Download Interactions between climate and desertification

INTERACTIONS BETWEEN
CLIMATE AND DESERTIFICATION
M.V.K. Sivakumar
Agricultural Meteorology Division
World Meteorological Organization
Presentation
• Use of the term “desertification”
• Problem of desertification
• Human impact on drylands
- Overgrazing
- Biomass burning
- Soil erosion
- Irrigation
• Impact of dryland climates on soils, ecosystems, water
balance and land use
• Climate change and desertification
• Recommendations
Use of the term “Desertification”
The term desertification was employed by French forester
Aubreville in 1949. He used the term to refer to the
replacement of tropical rainforest by secondary savanna and
scrub in those parts of Africa where forest was being cleared
and burned to provide land for cultivation.
“true deserts are being born in front of us at present time, in
regions where the rainfall ranges from 700 mm to more than
1500 mm per year”
Use of the term “Desertification”
UNEP’s definition in 1990 attributed all desertification to
human activity:
“land degradation in arid, semi-arid and dry sub-humid
areas resulting from adverse human impact”
UNCCD (1995) definition of desertification allows a role
for
“various factors, including climatic variations and human
activities”.
Hence we should look at two-way interactions
between climate and desertification
• Look at ways in which human activities modify the
surface characteristics and atmospheric composition
of drylands and consider how these may influence
local and regional dryland climates.
• Evaluate the impact of dryland climates on soils,
ecosystems, water balance and human land use in
the dryland regions.
Problem of desertification
•
•
Total land area of the earth – 145 m km2
Drylands occupy 6.31 billion ha (Bha) or 47.2% of
the total land area (UNEP, 1992).
- Africa
- Asia
- Australasia
- North America
- South America
- Europe
2 Bha
2 Bha
0.68 Bha
0.76 Bha
0.56 Bha
0.30 Bha
GLASOD estimates of desertification
(Oldeman and Van Lynden 1998)
Type of degradation
Water erosion
Wind erosion
Chemical degradation
Physical degradation
Total
Area (Bha)
0.478
0.513
0.111
0.035
1.137
Estimate of annual rate of land
degradation in mid latitude drylands
Land Use
Total land
area (Mha)
Irrigated
land
Rangeland
131
Rainfed
cropland
Total
Rate of desertification
Mha y-1 % of total y-1
0.125
0.095
3700
3.200
0.086
570
2.500
0.439
4401
5.825
0.132
Human Impact on drylands - surface and
atmospheric conditions
• Locally severe overgrazing can aggravate the impact of drought and
desertification by modifying soil microclimate, altering soil-waterplant relationships and exposing bare soil to erosion.
• Biomass burning contributes significantly to gross global emissions
of trace gases and particulates from all sources to atmosphere.
• Forest and woodland clearing leads to accelerated soil erosion.
• Expansion of irrigation leads to waterlogging and salinisation,
important agents of desertification.
Impact of Human Activities in Drylands on Climate
• Human
induced changes in dryland surface
conditions and atmospheric composition directly
affect the energy budget of the surface and
atmosphere column.
• Perturbations in energy balance may affect nearsurface and surface temperature in many ways.
Simulated temperature changes caused by desertification
0.3
0.25
0.2
0.1
0.05
(°
C
)
Temperature
0.15
0
-0.05
-0.1
-55
-45
-35
-25
-15
-5
Latitude
5
15
25
35
45
55
Impact of Human Activities in Drylands
on Climate (contd.)
• Human influence on local and regional precipitation levels
has been more difficult to identify.
• Climatic consequences associated with surface changes in
drylands are local as well as often regional. It is difficult to
detect a global-scale climate impact.
