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Lectures 3 -6: Climate and Agriculture
Prof Shellemiah O keya
6th June 2013
The Earth’s Environmental Woes:
Is Agriculture Part of the Problem or Part
of the Solution?
Reported by Ellen Wilson
Chapter 24: Global Warming changes the
focus for Agriculture
 Gommes, R. 1993. Current climate and population
constraints on world agriculture. In: Agricultural Dimensions
of Global Climate Change. H.M. Kaiser and T.E. Drennen
(eds.). pp. 67-86.
 Holmes, R. 1995. Arctic ice shows speed of climate 'flips'.
New Scientist 145 (1967): 13.
 Houghton, J.T., Meira Filho, L.G., Bruce, J., Lee, H.,
Callander, B.A., Haites, E., Harris, N. and Maskell, K. (eds.).
1995. Climate Change 1994. Radiative forcing of climate
change; and an evaluation of the IPCC IS92 emission
scenarios. Cambridge University Press, Cambridge, New
York, Melbourne.

 Kaiser, H.M. and Drennen T.E. (eds.). 1993. Agricultural
Dimensions of Global Climate Change. St. Lucie Press,
Delray Beach, Florida. 311 p.
 Katz, R.W. and Brown, B.G. 1992. Extreme events in a
changing climate: variability is more important than
averages. Clim. Change 21: 289-302.
 Keeling, C.D., Whorf, T.P, Wahlen, M. and van der
Plicht, J. 1995. Interannual extremes in the rate of rise of
atmospheric carbon dioxide since 1980. Nature 375:
666-670.
 Kukla, G. and Karl, T.R. 1993. Nighttime warming and
the greenhouse effect. Envir. Sci. Technol. 27 (8): 14681474.
 Comprehend the close relationship
between climate and agriculture
 Establish that climate change affects
agriculture and vice versa
 Understand that adaptation to climate
change is central in agriculture
Soils and Great/ Early Civilizations
 Great civilizations had good soils as one of their natural
resources
 In Kenya the highest population density are in the counties
with good/fertile soils
Other examples of fertile soils
 The ancient dynasties of the Nile were made possible by food
producing capacity of the fertile soils of the valley and
associated irrigation system. Tigris and Euphrates rivers in
Mesopotamia and Indus, Yangtse and Huang Ho rivers in
India and China respectively represent inhabitants of
flourishing civilization
 These fertile soils made possible stable and
organized communities and even cities
 In contrast to nomadic, shifting societies
associated upland soils with concomitant animal
grazing
 Soils destruction or degradation or
mismanagement was also associated with the
downfall of some of these civilization that good
soils had helped to build
 In the Euphrates and Tigris the elaborate irrigation
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and drainage systems were not maintained – resulting
in accumulation of salts;
And once the productive soils became barren and
useless;
The flourishing cities that had occupied these areas
fell into ruins and the people migrated elsewhere;
The ruins of Babylon in Syria is a living example;
Thus people are dependent on soils and to an extent
good soils are dependent on people.
Most people settle where the
best soils are...
 Soil formation is intrinsically linked to
climate
 Soil formation is a function of:
Climate + Vegetation+ Parent material
+Topography + Time
Climate has a dominant factor in the soils that
you see
 Agriculture systems : Climate system
 Crop agriculture,
 Animal husbandry,
Cloudiness, wind, rain ,
evaporation
Temperature
 Forestry and
 Fisheries can be defined as one of the systems, and
climate the other.
 If these systems are treated independently, this would
lead to an approach which is too fragmentary
 Tropical rainforest – found particularly in the
centre of the continent, and along the eastern coast
of Madagascar.
 Humid sub-tropical – found in the south-west.
 Mediterranean – mostly on the north-west
(Mediterranean) coast and in the south-east
 Savannah – found to the north and south where it
replaces the rain forest. There are distinct wet and
dry seasons.
