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For agriculture, there are three major options for mitigating greenhouse gas (GHG) emissions: 1) productivity improvements, particularly in the livestock sector; 2) dedicated technical mitigation measures; and 3) human dietary changes. The aim of the paper is to estimate long-term agricultural GHG emissions, under different mitigation scenarios, and to relate them to the emissions space compatible with the 2 °C temperature target. Our estimates include emissions up to 2070 from agricultural soils, manure management, enteric fermentation and paddy rice fields, and are based on IPCC Tier 2 methodology. We find that baseline agricultural CO2-equivalent emissions (using Global Warming Potentials with a 100 year time horizon) will be approximately 13 Gton CO2eq/year in 2070, compared to 7.1 Gton CO2eq/year 2000. However, if faster growth in livestock productivity is combined with dedicated technical mitigation measures, emissions may be kept to 7.7 Gton CO2eq/year in 2070. If structural changes in human diets are included, emissions may be reduced further, to 3–5 Gton CO2eq/year in 2070. The total annual emissions for meeting the 2 °C target with a chance above 50 % is in the order of 13 Gton CO2eq/year or less in 2070, for all sectors combined. We conclude that reduced ruminant meat and dairy consumption will be indispensable for reaching the 2 °C target with a high probability, unless unprecedented advances in technology take place.
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The importance of reduced meat and dairy consumption
for meeting stringent climate change targets
Fredrik Hedenus &Stefan Wirsenius &
Daniel J. A. Johansson
Received: 5 July 2013 /Accepted: 3 March 2014 / Published online: 28 March 2014
#The Author(s) 2014. This article is published with open access at Springerlink.com
Abstract For agriculture, there are three major options for mitigating greenhouse gas (GHG)
emissions: 1) productivity improvements, particularly in the livestock sector; 2) dedicated
technical mitigation measures; and 3) human dietary changes. The aim of the paper is to
estimate long-term agricultural GHG emissions, under different mitigation scenarios, and to
relate them to the emissions space compatible with the 2 °C temperature target. Our estimates
include emissions up to 2070 from agricultural soils, manure management, enteric fermenta-
tion and paddy rice fields, and are based on IPCC Tier 2 methodology. We find that baseline
agricultural CO
2
-equivalent emissions (using Global Warming Potentials with a 100 year time
horizon) will be approximately 13 Gton CO
2
eq/year in 2070, compared to 7.1 Gton CO
2
eq/year
2000. However, if faster growth in livestock productivity is combined with dedicated technical
mitigation measures, emissions may be kept to 7.7 Gton CO
2
eq/year in 2070. If structural
changes in human diets are included, emissions may be reduced further, to 35GtonCO
2
eq/year
in 2070. The total annual emissions for meeting the 2 °C target with a chance above 50 % is in
the order of 13 Gton CO
2
eq/year or less in 2070, for all sectors combined. We conclude that
reduced ruminant meat and dairy consumption will be indispensable for reaching the 2 °C target
with a high probability, unless unprecedented advances in technology take place.
1 Introduction
To keep the global average surface temperatures from increasing by more than 2 °C above the
pre-industrial level (UNFCCC 2010), global greenhouse gas (GHG) emissions will have to
decrease greatly by the end of this century (Rogelj et al. 2011). However, emissions have yet to
peak, and the expected growth in global population and per-capita income will continue to
exert a strong upward pressure on emission levels for many decades to come.
In agriculture, several dedicated,technical options with significant mitigation potential
exist, e.g. soil carbon sequestration, increased nitrogen (N)-use efficiency and nitrification
inhibitors for reduction of nitrous oxide (N
2
O) emissions from soils (Snyder et al. 2009;
Akiyama et al. 2010; Luo et al. 2010), drainage of paddy rice fields (Fumoto et al. 2010), and
Climatic Change (2014) 124:7991
DOI 10.1007/s10584-014-1104-5
Electronic supple mentary m aterial The online version of this article (doi:10.1007/s10584-014-1104-5)
contains supplementary material, which is available to authorized users.
F. Hedenus (*):S. Wirsenius :D. J. A. Johansson
Department of Energy and Environment, Chalmers University of Technology, Gothenburg, Sweden
e-mail: hedenus@chalmers.se
fat additives in ruminant feed rations to reduce methane (CH
4
) (Grainger and Beauchemin 2011).
However, it is highly uncertain whether technical measures alone can offer sufficiently deep
emission reductions in the long term. In the midterm, the global mitigation potential for CH
4
and
N
2
O in agriculture through such measures is estimated to be rather small (DeAngelo et al. 2006;
Beach et al. 2008; Smith et al. 2008).
