing seasons, and winter
temperatures, can strongly influence plant and animal
species (Prentice et al. 1992; Woodward et al. 1995).
Changes in climate may therefore have a profound impact
on terrestrial ecosystems worldwide. Many studies have
shown how climatic variations in the recent geologic past
have caused major shifts in the composition and distribution of terrestrial ecosystems (eg COHMAP 1988). The
possibility of future climate change, fueled by increases in
greenhouse gases, has also spurred speculation on the
future makeup of the biosphere (Cramer et al. 2001).
While it may be obvious that the climate affects terrestrial ecosystems, terrestrial ecosystems can also affect the
climate, particularly through their vegetative cover and
soils. This may happen by means of processes that are both
biophysical (ie changes in water, energy, or momentum
balance) and biogeochemical, including changes in the
proportion of important trace gases, such as CO2 and
methane, in the atmosphere.
We have become increasingly aware of this two-way
link. A new generation of computer models has been
developed over the past decade to analyze this coupled behavior (eg Foley et al. 1998, 2000; Cox et al. 2000;
Delire et al. in press). These models are useful for examining how variations in the structure and functioning of terrestrial ecosystems can affect the evolution of the climate
system, either through human land-use practices or the
impacts of global climate change. The atmosphere responds to the exchange of energy,
water, and momentum from the land, ocean, or ice. Any
change in this surface flux can strongly affect atmospheric
thermodynamics and circulation – changes such as an
increase in ocean temperatures during an El Niño event,
the collapse of a massive ice sheet, or the replacement of
a tropical rainforest with pasture.
Changes in land use and cover can also affect biophysical
surface fluxes in several ways. First, they can modify the surface albedo (the fraction of solar radiation that is reflected),
thereby changing the energy balance and the surface temperature. This, in turn, affects how the surface cools itself,
by shifting the balance between sensible heat loss (the cooling of a warm surface by the wind) and latent heat loss
(cooling through evapotranspiration). Finally, vegetative
height and density affect the roughness of the land surface,
which itself influences the mixing of air close to the ground.
Rough surfaces mix air more efficiently, enhancing both
cooling processes. Changes in albedo, surface roughness,
and the ratio between sensible and latent heat loss can all
affect surface fluxes and, as a result, modify the climate.
Much of the world’s land surface has already been
cleared for agriculture or human settlements. Together,
croplands, pasture, and urban areas cover nearly 35%
(about 55 million km2
) of the continental surfaces
(Ramankutty and Foley 1999; Ramankutty et al. unpublished). To a large extent, the only two remaining areas to
be exploited for human use, excluding the desert and polar
regions, are the tropical rainforests of South America,
Africa, and Southeast Asia, and the boreal forests of Canada and Russia (Figure 1). Pressures to develop these
last frontiers are increasing, especially as growing human
populations continue to demand more and more forest
and agricultural products.
The effects of tropical deforestation on climate have
been analyzed many times in the past two decades
(Dickinson and Henderson-Sellers 1988; Lean and
Warrilow 1989; Costa and Foley 2000). Typically, the climatic impacts of tropical deforestation have been evaluated using a global climate model (GCM), linked to a biophysical land surface model that explicitly represents the
characteristics of changing vegetation cover (eg changes
in canopy height, leaf density, or rooting depth).
According to most climate modeling studies, large-scale
patterns of tropical deforestation will probably cause a considerable increase in surface temperature and a decrease in
annual rainfall and evapotranspiration (Bonan 2002; Costa
in press). These changes result from fundamental shifts in
the surface energy, water, and momentum balance that
accompany deforestation (Figure 2). In general, replacing a
tropical forest with a grassland or pasture increases albedo
and decreases roughness, green leaf area index, and vegetation rooting depth (Costa and Foley 2000).
Changes in temperature as a result of tropical deforestation can affect both surface energy and water budgets.
First, the increase in albedo tends to cool the surface, by
reducing the amount of solar radiation it can absorb.