Climate change-induced change in Aridity Index
(P/PET) - productivity reduction
Vicious circle
1931-1960
Africa
1961-1990
Land degradation
Dry sub-humid
25 Mha Humid
3 Mha Semi arid
Arid
Hyper arid
51 Mha Arid
Hulme, M. et al., Climate Research, 1992
1931-1960
Humid
Climate change
Aridity Index 1961-2000
AI < 0.05
0.05 < AI < 0.20
0.20 < AI < 0.50
0.50 < AI < 0.65
AI > 0.65
Humid
Biodiversity loss
Sicily
Semiarid
•Precipitation
= Aridity Index ~
•Evapopotential productivity
transpiration
Sciortino, M. et al. 2010 (submitted)
Vicious circle
Land degradation
Reduced carbon
fixation and
sequestration
Global
carbon
stocks
Reduced
Soil erosion
Droughts
fires, floods
Increased
• frequency
• intensity
SOC in
eroded soil
oxidizes
Increased
emissions
Climate change
& increased CO2
•Crop failure
•Forage decline
Reduced biological
productivity of
economic value
•Precipitation
change
•Evapotranspiration
Soil
protection
Degraded
SOC depletion
Loss of •nutrients
•soil moisture
Diversity of
soil
vegetation
cover
Reduced
Biodiversity loss
& agrodiversity failure
Species differential
• sensitivity to
climate change
increase
•Soil
moisutre • response
to
decrease elevated CO2
 Steppe – away from the Equator, to the north and
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south, the savannah grades into drier steppe.
Desert – little rainfall, and big daily differences
between day and night temperatures.
The Sahara in the north is the world’s biggest desert.
Only three countries cover a greater area – Russia,
Canada and China.
The Kalahari in Southern Africa covers an area larger
than France.
Highland – largely found in the east, below the Horn
of Africa.
Marine – largely in the south-east.
 Becoming more global.
 It is now widely held view that human
activities can affect climate, one of the
components of the environment.
 Climate in turn affects agriculture, the source
of all food consumed by human beings and
domestic animals.
 Climate may be changing,
 Human societies and agriculture development
trends constraints climate
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THE CLIMATE 'COMPLEX
Climate variability is likely to increase under global
warming (Katz and Brown, 1992),
The rate of change itself is extremely important
Changes would be associated with dramatic effects
Ocean waters and associated products
Cause havoc to established national fishery activities
Would make adaptation to climate change difficult
Most agricultural planning would be
extremely difficult.
Important greenhouse gases are:
Carbon dioxide (CO2),
Methane (CH4),
Nitrous oxide (N2O),
Troposphere ozone (O3) and
Chlorofluorocarbons (CFCs).
Basic characteristics of the first three gases are given later
The degree to which these greenhouse gases stem from
agricultural sources is also important
 Deforestation
 Wetland rice
 Ruminants
 Biomass burning
 Synthetic fertilizers
 Manures and animal excreta
The following are significant trends for the
near future:
 World production of cereals will continue to grow,
 Export of cereals will undergo a modest growth in demand;
 The livestock sector in developing countries will continue to
grow;
 Root crops, tubers and plantains will retain their
importance;
 Oil crops will undergo rapid growth in developing countries;
most importantly,
 Many developing countries will become net agricultural
importers.
 Overall predictability of weather and climate would
decrease, making the day-to-day and medium-term
planning of farm operations more difficult;
 Loss of biodiversity from some of the most fragile
environments, such as tropical forests and
mangroves;
 Sea-level rise (40 cm in the coming 100 years) would
submerge some valuable coastal agricultural land;
 Incidence of diseases and pests, especially alien ones,
could increase
 Present (agro) ecological zones could shift in some cases over
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hundreds of kilometers horizontally, and hundreds of meters
attitudinally,
With the hazard that some plants, especially trees, and
animal species cannot follow in time, and that farming
systems cannot adjust themselves in time;
Higher temperatures would allow seasonally longer plant
growth and crop growing in cool and mountainous areas,
allowing in some cases increased cropping and production.
In contrast, in already warm areas climate change can cause
reduced productivity;
The current imbalance of food production between cool and
temperate regions and tropical and subtropical regions could
worsen.
 The greenhouse gases CH4, N2O and

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
chlorofluorocarbons (CFCs) have no known direct
effects on plant physiological processes.
They only change global temperature and are
therefore not discussed further.
Instead, concentration should be on the effects of
increased CO2 tropospheric O3, increased UV-B
through depleted stratospheric ozone,
Increased temperatures and the associated
intensification of the hydrological cycle.
CARBON DIOXIDE
 CO2 is an essential plant 'nutrient',
 In addition to light,
 Suitable temperature, water and chemical elements such
as N, P and K, and it is currently in short supply.
 Higher concentrations of atmospheric CO2 due to
increased use of fossil fuels, deforestation and biomass
burning, can have a positive influence on photosynthesis
(Figure 1.2)
 Under optimal growing conditions of light, temperature,
nutrient and moisture supply, biomass production can
increase, especially of plants with C3 photo-synthetic
metabolism
 With increased atmospheric CO2 the
consumptive use of water becomes more
efficient because of reduced transpiration
 This is induced by a contraction of plant
stomata and/or a decrease in the number of
stomata per unit leaf area.
