Download Implications of climate change for grassland: impacts, adaptations

Implications of climate change for grassland: impacts, adaptations and
mitigation options
Hopkins A. and Del Prado A.
Institute of Grassland and Environmental Research, North Wyke, Okehampton, Devon EX202SB, UK.
Abstract
Climate change associated with greenhouse gas (GHG) emissions has important implications for
Europe’s grasslands. Projected scenarios indicate that increased temperatures and CO2 concentrations
have potential to increase herbage growth and to favour legumes more than grasses, but changes in
seasonal precipitation would reduce these benefits particularly in areas with low summer rainfall.
Further problems may arise from increased frequency of droughts, storms and other extreme events.
Farm-scale adaptive responses to climate change are identified. Grassland agriculture also contributes to
GHG emissions, particularly CH4 and N2O, and agricultural land management affects net carbon
balances and C sequestration. Management options are identified for mitigating grassland agriculture’s
contribution to GHG emissions which need to be developed further in a holistic way that considers other
pressures.
Key words: climate, grassland, Europe, carbon dioxide, methane, nitrous oxide
Introduction
There is increasing evidence that the world’s climate is changing and that the rate of change since the
onset of the industrial revolution is greater than would be expected from natural variability alone. The
Third Assessment Report (TAR) of the Intergovernmental Panel on Climate Change (IPCC, 2001a)
details the extent of recent global change. Key indicators of change (increased mean temperatures,
changes in patterns of precipitation, increased cloud cover, more frequent floods and other extreme
events) are widely accepted to be due, at least in part, to the radiative forcing effects of increased
concentrations of atmospheric greenhouse gases (GHGs), exacerbated by land-use changes. Carbon
dioxide (CO2) is the principal GHG by virtue of its high atmospheric concentration (at the present time
about 365 ppmv, having increased by >30% since ca. 1750). Methane, nitrous oxide and halogens are
the other main GHGs and have also increased over the same period.
Changes in climate have important implications for agriculture generally, including grassland-based
livestock production and the wider objectives of grassland management. While the agricultural industry
is well adapted to some seasonal variability, departures from average weather conditions have, in recent
years, resulted in a range of management problems including issues of heat stress and animal welfare,
storage and applications of manures, and coping with storms, floods and droughts. As these issues
increase in frequency and severity they present increasing challenges for farmers, researchers and policy
makers. Although the major causes of increased GHG emissions are due to population growth and
industrialization, agriculture contributes to emissions of CO2 through its use of fossil fuels during
cultivations, and indirectly through energy-intensive inputs, such as fertilizers. Of the other GHGs,
methane has increased by over 150% and nitrous oxide by about 17% since ca 1750. Grassland
agriculture is a significant contributor to increased emissions of these gases.
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Table 1. Some key features of climate scenarios for Europe.
Temperature
•
Annual temperatures increase by 0.1-0.4°C per decade. Effect greatest over
southern Europe (Spain, Italy, Greece) and northeast Europe (Finland, western
Russia) and least along the Atlantic coastline.
•
Rapid winter warming (0.15–0.6°C per decade) of continental interior of eastern
Europe. In summer, southern Europe warming 0.2-0.6°C per decade and
northern Europe warming 0.08-0.3°C per decade.
•
Less frequent cold winters (i.e. those that occurred 1 year in 10 during 1961–
1990) and disappearing almost entirely by the 2080s. Hot summers become
much more frequent. Under the 2080s scenario, nearly every summer is hotter
than the 1-in-10 hot summer as defined under the present climate.
•
All model simulations show warming in the future across the whole of Europe
and in all seasons.
•
Alternative scenario, at present considered low-risk (but high potential impact)
of regional cooling due to shutdown of Atlantic thermohaline.
Precipitation
•
Increased annual precipitation in northern Europe (+1-2% per decade), small
annual decreases across southern Europe (maximum –1% per decade), and small
or ambiguous changes in central Europe.
•
Marked contrast between winter and summer patterns of precipitation change.
Most of Europe gets wetter in the winter season (+1-4% per decade), except in
Turkey and Balkan region. Pronounced summer north-south gradient: northern
Europe increases by 2% per decade, and southern Europe decreases (up to -5%
per decade).
•
Only for the A2-high GHG emissions scenario are there substantial areas
(Fennoscandia and north-west Europe) where precipitation changes by the
2020s are larger than might occur within natural climate variability. Even for
this scenario with rapid global warming, not all regions in Europe have welldefined precipitation signals from GHG-induced climate change by the 2080s.
