Global Trends in Land Degradation

(1) Past assessments of trends

(2) New definitions and approaches

(3) Projected trends

(4) Research needs

GLASOD

Soil degradation severity

Soil degradation severity and vegetation

So

il d

egra

da

tio

n

High

Low

HighLowVegetation index

Soil degradation in Drylands

Where are the drylands?precipitation

~ 1.5 lower than evapotranspiration

= DesertificationSoil degradation in Drylands

How much of drylands is desertified?

 

Source

Land areaPopulation size

(year 2000)

Million km2

%of dryland

%of global

Million people

%of drylands

%of global

DrylandsMA61 412001 35.5

Deser-tified

GLASOD )Soil(12208117.5 2.0

GLASOD )Soil and

Veg.(437029   

MA-LUCC6104.1201.00.3

GLADOD )soil(: expert opinion, travelers’ descriptions, research reports

GLADOD )+ Veg. – mean of 100 monthly [1983-1990] NDVIs of highest weekly value(

• Relatively reliable literature data • Change through 1980-2000

MA-commissioned desk study – Erica Lepers )2003(, Land-Use Land-Cover Change (IGBP’s LUCC)

Dryland not covered by data

Not dryland

Not degraded

Hyperarid

How much of drylands is desertified?

 

Source

Land areaPopulation size

(year 2000)

Million km2

%of dryland

%of global

Million people

%of drylands

%of global

DrylandsMA61 412001 35.5

Deser-tified

GLASOD )Soil(12208117.5 2.0

GLASOD )Soil and

Veg.(437029   

MA-LUCC6104201.00.3

How many people are affected?

The source of differences?

Dryland not covered by

data

susceptiblemay

DesertificationDegradationSusceptabilityDrylands

Hyperarid

Arid

Semiarid

Dry subhumid

Humid

Cold climate

•Misuse of terms•Controversial definitions•Disputed methodologies

Land degradation in the drylands

“terrestrial bio-productive system that comprises soil, vegetation, other biota, and the ecological and hydrological processes that operate within the system”

A terrestrial ecosystem Loss of ecosystem services, most notably – primary production

Ecosystem Services

Supporting services• Nutrient cycling • Soil conservation• Soil formation• Supporting biodiversity

• Primary production

Provisioning services• Food, Forage, fiber• Fuelwood • Freshwater• biochemicalsRegulating services• Pollination, seed dispersal• Water regulation• Climate regulation• Carbon sequestrationCultural services• Spiritual, religious, cultural heritage• Indigenous ecological knowledge• Ecotourism

“reduction or loss … of the biological … productivity … resulting from land uses …. or … combination of )other( processes, such as…”

Primary production

Soil conservation

FoodFuelwoodFreshwater

Water regulation

Dryland

Reduction inproductivity

desertificationExpression of

below its potential

Net Primary Productivity (NPP)

Normalized Difference Vegetation Index (NDVI)

1. Define (large) region2. Obtain digitized thematic maps:

• Soils• Climate• Vegetation structure

3. Classify region into homogenous land classes4. Overlay a layer of several-years’ mean NDVIs 5. Highest NPPs of each land class - estimators of it potential NPP6. Normalize NPP values; potential for each class = 100%7. All other pixels in the class represent percentage of potential8. Lowest percentages represent sites undergoing desertification

Pixels of Potential NPP, non-degraded

Pixels of degradation Zimbabwe

Local NPP Scaling (LNS) – Stephen Prince, Inbal Reshef

Mean NDVI of 5 years (1998-2002) SPOT-VEGETATION, 1 km2 resolution

Not recordedLowModerateHighVery high

Risk

b.

Biomes, soils, climate, population (NRCS )

Risk

c.

GLASOD

Mean NDVI of 1998-2002 What is the trend?