Simulated precipitation changes caused by desertification
10.0
5.0
Rainfall (cm/yr)
0.0
-5.0
-10.0
-15.0
-20.0
-25.0
-55
-45
-35
-25
-15
-5
Latitude
5
15
25
35
45
55
Overgrazing in rangelands
•
•
•
Widely considered to be a
major cause of desertification
in rangelands due to depletion
of grass and shrub cover and
ensuring the accelerated loss
of top soil
Attributed to a combination of
overstocking and poor land
management
Aggravated by periodic
droughts
Hypothetical impact of overgrazing and reduced grazing,
on rainfall in drylands (After Krebs and Coe 1985)
Biomass burning and atmospheric emissions
•
Includes wild and prescribed fires in
forested areas as well as annual burns
of savanna, burning of agricultural
waste and use of fuelwood as energy
source
•
Common practice in the tropics and
subtropics
•
Affects Africa all year round, but
particularly prevelant in the dry season
•
Emissions from biomass burning are
considerable and contribute
significantly to gross global emissions
of trace gases and particulates from all
sources to atmosphere.
Estimates of gross global atmospheric emissions from
all sources and biomass burning (Cachier 1992)
Gas or
aerosol
CO2
All sources
(1012 g/yr)
8,700
Biomass burning
1012 g/yr
(%)
3,700
42
CO
1,100
350
32
CH4
500
38
8
NH3
44
5
12
Estimates of biomass burning
•
Biomass burning from savanna fires annually affect an
area about 100 times larger than that from which forest is
cleared.
•
80% of the biomass burning now occurs in the intertropical zone, of which nearly half is from agricultural
burning and use of wood for fuel.
•
Most of the fires are deliberately started by humans.
Biomass burning by region
(Adopted from Andreae 1993)
Region
Africa
South America
Asia
Australia and Oceania
Central America and
Mexico
North America
Europe
%
42.6
26.5
12.5
6.6
6.8
4.2
0.8
Emissions from biomass burning
•
Biomass burning contributes
significant amounts of trace gases
and particulates to the atmosphere.
•
Estimates of smoke emissions from
tropical America, Africa, Asia and
Australia range between 25 and 79
x 1012 g/yr.
•
For a comparison, smoke emissions
from fossil fuel burning range
between 22.5 and 24 x 1012 g/yr,
the dominant source of smoke in
the Northern Hemisphere (Ghan
and Penner 1992).
Agriculture’s contribution to air pollution
Public attention tends to focus on the more
visible signs of agriculture’s impact on the
environment, whereas it seems likely that
the non-visible or less obvious impacts of
air pollution cause the greatest economic
costs
Pretty et al. 2001
Effects of biomass burning
•
Soot, dust and trace gases are released by biomass burning during
forest, bush or rangeland clearance for agriculture.
•
Biomass burning results in globally important contributions to the
atmospheric budget of several trace gases (Crutzen and Andreae
1990).
•
Ozone from biomass burning makes up 38% of all tropospheric
ozone.
•
About 50% of all nitrogen in the biomass is released as N2 during
burning, causing sizeable loss of fixed nitrogen in tropical
ecosystems in the range of 10 to 20 x 1012 g/yr.
•
Trace gases and particulates can modify atmospheric chemistry.
Climate change itself, however, may cause
temperatures to rise in the dry season,
increasing fire risks and thus increasing
pollution from biomass burning in some
areas
Lavorel et al. 2001
Forest and woodland clearing and
accelerated soil erosion
•
•
If the soil surface is left bare through
clearing for agriculture, the erosive
impact of early season convective rains
can increase erosion by a factor of 50
or above the normal long-term
disturbed rate.
Accelerated soil erosion affects the C
pool and fluxes because of breakdown
of soil aggregates, exposure of C to
climatic elements, mineralization of
organic matter in disrupted aggregates
and redistributed soil, and transport of
sediments rich in SOC downslope to
protected areas of the landscape.