Weather Extremes
•
It is considered likely that frequencies and intensities of summer heat waves will
increase throughout Europe; that intense precipitation events will increase in
frequency, especially in winter, and that summer drought risk will increase in
central and southern Europe; and possible that gale frequencies will increase.
Sea Level
•
Although global-mean sea level may rise by up to 70 cm during 21st century,
due to thermal expansion and glacial melt, regional effects in Europe will vary
because of post-glacial tectonic movements.
•
Potential interactions of sea-level rise with increased storms and tidal surges.
Sources: Kundzewicz and Parry (2001), Hulme et al. (2002), Wood et al. (2005)
As these have a much greater radiative forcing potential than CO2, there is now increasing pressure to
curb their emissions.
In this paper we review the key features of future climate change in the context of grassland in Europe,
and consider the possible impacts and adaptations and the need for integrated management options for
reducing the emissions of GHGs from grassland.
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Impacts and adaptations of anticipated future climate change for grassland in Europe
The TAR Report (IPCC, 2001a) estimated that by 2100 global temperatures could increase by between
1.4 and 5.8oC, and that CO2 concentrations could reach 500-900 ppmv. Table 1 summarizes the key
features of 21st century climate change for Europe and highlights some of the geographical variation in
anticipated effects. In general terms vulnerability is greatest in Mediterranean and southern Europe
(Schroter et al., 2005), due to summer heat and drought, and also at the highest latitudes and elevations,
where natural ecosystems such as wetlands, tundra and periglacial vegetation are threatened, and
changes in snowfall affect summer water availability. The adaptation potential for natural systems is
low, even though these systems contribute to a range of ecosystem services. In contrast, the adaptive
capacity of many managed land-use systems in Europe is relatively high (IPCC, 2001b). Climatechange induced effects such as coastal and river flooding, droughts, and threats to some vulnerable
habitats are likely to be felt locally, whereas the effects of higher temperatures and enhanced CO2 in
situations where water is not the limiting resource, e.g. in northern and north-west Europe, could be
generally positive for agricultural production. Increased concentrations of CO2 may modify the
responses of grassland to the effects of changes in temperature and in water availability.
A recent major UK study involving sward growth experiments under elevated CO2 and its interaction
with temperature, linked to regional and livestock enterprise scenarios applied to the LEGSIL grassland
model, has revealed the likelihood of several impacts and highlighted possible farmer responses (Topp
and Doyle, 1996; Hopkins et al., 2003; Harmens et al., 2004; Hopkins, 2004; Scholefield et al., 2005).
These include:
•
•
•
•
•
•
•
Increased herbage growth potential: compared with ambient CO2 and temperature
(ACAT) DM yield for swards cut frequently was enhanced by elevated temperature (ET),
elevated CO2 (EC) and elevated CO2 + temperature (ECET) by 30%, 46% and 56%,
respectively.
Changes favour legumes, particularly red clover and lucerne, more than grasses, but
reduced opportunities for grazing and harvesting on wetter soils.
Changes in herbage quality with higher content of water-soluble carbohydrate and lower
N content at a given yield.
Greater incidence of summer drought (which at its most serious would off-set the
advantages that may arise).
Leaching may be increased due to increased winter rainfall, but some potential for
improved N use by the growing sward and the animal.
Farm-scale adaptive responses could include greater reliance on conserved food for
housed livestock, increased use of maize, greater use of forage legumes in place of Nfertilized grass, increased need for manure storage and improved applications, increased
demand for irrigation (although with greater pressure on water resources this option may
be unavailable in many areas). Feed budgeting for dry seasons in northern Europe could
require alternative forage species / mixtures adapted to drought and managed
accordingly.
Adaptive responses have further implications for N2O and CH4 emissions.
Aside from grassland agriculture, there are also important challenges for the management of grassland
and related habitats that contribute to amenity and ecosystem services, including their role in soil and
water conservation, and in nature and biodiversity protection. There are potential threats to many
species and habitats; extinctions are linked to the frequency of extreme weather events (Easterling et al.,
2000). Species with bounded distributions are under high risk, as are those with poor dispersal
capability and restricted range (Green et al., 2003).
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Grassland management to mitigate greenhouse gas emissions
Grassland-based agricultural systems contribute to the biosphere-atmosphere exchange of radiativeforcing gases, with fluxes intimately linked to management practices (Soussana et al., 2004). Carbon
dioxide is exchanged with the soil and vegetation, N2O is emitted by soils and manures and CH4 is
emitted by livestock and manure.