However, surface roughness, leaf area, and root depth are
lower in pastures than in forests; this dramatically reduces
evapotranspiration from the smoother surface, which in
turn substantially increases its temperature. As a result,
the cooling effect of the higher albedo is completely offset,
and often surpassed, by the reduction in evaporative cool ing. The net effect is a warming of approximately 1–2 ˚C
in tropical regions undergoing large-scale deforestation
(Costa and Foley 2000).
The reduction in precipitation in these huge cleared
areas is also a consequence of the changes in the energy
and water balance. First, the reduction in absorbed solar
radiation and the increase in surface temperature both lead
to a decrease in the net radiative heating of the land
surface, defined as the difference between absorbed solar
radiation and outgoing longwave (infrared) radiation. This
leaves less energy to fuel atmospheric circulation, which
ultimately results in a cooling of the upper atmosphere,
followed by subsidence (sinking air) and less precipitation
over the deforested region (Eltahir 1996). Second, the
reduction in surface roughness, leaf area, and root depth
dramatically limits how much water vapor can be recycled
into the atmosphere locally through evapotranspiration –
an important component of the hydrologic cycle of tropical rainforests (Salati et al. 1979; Eltahir and Bras 1994;
Costa and Foley 1999). By reducing evapotranspiration,
deforestation results in less water being pumped into the
atmosphere, thereby contributing to the decrease in
rainfall.
The effects of deforestation on the tropical climate may
enhance the impacts of global warming. Costa and Foley
(2000) suggested that the increases in temperature associated with deforestation in the Amazon basin may be
around 1.4 ˚C, compared to a warming of approximately
2.0 ˚C that would be expected from a doubling of atmospheric CO2. They also showed that tropical deforestation
would lead to a substantial decrease in rainfall over the
Amazon basin, while global warming might increase rainfall. The net effect of global warming and widespread deforestation in the Amazon may therefore be a significantly warmer and somewhat drier local climate.
While the impacts of tropical deforestation have
received considerable attention, other studies have shown
that land-use and land-cover change in semi-arid (Zeng et
al. 1999; Wang and Eltahir 2000a, 2000b, 2000c, 2000d),
temperate (Copeland et al. 1996; Bonan et al. 1997, 1999;
Reale and Dirmeyer 2000; Reale and Shukla 2000; Heck et
al. 2001) and boreal ecosystems (Bonan et al. 1992, 1995)
can also greatly affect the climate.
In addition, several modeling exercises have examined
the consequences of global-scale land-cover changes on
climate systems (eg Betts 1999; Bounoua et al. 2002;
Brovkin et al. 1999; Zhao et al. 2001). These studies have
shown that such changes can substantially affect the climate over large regions – sometimes more than the
changes expected from global warming. These effects are
mainly confined to a few geographic regions, however.
Characterizing the climatic significance of land-cover
change is not as easy as discussing global warming. There
are no simple generalities; some regions become warmer
while others become colder, some become wetter as others
become drier. As a result, the impacts on the global climate are fairly small on average. Unlike the warming
effect of increasing greenhouse gases, these variations are
extremely dependent on geography, and cannot simply be
reduced to an average value. Any assessments of future climate change should therefore consider both the impacts
of greenhouse gases and land-use practices on local,
regional, and global scales. What about indirect effects on vegetation cover that
might result from global warming? Could shifting plant
patterns also affect the climate, and would these changes
amplify or reduce the warming effects?
Biophysical feedbacks can occur through the two-way
interactions between climate and vegetation cover
(Figure 3). For example, global warming may affect the
structure and distribution of terrestrial ecosystems around
the world. These climate-induced changes may affect biophysical processes at the land–atmosphere boundary and,
as a result, create a positive or negative feedback effect.
