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Vegetation and Climate The Biosphere is the part of the Earth where living organisms live.
Biomass generally refers to all vegetation on the planet, in the oceans and
on the continents, live or dead. Although biomass has a large influence
on the Earth’s climate, termites, cows, sheep also play a role in the climate
system.
Jan 2004
Jul 2004
The strong connection between surface vegetation and climate was first demostrated in 1947 by Holdridge who showed that potential distribution of global vegetation types depends primarily on two parameters: temperature and precipitation.
The vegetated surface of the Earth is part of the lower boundary of
the atmosphere where important exchanges of heat, radiation,
momentum, moisture, trace gases and aerosols occur.
The wavelength dependence of surface reflectance is also significant.
The chlorophyll in vegetation absorbs strongly at visible wavelengths;
consequently, leaves may reflect only about 0.05 of the solar radiation
shorter than 0.7m, but approximately 0.25 of longer wavelengths.
Soils also increase their reflectance with increasing wavelength, although
more gradually.
Cooling of the boundary
layer is achieved by
evapotranspiration. Under
normal conditions, plant
leaves take in carbon dioxide
from the air and
release moisture as part of
the photosynthesis process.
The release of
moisture, through
evapotranspiration, causes a
cooling of the air, since
energy normally used to heat
the surface is now used to
evaporate water.
The removal of vegetation
results in the warming of the
surface.
The rates of the fluxes of gases are modulated by the ability of roots to extract
water from the soil and by the resistances imposed by the stomatal controls
of leaves. Leaf surfaces are equipped with small openings or pores called
stomata which allow CO2 to enter the leaf and O2 to escape during
photosynthesis. In addition, water is lost through the stomata during
transpiration. It is estimated that 99% of the water absorbed by the roots of
the plant is lost by the leaves via the stomata.
In times of warm climate, when the atmosphere is rich in CO2, plants
need relatively few pores to take in all the CO2 they need, so their leaves
develop comparatively few stomata. In times of cold climate, more
stomata are needed because less CO2 is readily available. The stomata
are also important in conserving the plants moisture in times of dry
conditions. Warmer conditions result in the stomata closing to prevent
the loss of water.
Storage of Carbon:
The absorbed CO2 from the atmosphere is used partially for the
production, within plants, of a wide variety of organic materials. Some,
such as leaves and fine roots, are generally short-lived and become
available to decomposers within a few years of production. Therefore
the carbon is returned to the atmosphere fairly quickly. However, woody
branches and stems of trees can live for decades to centuries, storing the
Carbon in the biosphere instead of in the atmosphere.
Photosynthesis in
C4 plants is 6 times
faster than in C3
plants
Will increasing atmospheric CO2
green the planet?
The biosphere is a major sink for atmospheric CO2. There is a clear annual
cycle in the global CO2 atmospheric concentrations resulting from the
withdrawal and production of CO2 by the terrestrial and oceanic biosphere.
Human use of the landscape generally reduces carbon storage. The
harvesting of forests (deforestation) generates CO2 both from the decay
and burning of wood produces. Conversion from forests or grasslands to
agriculture releases CO2 both through the loss of vegetation and litter
inputs. However, regrowth of forests and the reverting of agricultural land
to native vegetation results in a net sink of CO2 in the atmosphere.
Historical trends
There is an indication that over the last
century the contribution of the terrestrial
biosphere to atmospheric CO2 has been
increasing. However, the main source of
this increase is from Latin America and
tropical Africa resulting from land use
conversion, as well as deforestation. The
contribution from Europe and North
America is actually decreasing over time.
It is estimated that 20-40% of the increasing
trend in CO2 results from changes in the
biosphere. It is also believed that only within
the last 60 years has the input from the
burning of fossil fuels been greater than the
input from terrestrial ecosystems. The
largest single component of the Carbon flux
from the biosphere is presently due to tropical
deforestation.
Land Use change
Increasing atmospheric CO2 trend
The land biosphere may be responsible for the uptake of about 1/3 of all
the CO2 that is released into the atmosphere. On an annual basis, land
vegetation removes from the atmosphere about 100 Gt of carbon, compared
to the 6 Gt released by the burning of fossil fuels. However, about the
same flux of Carbon returns to the atmosphere from the respiration of
live plants and the decay of dead ones. The gross annual flux of Carbon
through the biosphere is very similar to that of the world’s oceans.
