Green surprise? How terrestrial ecosystems could affect earth's climate? Flashcards
What climatic conditions can strongly influence plant and animal species?
Rainfall, seasonal water
balance, the length of growing seasons, and winter
temperature.
While it may be obvious that the climate affects terrestrial
ecosystems, terrestrial ecosystems can also affect the
climate, particularly though 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.
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.
Modify the surface
albedo 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.
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.
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 long wave (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.
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
The “vegetation feedback” on global warming.
(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.
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…
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.