A new suite of computer models has recently been
developed to consider the interactions between vegetation and climate. Most of these models are based on existing GCMs of the atmosphere, linked to representations of
land surface processes and global vegetation dynamics
(Foley et al. 1998, 2000). These models build on a long
tradition of global climate modeling and the more recent
inclusions of detailed, mechanistic models of surface biophysical processes. Coupled climate–vegetation models
are still in the early development stages, however, and their representation of ecological processes (including
plant growth and competition, succession, and disturbance) remains somewhat crude.
Levis et al. (1999, 2000) used a fully coupled climate–vegetation model to investigate the potential effects of altered
vegetation cover on future climate change. They considered
global warming scenarios in which vegetation patterns
either remained fixed in their current state or were allowed
to vary in response to the shifting climate patterns. By comparing the two scenarios, the authors were able to identify the additional climate variations induced by changing vegetation – the “vegetation feedback” on global warming. They
demonstrated that changes in vegetation cover could substantially amplify global warming in high northern latitude
regions, through a positive feedback process involving several steps (Figure 4):
(1) Increases in greenhouse gases warm the planet, especially in the high northern latitudes.
(2) As the high latitudes begin to warm, the evergreendominated boreal forests of North America and
Eurasia move northward, replacing treeless tundra.
(3) The expansion of evergreen forests, which are much
darker than tundra (especially in the snowy winter and
early spring), causes a significant decrease in albedo.
(4) The darker forest cover absorbs more sunlight than
the tundra, warming the region even more (Bonan et
al. 1992; Foley et al. 1994).
According to Levis et al., a doubling of atmospheric CO2
would warm landmasses between 45˚ and 90˚ N latitude
by about 3.3 ˚C in the spring, even without the vegetation
feedbacks, which would increase the warming by an
additional 1.1–1.6 ˚C. While the impacts of vegetation
feedbacks on global warming may be localized, they are
nonetheless important to helping us understand the future
of the climate system. Terrestrial ecosystems may also affect the climate by changing the amount of CO2 in the atmosphere. These biogeochemical feedbacks occur as a result of the two-way interaction between climate and terrestrial
ecosystems (Figure 5). In particular,
global warming could affect the ability
of terrestrial ecosystems to process C
through photosynthesis and respiration
and store it in biomass and soil organic
matter. Such climate-induced changes
in terrestrial C storage may affect the
amount of CO2 in the atmosphere and,
as a result, create a feedback effect on
the climate.
In the 1990s, humans were releasing
roughly 6 billion tons of C into the
atmosphere annually by burning fossil
fuels (Marland 2000), and another 1–2
billion tons through the burning associated with deforestation (Bolin et al.
2000; Houghton 2000). This C
becomes CO2 in the atmosphere. The
other half is – temporarily at least –
absorbed by the oceans and the terrestrial biosphere (Rayner et al. 1999;
Bousquet et al. 2000; Prentice et al.
2001; Schimel et al. 2001).
As we continue to pump more CO2
into the atmosphere, we should remember that the terrestrial biosphere and
the oceans may not always be able to
absorb such a large amount of our emissions. What if the
terrestrial biosphere suddenly stopped absorbing so much
CO2 – or actually started releasing it instead?
The amount of C absorbed by the biosphere is the difference between how much C plants absorb through photosynthesis and how much is released to the atmosphere by
plant and microbial respiration. In addition, natural disturbances such as fire, blowdowns, insect outbreaks, and
human-caused alterations in land cover, can dramatically
alter terrestrial C storage. Generally speaking, these events
tend to release C from terrestrial ecosystems. Long-term
changes in land use will therefore affect the net C balance.
The terrestrial biosphere appears to have acted as a
net C sink for the last few decades, absorbing roughly
1.4 billion tons of C per year in the 1990s, or about
22% of anthropogenic emissions (Prentice et al. 2001;
Schimel et al. 2001). Will this continue in the future, as
climatic variations become more pronounced? Changes
in climate and atmospheric CO2 concentrations are
likely to alter many aspects of the terrestrial C balance
by affecting the rates of photosynthesis, respiration,
vegetation mortality, and disturbance.