But on time scales of decades or more, the CO2 concentrations are
controlled mainly by the exchange with the oceans. Deforestation and
other conversions and uses of land surface are thought to contribute a
net flux of 1-2 Gt of carbon per year to the atmosphere, 20-40% of the
contribution due to the combustion of fossil fuels.
The biosphere is also a source region for two other important greenhouse
gases: methane (CH4) and nitrous oxide (N2O). Both methane and nitrous
oxide are increasing in the atmosphere. The major anthropogenic sources
of N2O are from fertilization of agriculture.
The production of CH4 in terrestrial ecosystems is limited to highly
reduced conditions, as are found in wetland soils and sediments, and the
digestive system of animals. On a global scale methane production is
dominated by fluxes from natural wetlands and bog areas, and agricultural
wetlands such as rice paddies. Anthropogenic sources of CH4 contribute
345 Gt CH4 to the atmosphere annually, compared to natural sources of
180 Gt. Rice paddies are
the most important
anthropogenic source of
methane, followed by
biomass burning
(especially in the tropics),
gas and coal drilling, and
landfills.
Methane concentrations are increasing in the atmosphere at a rate of nearly
1% per year. The rate has slowed in recent years. There is a risk that large
regions of permanently frozen soil (permafrost) in high latitude boreal and
tundra regions may become a major additional source of CH4 if
temperatures warm in these regions. In many areas, deep layers of peat
exist below these frozen soils. Higher soil temperatures and longer frost-
free seasons could increase the CH4 emissions significantly.
Aerosols
The biosphere is also a major source of aerosol particles that can be used
as cloud condensation nuclei, and ice nuclei in the production of
precipitation. In addition, these particles can interfere with the direct
incoming solar radiation to reduce the radiation at the surface (cooling
effect). The oceanic biosphere plays a major role in the production of
sulfate aerosols. Through the production of Dimethyl Sulfide (DMS),
which are the dominant aerosols in the atmosphere. Release of gases such
as isoprene from vegetation can also result in the formation of organic
aerosols. Finally, burning of biomass results in large amounts of aerosols
pumped into the atmosphere on seasonal time scales.
Possible consequences of future climate change on vegetation:
Long term climate change (over thousands of years) result in the migration
of species to new climatic regions favourable for the type of vegetation.
Short term rapid climate change (hundreds of years), as is being observed
today, may be too rapid for natural migration of species, resulting in many
species becoming extinct. Natural migration of species occurs at a rate of
tens of kilometers per century, while future climate warming in mid- to
high latitudes would require the species to migrate at rates of hundreds of
kilometers per century.
In addition, there are also close
associations between the
distribution of major vegetation
types and the distribution of
major soil types which may hinder
the free movement of species
boundaries.
Today
2xCO2
Vegetation – Climate Feedbacks
SW-LW
SW-LW
HL
HL
HS
HS
soil
soil
Ts P
Climate effects of
deforestation
Positive feedback due to vegetation changes
BIOTIC RESPONSE TO RECENT CLIMATE CHANGE
Biota
Location
Change
Climate Link
Treeline
Europe, New Zealand
Upward shift
Warming
Arctic Shrub Tundra
Alaska
Spread
Warming
Alpine Plants
Alps
Upward shift
Warming
Biota
Antarctica
Spread
Melting
Zooplankton
Calif. & N. Atlantic
Population increase
Warming
Butterflies (39 spp)
N Amer. & Europe
Northward shift, 200 km
Warming
Birds, lowland
Costa Rica
Expansion upward
Dry-season mists
Birds, migratory
England
Northward shift , km20 Winter Warming
Foxes, red, white
Canada
Northward boundary shift
Warming
Gaia Hypothesis:
James Lovelock suggested that the biosphere always acts in a way to
regulate the Earth’s environment and climate so that it remains in some
type of equilibrium. The adjustments by the biosphere to any perturbation
of the system is done by the process of natural selection (e.g. DMS-CNN-
cloud albedo cycle). However, Gaia does not explain why the biosphere
did not regulate the climate during the ice ages.
Homework
Groenigen et al., 2011: Increased soil emissions of potent greenhouse gases under increased atmospheric CO2, Nature, 475, 214-218.