In addition to the GHG emissions generated naturally from these activities, mechanical operations on
farms consume fossil fuel, and there is an indirect consumption of fuel, and therefore to CO2 emissions,
from the manufacture and transport of farm inputs, particularly fertilizers. On the other hand, the
growing of crops and pasture, the maintenance of other farmland vegetation (hedges, trees, scrub etc.)
and the accumulation of carbon as organic matter in soils can all contribute to the temporary removal,
and in some cases to the long-term sequestration, of CO2 from the atmosphere. There is the potential to
increase the sequestration of CO2 (Freibauer et al., 2004) and also to reduce the emissions of methane
and nitrous oxides from changes in agricultural and other land management.
However, due to the multiple interactions, these systems are quite complex. Hence, they require an
integrated approach as most mitigation options to reduce emissions involve important trade-offs. As
much as one-half of the climate mitigation potential of some C sequestration options could be lost when
increased emissions of other greenhouse gases (N2O and CH4) were included (Smith et al., 2000, 2001).
Opportunities to mitigate CH4, N2O and CO2 emissions from agriculture therefore need to be developed.
Methane
Enteric fermentation in livestock is the main agricultural source of methane in Europe, with emissions
from livestock manures accounting for most of the rest. Methane is produced as a by-product of
digestion of structural carbohydrates, principally cellulose, due the action of microbes (bacteria, fungi
and protozoa) in the rumen. During this digestion, mono-saccharides are fermented to H2, CO2 and
volatile fatty acids (VFAs) such as acetate, propionate or butyrate. As part of this stage of ruminant
digestion some of the microbes (called methanogens) produce CH4 from and CO2.
Several studies have formulated abatement strategies to mitigate CH4 emissions. Mitigations can
generally be split into two groups, (i) those aimed at enteric fermentation and (ii) those targeting manure
management.
Enteric fermentation strategies
These may be addressed at three different levels (Jarvis, 2001): dietary changes, direct rumen
manipulation and systematic changes. The dietary changes suggested so far involve measures which
enhance the efficiency of feed energy use. Even assuming a constant percentage of methane loss, this
strategy will decrease methane loss per unit of product and probably decrease CH4 emissions in the long
term (Johnson and Johnson, 1995). The most natural way to depress CH4 production would be to
manipulate the diet to give high rates of fermentation and/or passage through the rumen, which might
increase the molar percentage of propionate and decrease that of acetate in the rumen VFAs. These
changes in VFA proportions have been associated with a decrease in the fibre content of the diet (i.e.
switching to high starch concentrates or maize silage). Ingestion of organic acids (aspartate, malate and
fumarate) and yeast culture has been associated with reduced emissions in total CH4 per cow and also
with beneficial increases in animal product (i.e. milk yield). Their reactions in the rumen produce
propionate and butyrate, which behave as electron metabolic sinks competing for the H2 and thus
minimising the chances of methanogens to produce CH4 from H2.
The use of some plant extracts (i.e. tannins, saponins) has also been associated with CH4 reduction
(Sliwinski et al., 2002; Hess et al., 2003; Carulla et al., 2005; Hu et al., 2005; Puchala et al., 2005). As
yet there is no consensus on its efficacy (Newbold and Rode, 2005).
There are some drawbacks to using these dietary supplements. The organic acids are not commonly
used as yet, and they may also trigger pH problems in the rumen. Wallace (2005) showed that the pH
problem may be overcome by the encapsulation of the organic acid (i.e. fumarate). Plant extracts may
also have anti-nutritional effects and even be toxic (Teferedegne, 2000). For instance, in a study by Hess
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(2005), extracted tannins had a positive effect on feed rates and hence a possible reduction of CH4 per
kg product, whereas the use of shrub legumes rich in tannins resulted in decreased feed rates. Yeast
culture, on the other hand, although variable, may be promising as a successful mitigation option as it is
already in common use.
There have been several attempts to reduce enteric CH4 production through direct rumen manipulation;
for instance, defaunation of protozoa may decrease the number of methanogenic bacteria as an
important proportion of rumen methanogenic bacteria are parasitic to protozoa (Takahashi, 2005). The
main drawbacks though are that protozoa defaunation may trigger some metabolic diseases.
The ingestion of ionophores acts as propionate enhancers and hence increases the ratio of propionate:
acetate. Their use is very limited as they are antibiotics (i.e. monensin) and their main drawback is that
they may enhance bacterial resistance to antibiotics.
Some changes in the dietary fat contents of the ration have been described to reduce CH4 emissions
from ruminants (Dong et al., 1997; Machmüller et al., 1998; Johnson et al., 2002; Giger-Reverdin et al.,
2003) as some fats alter the ruminal microbial ecosystem and, in particular, the competition for
metabolic H2 between the CH4 and propionate production pathways (Czerkawski, 1972).