Two recent modeling studies have shown how this
might occur. Research by the UK Hadley Center (Cox
et al. 2000) suggests that global warming could cause
the biosphere to switch from being a net sink to a net
source of CO2. In their simulation, global warming
causes a large reduction in rainfall over the Amazon,
which causes a severe dieback of tropical rainforests by around 2050. Combined with increasing losses from soil
organic material, this dieback releases CO2 back into
the atmosphere and further accelerates global warming.
Overall, the simulation indicates a global average temperature increase of 4 ˚C (global warming alone with
no feedback from terrestrial ecosystems) to 5.5 ˚C
(including the feedbacks).
Another modeling study obtained qualitatively similar
(although less dramatic) results. Friedlingstein et al.
(2001) showed that global warming could reduce the
ability of the biosphere to absorb CO2 by as much as
25%. In this study, the biosphere did not become a net
source of C, but still became much less efficient at
absorbing it.
While these results differ in the details, the central
message is clear. The amount of CO2 in the atmosphere
will be determined not only by human activity, but also
by the terrestrial biosphere and the ocean. Global warming could be substantially accelerated in a warmer world
with more CO2, if the terrestrial biosphere cannot absorb
as much CO2, or terrestrial ecosystems start releasing it
from dying vegetation. As we consider the future of the climate and how it may
be affected by human activities, we must remember that
the atmosphere does not operate in isolation. It is also
affected by the ocean and by terrestrial ecosystems. We are beginning to realize that the biosphere exerts considerable control over the physical systems in the atmosphere and the ocean. A new generation of climate models
that allow for the two-way interactions between physical
and biological systems is starting to show how important
these processes are.
The role of the biosphere cannot be ignored when forecasting the possible climatic changes of the century. We
must take into account the potential for ecological feedbacks on climate, which may enhance or reduce the
effects of global warming. So far, however, only a few
exploratory studies have considered whether changes in
the biosphere could affect global warming. At this stage, it
is impossible to come to any general conclusions about the
magnitude and implications of these feedbacks. It is
intriguing to note, however, that the studies published to
date suggest that ecological processes would amplify the
warming caused by greenhouse gases, not reduce it.
These studies provide a powerful motivation to further examine the interaction between climate and the
biosphere. In particular, we must reevaluate future scenarios of global change to consider the potential for vegetation feedback mechanisms. For example, scenarios of
CO2-induced global warming, already amplified in the
high latitudes by snow and sea-ice feedbacks, may be
substantially modified by long-term changes in the
boundaries of evergreen forests and tundra.
Furthermore, several GCM simulations have indicated
that continental interiors may become much drier in
response to global warming, but this prediction does not
take into account the potential feedbacks caused by
alterations in vegetation cover. Clearly, models used to
simulate the future climate must be improved to take
into account variations in ecosystems, and their consequent feedbacks on the atmosphere.
Future research efforts should be concentrated along
several lines. First, the research community must continue
to develop fully coupled models of climate and terrestrial
ecosystems, which will allow us to explore the interactions
between atmosphere, oceans, ice, and the terrestrial biosphere. Many modeling groups are already moving in this
direction, but a range of models would ideally be used,
including simple analytical models, earth system models of
“intermediate complexity” (simpler, computationally efficient climate models), and the highly complex and computationally expensive global climate models.
We must also put more effort into measuring the effects
of terrestrial ecosystems on climate. For example, can we
document the effects of land-use and land-cover change
on climate? Can we see biophysical vegetation feedbacks
operating in the current climate system, on seasonal or
multi-year timescales? Can we monitor long-term changes
in terrestrial C storage, and how terrestrial ecosystems
respond to long-term climate changes?
Finally, policy analysts must determine how important
the uncertainties ecological processes introduce into future
climate scenarios ultimately are. How will these uncertainties play out in the policy arena, and how are they viewed
through the lens of the “precautionary principle”?