 This restricts the escape of water vapour
from the leaf more than it restricts
photosynthesis
(improved water-use efficiency WUE)
 With the same amount of available water,
there could be more leaf area and biomass
production by crops and natural vegetation.
Plants could survive in areas hitherto too
dry for their growth.
 Increased ultraviolet radiation (UV-B, between 280 and 320
nanometers), due to depletion of the stratospheric ozone layer,
 Mainly in the Antarctic region, may negatively affect
terrestrial and aquatic photosynthesis and animal health.
 Over the last decade, a decrease of stratospheric ozone was
observed at all latitudes (about 10% in winter, 0% during
summer and intermediate values during spring and autumn).
 However, the 'Biological Action Factor' of UV-B can vary over
several orders of magnitude with even slight changes in the
amount and wavelength of UV-B.
 There are damaging effects of increasing UV-B on
crops, animals and plankton growth. It has been
reported that UV-B affects the ability of plankton
organisms to control their vertical movements and
to adjust to light levels;
 Reductions in yield of up to 10% have been
measured at experimentally very high UV-B values,
and would be particularly effective in plants where
the CO2 fertilization effect is strongest.
 On the other hand, UV-B increase could increase
the amount of plant internal compounds that act
against pests.
 Tropospheric ozone originates about half from

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photochemical reactions involving nitrogen oxides (NOx),
methane or carbon monoxide, and half by downward
movement of stratospheric ozone.
High ozone concentrations have toxic effects on both
plant and animal life (German Bundestag, 1991;
It is likely that ozone, in conjunction with other photooxidants, is contributing towards the 'new type of forest
damage' observed in Europe and the United States
In the tropics, tropospheric ozone concentrations are
generally lower than at northern mid-latitudes.
However, this does not apply to periods when biomass
burning releases precursor substances for the
photochemical formation of ozone.
 Rising temperatures - now estimated to be 0.2°C
per decade, or 1 °C by 2040
 Would diminish the yields of some crops,
especially if night temperatures are increased
the temperature increase since the mid-1940s is
mainly due to increasing night-time
temperatures,
 While CO2-induced warming would result in an
almost equally large rise in minimum and
maximum temperatures
 Higher temperatures could have a positive
effect on growth of plants of the CAM
type. They would also strengthen the CO2
fertilization effect and the CO2 antitranspirant effect of C3 and C4 plants
 Higher night temperature may increase
dark respiration of plants, diminishing
net biomass production;
 Higher night temperature may increase dark
respiration of plants, diminishing net biomass
production;
 Higher cold-season temperatures may lead to
earlier ripening of annual crops, diminishing
yield per crop, but would allow locally for the
growth of more crops per year due to lengthening
of the growing season. Winter kill of pests is
likely to be reduced at high latitudes, resulting in
greater crop losses and higher need for pest
control;
 Higher temperatures will allow for more plant
growth at high latitudes and altitudes.
What can be done in drylands?
and likely to apply to non-drylands too
Restoration
Soil depleted Soil salinized Range degraded
Afforestation
Runoff harvesting
•Builds soil
•Halts erosion
•Regulates water
•Promotes forage
•Provides firewood
Transfer to patch
cultivation -agroforestry
Below-ground SOM
Above-ground stand
Arid dryland
After 35 y - twice as much SOM
as the adjacent non-forested,
degraded land
Increasing C Reducing C Reducing Food
stocks
emissions poverty
security
 The extra precipitation on land, if
indeed including present sub humid to
semi-arid areas, will increase plant
growth in these areas, leading to an
improved protection of the land surface
and increased rain fed agricultural
production; in already humid areas the
extra rainfall may, however, impair
adequate crop drying and storage;
 The extra precipitation predicted to occur in
some regions provides possibilities for off-site
extra storage in rivers, lakes and artificial
reservoirs (on-farm or at sub catchment level)
for the benefit of improved rural water supply
and expanded or more intensive irrigated
agriculture and inland fisheries:
 The effects on water resources and water
apportioning of international river and lake
basins can be very substantial, with political
overtones
 Increased temperatures may lead to more
decomposition of soil organic matter;
 Increased plant growth due to the CO2
fertilization effect may cause other plant
nutrients such as N and P to become in short
supply; however, CO2 increase would stimulate
mycorrhizal activity (making soil phosphorus
more easily available), and also biological
nitrogen fixation (whether or not symbiotic).