Systematic changes may involve identifying animal breeds which result in a reduction of CH4 output per
animal. However, and so far, no clear evidence has been found (Münger and Kreuzer, 2005). Waghorn
et al. (2005), for instance, found that variability within animals is greater than between animals, and it is
therefore difficult to come to any conclusions. Increasing productivity per head (i.e. milk yield per cow),
or increasing the number of lactations for which the average cow remains economically productive,
would decrease CH4 production per unit of milk, and within the framework of production quotas would
decrease total CH4 emissions. Although more intensive forms of animal production tend to decrease
total CH4 output, they might not be compatible with other issues for water, atmosphere, soil,
biodiversity, landscape or animal welfare.
Manure management
Opportunities to decrease total CH4 outputs from farming systems are limited to either increasing the O2
supply to restrict methanogenesis, minimizing the release of CH4 to the environment (e.g. covered
lagoons) or using anaerobic digesters to produce more CH4 in a controlled environment and hence use
this CH4 as a source of energy. This last technique could represent a future sustainable option, but
currently has the drawback of high capital cost.
Nitrous oxide
Nitrous oxide is formed in the soil through nitrification and denitrification and is controlled by a number
of soil factors, including moisture content (del Prado et al., in press), temperature (Hatch et al., 2005),
fertiliser additions (del Prado et al., in press), pH (Merino et al., 2000), organic matter content (Smith et
al., 1997; Chadwick et al., 1998; Estavillo et al., 2002), nitrate (NO3-) and ammonium (NH4+) (Tiedje,
1988; Granli and Bockman, 1994).
Nitrate and NH4+ in the soil are subject to the following process dynamics. Nitrate may: (1) undergo
denitrification to gaseous oxides of N and to N2; (2) be taken up by organisms (assimilatory reduction);
(3) be used by micro-organisms as an electron acceptor and become reduced to NH4+ (dissimilatory
reduction); (4) be leached or removed in run off; or (5) accumulate in the soil. Ammonium may: (1) be
taken up by plants; (2) be immobilised in microbial biomass; (3) nitrify to NO3- and be partially lost as
gaseous oxides of N (4); be leached; (5) accumulate in the soil (Paul and Clark, 1996); or (6) be
volatilised as ammonia (NH3).
The regulation of trace N-gas production via nitrification and denitrification has been described by the
‘hole-in-the-pipe’ conceptual model (Firestone and Davidson, 1989). The rate of the processes
(denitrification and nitrification) and the relative proportions of end products are controlled at two
different levels. First-level factors control the movement of N through the ‘pipe’. Second-level factors
control the partitioning of the reacting N species to N2, N2O, NO, etc. and therefore control the size of
the ‘holes’ in the pipe through which the gases ‘leak’.
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In general, N2O emissions can be reduced by implementing practices aimed at enhancing the ability of
the crop to compete with processes that lead to the escape of N from the soil-plant system (Freney,
1997). For instance, there are several methods for increasing the efficiency of the crop to remove
mineral N from the soil. These include improving fertilizer efficiency (Brown et al., 2005), optimising
methods and timing of applications (Dosch and Gutser, 1996), using ammonium-based fertilisers rather
than nitrate-based ones (Eichner, 1990; Dobbie and Smith, 2003) and employing nitrification chemical
inhibitors (Dittert et al., 2001; Merino et al., 2002; Macadam et al., 2003).
Increasing the soil aeration may significantly reduce N2O emissions. Improving drainage would be
particularly beneficial on grazed grassland (Monteny et al. in press). Hence, avoiding compaction by
traffic, tillage (Pinto et al., 2004) and grazing livestock may help to reduce N2O emissions. Housing
system and management will also influence N2O emissions, e.g. straw-based manures result in greater
N2O emissions than slurry-based ones (Groenestein and Van Faassen, 1996). Minimizing the grazing
period is likely to reduce N2O emissions as long as the slurry produced during the housing period is
uniformly spread.
Carbon dioxide
Carbon dioxide is formed naturally in grassland systems through respiration (below-ground soil, shoot
plants and herbivores) and is fixed into carbohydrates via photosynthesis. Grasslands are generally
regarded as potential sinks for CO2 although soil management factors such as the frequency of
ploughing and reseeding might alter the potential for carbon sequestration. Other management factors
may also affect on the whole C balance of a farm, including emissions from farm machinery, and
indirect CO2 emissions associated with fertiliser manufacturing, transport of feed and other inputs to the
farm. There is also the issue of the C balance of the whole farming and food supply business, including
the fuel consumption issues of food chains and connectivity between the farm and the consumer (Jones,
2001).