Rates of erosion in selected countries (kg/m2/yr)
Country
Natural
Cultivated
Bare soil
China
< 0.20
15-20
28-36
0.01-9
1-75
Cote d’Ivoire 0.003-0.02
Nigeria
0.05-0.1
0.01-3.5
0.3-15
India
0.05-0.1
0.03-2
1-2
Impact of drought and removal of vegetation
on sand and dust transport
•
It has been estimated that in the arid
and semi-arid zones of the world, 24%
of the cultivated land and 41% of the
pasture land are affected by moderate to
severe land degradation from wind
erosion
• The world-wide total annual production
of dust by deflation of soils and
sediments was estimated to be 61 to 366
million tonnes.
• For Africa alone, more than 100 million
tonnes of dust per annum is blown
westward over the Atlantic.
Model to show relationship of drought, human activities
and desertification to increasing duststorm activity
Anthropogenic land disturbances and wind erosion
• Human-induced change is by far the most significant factor in
the alarming increase of dust storms in some regions.
• According to previous studies, wind erosion in the semi-arid
regions of America, Africa, Australia, the Near East and many
parts of Central Asia could only reach threatening proportions
when man disturbed the ecosystem balance.
• Past policies on land-use and the promotion of farming
systems that were unsustainable were the root cause of most
disasters.
Increasing frequency of sand and dust storms
• In recent years, there is evidence that the frequency of
sandstorms is increasing.
• Annual frequency of strong and extremely strong sandstorms
in China
- 5 times in the 1950s
- 8 times in the 1960s
- 13 times in the 1970s
- 14 times in the 1980s
- 20 times in the 1990s.
• Every year desert encroachment caused by wind erosion
buries 210,000 hectares in China ( PRC, 1994).
Wind erosion in the Sahel
• Rapid population growth of 3% during recent decades, has
increased demand for food and farmers have tried to enhance
production by extending cropping to more marginal areas.
• Consequently, over- exploitation has resulted in land
degradation , or desertification, on a large scale.
• The amount of dust arising from the Sahel zone has been
reported to be around or above 270 million tons per year which
corresponds to a loss of 30 mm per m2 per year or a layer of
20mm over the entire area.
Impacts of dust storms on climate
• Dust particles exert a radiative influence on climate directly
through reflection and absorption of solar radiation and
indirectly through modifying the optical properties and
longevity of clouds.
• Dust particles can cause cooling in two ways. Directly,
through radiative influence and indirectly by acting as
condensation nuclei, resulting in cloud formation.
• Dust storms have local, national and international
implications concerning global warming. Climatic changes in
turn can modify the location and strength of dust sources.
Impact of irrigated agriculture on surface
conditions in drylands
•
About 30% of all irrigated lands are
considered to be degraded to varying
degrees.
•
Inadequate drainage and ineffective
leaching of the soil, can cause problems
of water logging and salinisation which
are becoming increasingly frequent.
•
A secondary and equally incidious
problem is the dispersion of sodic soils
leading to a reduction in soil infiltration
capacity and permeability.
•
Salt-affected lands are reflected as saline
seeps in dryland agriculture and
secondarily salinized irrigated lands
(Tanji 1995).
Extent of salinisation in the world
• Total area of saline soils is 397 million ha and of sodic
soils 434 million ha at global level.
• Of the current 230 million ha of irrigated land, 45 million
ha are salt-affected soils (19.5 percent)
• Of the almost 1 500 million ha of dryland agriculture, 32
million are salt-affected soils (2.1 percent) to varying
degrees by human-induced processes.
Impact of saline soils on local climate
•
•
•
•
Albedo over salt pans and dry
salt lakes is higher (60%).
Greater contrast in surface
temperatures
- day temp 40° C on sand vs
24 ° C on salt crust
- night temp 0.4 ° C on sand
and 8 ° C on salt
Nocturnal air flow
convergence and daytime
divergence
Changes in airflow could
accelerate wind erosion and
dust deflation.
Two-way interactions between climate and
desertification
• Ways in which human activities modify the surface
characteristics and atmospheric composition of
drylands and consider how these may influence local
and regional dryland climates.
• Evaluate the impact of dryland climates on soils,
ecosystems, water balance and human land use in
the dryland regions.
