Climate Change Information Sheet 27
New approaches to forestry and agriculture
Forestry and agriculture are important sources of carbon dioxide, methane,
and nitrous oxide. The world's forests contain vast quantities of carbon.
Some forests act as "sinks" by absorbing carbon from the air, while forests
whose carbon flows are in balance act as "reservoirs". At the global level,
deforestation and changes in land use make forests a net source of carbon
dioxide. As for agriculture, it accounts for about 20% of the human-enhanced
greenhouse effect. Intensive agricultural practices such as livestock rearing,
wet rice cultivation, and fertilizer use emit 50% of human-related methane
and 70% of our nitrous oxide. Fortunately, measures and technologies that
are currently available could significantly reduce net emissions from both
forests and agriculture - and in many cases cut production costs, increase
yields, or offer other socio-economic benefits.
Forests will need better protection and management if their carbon dioxide
emissions are to be reduced. While legally protected preserves have a
role, deforestation should also be tackled through policies that lessen the
economic pressures on forest lands. A great deal of forest destruction and
degradation is caused by the expansion of farming and grazing. Other forces
are the market demand for wood as a commodity and the local demand for fuel-wood
and other forest resources for subsistence living. These pressures may be
eased by boosting agricultural productivity, slowing the rate of population
growth, involving local people in sustainable forest management, adopting
policies to ensure that commercial timber is harvested sustainably, and
addressing the underlying socio-economic and political forces that spur migration
into forest areas.
The carbon stored in trees, vegetation, soils, and durable wood products
can be maximized through "storage management". When secondary forests
and degraded lands are protected, they usually regenerate naturally and start
to absorb significant amounts of carbon. Their soils can hold additional
carbon if they are deliberately enriched, for example with fertilizers, and
new trees can be planted. The amount of carbon stored in wood products can
be increased by designing products for the longest possible lifetimes, perhaps
even longer than what is normal for living wood.
Sustainable forest management can generate forest biomass as a renewable
resource. Some of this biomass can be substituted for fossil fuels; this
approach has a greater long-term potential for reducing net emissions than
does growing trees to store carbon. Establishing forests on degraded or
non-forested lands adds to the amount of carbon stored in trees and soils.
In addition, the use of sustainably-grown fuel-wood in place of coal or oil
can help to preserve the carbon reservoir contained in fossil fuels left
Agricultural soils are a net source of carbon dioxide - but they could
be made into a net sink. As much as 400800 million tonnes of carbon
could be taken up by agricultural soils every year through improved management
practices designed to increase agricultural productivity. Low-tech strategies
include the use of composting and low- or no-tillage practices, since carbon
is more easily liberated from soil that is turned over or left bare. In
the tropics, soil carbon can be increased by returning more crop residues
to the soil, introducing perennial (year-round) cropping practices, and reducing
periods when fallow fields lie bare. In semi-arid areas, the need for summer
fallow could be reduced through better water management or by the introduction
of perennial forage crops (which would also eliminate the need for tillage).
In temperate regions, soil carbon could be increased by the use of more animal
manure. One recent study suggests that reduced tillage practices alone could
convert US agricultural soils from an estimated net source of 200 million
tonnes of carbon per year to a net sink of 200300 million tonnes by
the year 2020, while improving yields for some crops.
Methane emissions from livestock could be cut with new feed mixtures.
Cattle and buffalo account for an estimated 80% of annual global methane
emissions from domestic livestock. Additives can increase the efficiency
of animal feed and boost animals' growth rates, leading to a net decrease
of 515% in methane emissions per unit of beef produced. In rural
development projects in India and Kenya, adding vitamin and mineral supplements
to the feed mixture of local dairy cows has significantly increased milk
production and decreased methane emissions. Laboratory experiments with bovine
somatotropin, a growth hormone for cows, have increased milk production in
dairy cows while reducing methane emissions by up to 9%.
Methane from wet rice cultivation can be reduced significantly through
changes in irrigation and fertilizer use. Some 50% of the total cropland
used to grow rice is irrigated. Today's rice farmers can only control flooding
and drainage in about one-third of the world's rice paddies, and methane
emissions are higher in continually flooded systems. Recent experiments suggest
that draining a field at specific times during the crop cycle can reduce
methane emissions by up to 50% without decreasing rice yields. Additional
technical options for reducing methane emissions are to add sodium sulfate
or coated calcium carbide to the urea-based fertilizers now in common use,
or to replace urea altogether with ammonium sulfate as a source of nitrogen
for rice crops.
Nitrous oxide emissions from agriculture can be minimized with new fertilizers
and practices. Fertilizing soils with mineral nitrogen and with animal
manure releases N2O into the atmosphere. By
increasing the efficiency with which crops use nitrogen, it is possible to
reduce the amount of nitrogen needed to produce a given quantity of food.
Other strategies aim to reduce the amount of nitrous oxide produced as a
result of fertilizer use and the amount of
N20 that then leaks from the agricultural
system into the atmosphere. One approach, for example, is to match the timing
and amount of nitrogen supply to a crop's specific demands. Another is to
use advanced fertilization techniques such as controlled-release fertilizers
and systems that deliver fertilizer to the plant's roots through its leaves
rather than through the soil (where most nitrous oxide production occurs).
The fertilizer's interactions with local soil and climate conditions can
also be influenced by optimizing tillage, irrigation, and drainage systems.