There are a number of possible C sequestration measures that may be applied on grasslands and some
present management schemes have probably helped to maintain soil-C stocks (Smith, 2004; Freibauer et
al., 2004). These include improving efficiency of animal manure and crop residue use, reducing soil
disturbance, maximising the C returns in manure, use of deeper rooting species, application of sewage
sludge or compost to land, irrigation, extensification and improved management to reduce wind and
water erosion. The increase in soil C content after a shift from arable to grassland is partly explained by
a greater supply of C to the soil under grass, mainly from the roots, but also from the shoot litter, partly
by the increased residence time of C due to the absence of tillage (Soussana et al., 2004). Whereas this
rate of increase of soil C after conversion to grassland is slow, the rate of C disappearance from soil
after returning grassland to arable is rapid. A grassland sward aged more than 20 years no longer acts as
a C sink (Frank, 2002). Converting grassland to forest can lead to an accumulation or to a release of soil
C, depending on the conditions (Post and Kwon, 2000). Introducing short duration leys tends to have an
intermediate C sequestering potential between crops and permanent grasslands (Soussana et al., 2004).
In this context we can also include the growing of biofuel and other industrial crops on agricultural land,
with the potential for non-food crops to further mitigate CO2 emissions by displacement of fossil fuels.
The need for integration
There is need to develop useful tools in order to explore agricultural systems in a holistic way. Climate
change, and consequently greenhouse gases, unfortunately, account for only a part of the whole
challenge in grassland systems. Agriculture is also subject to other pressures, including environmental
(i.e. eutrophication, erosion, and acidification), socio-economic and sustainability issues. Identification
of win-win strategies requires development of appropriate modelling systems together with the
acquisition of field and farm data (Scholefield et al., 2005).
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Protein
Butterfat
Milk Lha -1
NH3 L -1
% change from baseline farm scenario
NOx L -1
N2O L-1
NO3 L -1
-5
0
5
10
15
20
25
% reduction from baseline farm
Figure 1. Percentage reduction from baseline farm scenario in environmental losses per L of milk
(NO3– leaching, N2O, CH4, NOx and NH3) and in milk yield and properties.
So far, some modelling studies have been developed to (i) assess the effects of different dietary
strategies on the sustainability of a grassland system (del Prado et al., 2006), (ii) evaluate the economic
cost for implementing mitigation strategies for GHGs (Jarvis, 2001), (iii) evaluate the impact of NO3
leaching abatement measures on N2O, NH3 and CH4 emissions (Brink et al., 2005; del Prado et al.
2005), (iv) assess successful mitigation strategies for GHGs (Schils et al. 2005), and (v) evaluate not
only environment (N2O, CH4, NH3, NO3– and P leaching) and economics, but other attributes which
define the sustainability of a farm (e.g. SIMSDAIRY model: del Prado et al., this congress).
For instance, using the SIMSDAIRY model we compared environmental losses, milk yield and milk
properties in two typical dairy farms in the UK (2 LU ha-1, 30.5 ha grazed grass, 16.5 ha cut grass and 3
ha maize) which differ only in terms of their past grassland management. Whereas the baseline farm had
a history of long-term grassland with old swards (>11 years), the second farm had a history of shortterm grass leys with new swards (2-3 years). As indicated in the previous sections, the baseline farm’s
past history would offer more opportunities for C sequestration than would the history of the second
farm. However, if we compare the predicted environmental losses (Figure 1) of the SIMSDAIRY model,
we find that NO3- leaching (24%) and N2O (6%) per L of milk were significantly lower from the second
farm than from the baseline farm. Milk yield and milk properties (butterfat and protein) were also
slightly improved, which may have a modest impact on the economy of the dairy farm.
Conclusions
Climate change presents a number of challenges for the future management of grasslands in Europe,
including effects on biodiversity and ecosystem functions, but particularly in terms of agricultural
adaptations to ensure food production. This needs to be considered against a background of global
population growth and climate change impacts on agriculture in other continents which could have
profound impacts on world food supplies. There remain uncertainties about the effects and timescales of
climate change, and regional variations, and possible outcomes such as a shutdown of the Atlantic
thermohaline circulation, though at present a high impact, low probability event (Wood et al., 2005)
underline the potential dangerous consequences of failing to adapt in the short term, and to develop
long-term mitigation responses to reduce the causes of climate change for the longer term.
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