 Through increased root growth there would be
extra weathering of the substratum, hence a
fresh supply of potassium and micronutrients;
 The CO2 fertilization effect would produce more
litter of higher C/N ratio, hence more organic
matter for incorporation into the soil as humus;
litter with high C/N decomposes slowly and this
can act as a negative feedback on nutrient
availability;
 the 'CO2 anti-transpirant' effect would stimulate
plant growth in dryland areas, and more soil
protection against erosion and lower topsoil
temperatures, leading to an 'anti-desertification
effect'.
 Global climate change, if it occurs, will
definitely affect agriculture.
 Most mechanisms, and two-way interactions
between agriculture and climate, are known,
even if not always well understood.
 It is evident that the relationship between
climate change and agriculture is still very
much a matter of conjecture with many
uncertainties
 it remains largely a conundrum.
 Major uncertainties affect both the Global
Circulation Models (GCMs) and the response of
agriculture, as illustrated by differences among
models, especially as regards effects at the national
and sub regional levels.
 In addition, many of the models do not take into
consideration CO2 fertilization and improved wateruse efficiency, the effect of cloud cover (on both
climate and photosynthesis), or the transient nature
of climate change.
 It is also worth remembering that enormous
knowledge gaps still affect the carbon cycle
(with a missing sink of about 2 Gt of carbon),
the factors behind the recent near-stabilization
of the atmospheric methane concentrations or
the unexplained reduced rate of CO2 increase in
recent years, the effect of volcanic eruptions
(such as the recent Pinatubo eruption), the
effect of any increased cloudiness, etc.
1. Describe with appropriate examples the uses of soils:
a) in agricultural production
b) Non agricultural activities
2. Explain briefly why soil information is important for a
country’s development.
3. What is land degradation?
4. State five sources of green house gases
5. State five land qualities that constrain agriculture
6. Discuss the general issues of climate change on
agriculture
7. What is CO2 Fertilization effect?
8. Discuss the effect of increased UVR on: a) crops, b)
animals, c) tropospheric ozone, d) rising temperatures
9. Discuss the ecological and indirect climate change
effect on agriculture and the environment.
9. Discuss the principal types of land degradation.
CO2
CH4
N2O
Atmospheric lifetime
(yr)
120
14.5
120
Direct GWP 1
1
24.5 2
320
Pre-industrial
concentration 3
280 ppmv
0.8 ppmv
288 ppbv
Present-day levels
360 ppmv
1.72 ppmv
310 ppbv
Current annual increase
(%)
0.5
0.9
0.25
Major agricultural
sources 4
deforestation
- wetland rice
- synthetic N
fertilizers
- ruminants
- animal excreta
- biomass burning - biological N fixation
Percentage of global
source stemming from
agriculture
30
40
25
Predicted change 19902020
-
+
+
Table 1.3. Growth rates between 1961 and 1990 in agricultural sectors
responsible for greenhouse gas emissions (from FAO, 1990). Europe and
Asia do not include the former USSR. Domestic ruminant numbers were
computed as the sum of cattle, sheep, goats, camels and buffaloes
Continent
1961-1990 exponential growth rate (%)
Ruminant
numbers
Forested area
Rice area
Fertilizer
consumption
Africa
1.29
-0.43
2.23
6.21
N and C America
-0.07
-0.02
1.50
3.29
S. America
1.29
-0.49
1.65
9.15
Asia
1.18
-0.59
0.62
9.54
Europe
0.33
0.25
0.78
2.75
Oceania
0.07
-1.15
5.88
1.25
World
0.90
-0.26
0.74
5.35
Figure 1.2. Schematic effect of CO2 concentrations on C3 and C4
plants (after Wolfe and Erickson, 1993). The main mechanism of
CO2 fertilization is that it depresses photo-respiration, more so in
C3 than in C4 plants
Box 1.2. Some mechanisms likely to affect biomass production under global
change conditions. Note that the ratio between economic yield (e.g., grain,
fibre) and biomass may change relative to current conditions
ETP: Evapotranspiration potential
WHC: Soil water holding capacity
ETA: Actual evapotranspiration
OM: Organic matter
WUE: Water-use efficiency
LAI: Leaf area index
The heavy line indicates a hypothetical link between increased humidity and
cloudiness.
1985: Global Flood Archive – Dartmouth
Observatory