South AfricaDegradation criteria:

Former homelands

• Reduced Vegetation cover• Changed plant composition• Bush encroachment• Livestock density in communal

areas twice larger than in commercial farms

1. Define (large) region2. Obtain digitized thematic maps:

• Soils• Climate• Vegetation structure

3. Classify the region into homogenous land classes 4. Overlay a layer of several-years’ mean NDVIs4. Overlay a layer of NDVI values for each year of a long time-series

with non-degrading and degrading land uses

5. Calculate annual NDVIs for pairs (degraded, non-degraded) pixels of each land class for each year of the long time-series

Local NPP Scaling

86 89 92 94 98 00 03

50

40

60

70

80

sum

ND

VI

16 growing seasons

Non-degraded

degraded

What is the source of interannual variation?

86 89 92 94 98 00 03

600

1400

1000

200 Rai

nfal

l )m

m(

50

40

60

70

80Productivity

reduction in productivityPersistent

sum

ND

VI

16 growing seasons

Non-degraded

degraded

Residuals+

-

NP

P

Rainfall

Year

Ra

infa

ll

Re

sid

ua

l

• Small residuals – actual NPP close to potential NPP

• Large residuals – actual NPP deviates from potential NPP

• Negative residuals – NPP lower than potential NPP

• Positive residuals –NPP higher than potential NPP

• As time advances – residuals more negative

• Degradation increased with time during the studied period

Regression slope

Residual Trends (RESTREND) – Konrad Wessels and Stephen Prince

Local NPP Scaling (LNS)

-

+

Percentage of Potential Productivity

0 %

100% )mean 1998-2002(

Is this persistent productivity loss irreversible?

Temporal Trend of Deviation from Potential

ProactiveReactive

Tra

nsiti

on o

f Glo

bal

soci

ety

Ecosystem management approach

Glo

baliz

ed

Frag

men

ted,

Reg

iona

lized

GlobalizedReactive

RegionalizedReactive

RegionalizedProactive

GlobalizedProactive

Present Conditions &

Trends

50-year projections

Millennium Ecosystems Assessment Scenarios

Future trends

Rate of change in the extent of desertified areas

Time

De

ser

tifi

ca

tio

n t

ren

ds

Pressure of desertification drivers

Small increase Strong increasePoverty:

Climate Change: No increase Strong increase

Research needs

• Detect and distinguish desertification from desertification risk at all scales

• Identify and detect thresholds beyond which dryland productivity change irreversibly

• Decouple effects of desertification from effect of dryland’s low productivity on poverty

• Quantify the feedback loops between desertification and climate change

SinaiNegev

Tsoar et al. 1995

1948 border closed

1967 border opened

1982 border closed

1945 1956 1968 1976 1982 1984 1989

Years of airphotos

100

200

300

400

500

600

Nu

mb

er o

f sh

rub

s/km

2Recovery in Negev Negev:

delayed response of herders; Sinai: overgrazed

Negev and Sinai overgrazed

Negev recovers; Sinai overgrazed

NegevSinai

?

Desertification

Vegetation changes

Climate change Biodiversity loss

Persistent reduced productivity

Soil erosion

Desertification

Climate change

Arid drylandNegev Desert

watershed

Mid Pleistocene

60K 20KLate

Pleistocene last pluvial

phase

Loess sediments wind-transported from the Sahara

16KHolocene

Post-pluvial

climate change

Less dustLess but higher

intensity rain

3KBronze

age

Land management Agriculture

1.5K

Byzantine periodPeak

agriculture

1.4K 1.2KEarly

Islamic period

Cultivation abandoned

2m

5-10m thick4 m

Last few centuries – Bedouin use of Byzantine terraces

Current rates (1990-2001)/year• Gully incision 1-23 m• Soil loss 81-818 m3

Loss since Byzantine cultivation peak – 10% of arid Negev land

20011984

Some watersheds already lost most of their soil

Desertification NOT driven by human over-use

NOT driven by anthropogenic global climate changeBut due to NATURAL climate change

Years-15,000 +5,0000

Soil loss

0%

100%

Runoff in

crea

se

Soil for agriculture within the watershed

No soil for agriculture

No runoff for agriculture

Rocky surfaces

within the watershed

Agriculture window

Mid Pleistocene

60K 20KLate Pleistocene last pluvial

phase

16KHolocenePost-

pluvial climate change

3KBronze

age

1.5K

Byzantine periodPeak

agriculture

1.4K 1.2KEarly

Islamic period