Impact of dryland climates on soils
• High temperatures and low precipitation lead to poor organic matter
production and rapid oxidation. Low organic matter leads to poor
aggregation and low aggregate stability leading to a high potential for
wind and water erosion.
• Evapotranspiration greatly exceeds precipitation leading to
accumulation of salts on soil surface. Soils with natric horizon are
easily dispersed.
• Low moisture levels lead to limited biological activity.
• Structural crusts/seals formed by raindrop impact which could
decrease infiltration, increase runoff and generate overland flow and
erosion.
Marked seasonality and a tendency towards drought limits
the productive capacity of the land.
Impact of dryland climates on ecosystems
• Climate exerts a strong influence over dryland vegetation
type, biomass and diversity.
• Rainfall influences vegetation production, which in turn
controls the spatial and temporal occurrence of grazing and
favours nomadic lifestyle.
• Dryland plants and animals display a variety of
physiological, anatomical and behavioural adaptations to
moisture and temperature stresses brought about by large
diurnal and seasonal variations in temperature, rainfall and
soil moisture.
Impact of climate on the hydrological cycle
• Dryland precipitation is highly variable in time and space,
leading to corresponding variability in runoff, soil moisture,
and streamflow in the drylands of the world.
• Dryland rivers exhibit a high variability of volume and
discharge. One reason is the influence of ENSO.
• Owing to the infrequent and short-lived nature of rainfall,
runoff in most drylands is < 10% of rainfall
Impact of climate on human land use
• Rainfall variability and other dryland climate
characteristics greatly influence vegetation productivity,
carrying capacity of the land, susceptibility of the land to
erosion, surface water availability and aquifer recharge.
• Operations affected include crop planting dates, rangeland
management strategies, subsistence strategies among
peasant farmers, water management in irrigated agriculture
etc.,
Present Climate
“An increasing body of
observations gives a collective
picture of a warming world and
other changes to the climate
system”
Intergovernmental Panel on Climate Change, 2001
Agriculture’s contribution to global
greenhouse gas and other emissions
Gas
% of total
anthropog.
sources
Carbon Methane
dioxide
15
Nitrous
oxide
49
66
Climate
change
Nitric
oxides
27
Acidification
Ammonia
93
Main
effects
Climate
change
Climate
change
Acidification.
Eutrification
Agric.
Source
Land use
change,
deforest.
Ruminants Livestock
Biomass
Burning
Rice prod.
Fertilizer
Bio Burning Bio Burning Fertilizer
Livestock
Fertilizers
Bio Burning
Expected
Changes
to 2030
Stable or
declining
Rice: stable 35-60%
or declining increase
Livestock:
rising 60%
Livestock:
rising 60%
Role of drylands in greenhouse gas build-up
• Difficult to assess, but drylands are likely to contribute
between 5 to 10 per cent of overall greenhouse gas buildup.
• These values are small at the global level, but are
significant at the regional or national level.
• Many drylands have relatively small anthropogenic
emissions of carbon dioxide.
Why Climate Change can be of Significance
to the Drylands ?
• Most dryland ecosystems are already affected by
increasing
resource
demands
and
unsustainable
management practices and human-induced climate change
adds an important new stress.
• Most systems are sensitive to both the magnitude and rate
of climate change.
• Successful adaptation depends upon advances in
technology, institutional arrangements, availability of
financing and information exchange.
• Vulnerability increases as adaptive capacity decreases.
Recommendation for the Meeting and for the
Working Groups to discuss
• Establish CONASTAC Network to provide an integrated
approach to study the contribution of agriculture to the state
of climate.
• The integrated approach would help address the issue in a
holistic manner and would contribute significantly to the
IPCC process.
• Participating scientists in the network would benefit from
exchange of information and experiences.
• Means to disseminate information on the web would help
highlight the issue and help interest more scientists to
participate in the work of the network.
Thank you very much for your
attention