Aggregate productivity in the global agricultural sector has increased over time, in partic-
ular the past 50 years. This has entailed increased output of crops per unit of land, and meat per
unit of feed consumed by livestock. Such productivity increases have also mitigated GHG
emissions per unit of food, particularly if land-use-change effects on soil and vegetation carbon
stocks are included (Burney et al. 2010). Increased productivity is likely to be a major GHG
mitigation option in many low and middle-income countries, where crop yields and feed
conversion efficiencies are well below biophysical limits (Lobell et al. 2009; Wirsenius et al.
2010; Tilman et al. 2011; Valin et al. 2013). However, it should also be recognized that yields
of wheat and other major crops in some key regions have leveled off and been stagnant in the
past 1020 years (Grassini et al. 2013), which suggests that the potentials for further yield
increases are limited. Furthermore, realizing the productivity potentials that do exist will be
associated with higher N turnover per area unit and intensified livestock production, which
both may counteract GHG emission reductions.
Dietary changes that substitute vegetable products for animal products may hold a large
mitigation potential. However, the actual mitigation level is highly sensitive to which
products that are substituted since there is very large variation in the GHG intensity per
unit of food within both the animal and vegetable groups (see e.g. Berners-Lee et al. 2012).
In addition, a substitution from e.g. meat to vegetables is not consistent with historical and
current trends: Per capita consumption of meat continues to increase, also in many affluent
countries (FAOSTAT 2013). In most analyses of diets as a mitigation option, assumed
dietary changes are not based on specific changes in policy that influence demand (c.f.,
consumption taxes, as in Wirsenius et al. 2011 and Edjabou and Smed 2013), but instead
rely on hypothetical changes in consumer preferences. There is, therefore, poor knowledge
of the global mitigation potential through dietary changes under the constraints of consumer
preferences.
To our knowledge, no previous study has assessed the combined effect on agricultural
GHG emissions of the afore-mentioned mitigation categories increased productivity,
technical measures, and dietary changes. Popp et al. (2010) estimated the impact of
reducing meat and dairy demand, and Stehfest et al. (2009) found that the cost of reaching
stringent climate targets would be significantly lower if consumption of livestock products
was abandoned. However, none of these studies included analysis of the mitigation effect
of above-baseline increases in agricultural productivity. Valin et al. (2013)carriedout
analyses of different long-term scenarios of increased productivity, but did not include
assessments of the mitigation effect of dedicated mitigation technology or changes (from
baseline) in human diets.
This paper aims to contribute to the understanding of global agricultural GHG mitigation by
estimating long-term emissions under different mitigation scenarios and relate these emission
levels to those compatible with global temperature targets. More specifically, this paper
aims to:
1. Assess the global GHG mitigation potential in the agricultural sector from i) livestock
productivity increases above baseline; ii) technical measures; and iii) dietary changes from
baseline.
2. Assess the compatibility of emissions from the agricultural sector with the 2 °C target.
80 Climatic Change (2014) 124:7991
2 Methods and data
2.1 Overview of scenarios and methodology
In the analysis, we include CH
4
from enteric fermentation, paddy rice, and manure manage-
ment and N
2
O from agricultural soils and manure management. We consider CO
2
from fossil
fuels to be part of the energy system and not the food system, and it is therefore omitted. We
also do not include CO
2
from agricultural expansion and other land-use changes. Our
methodological approach is purely biophysical and cannot capture the political and socioeco-
nomic mechanisms behind deforestation and land use change.
To analyze future GHG emissions, we construct regionalized food consumption scenarios
and calculate the corresponding crop and livestock production in each region. Application
rates of N in fertilizer and manure in crop production are estimated from the N content in
harvested crops and assumptions of N use efficiencies for different crops and regions. N
2
O
from agricultural soils is estimated as an emission factor per mass unit N input in fertilizer and
manure as well as crop residues left on the fields. Feed intake in livestock production is
estimated from assumptions on feed energy conversion efficiencies and feed rations for
different livestock systems and regions. Based on the feed intake we estimate N
2
OandCH
4
from manure excretion and storage, as well as CH
4
from enteric fermentation. For details on
the scenario assumptions, see the online supplementary material.
We present GHG emission estimates for the years 2000, 2030, 2050 and 2070. The estimate
for year 2000 is produced for validation purposes. Our main focus is 2050 and 2070 since the
global aggregated emission trajectories to reach the 2 °C target at a chance more likely than not
exhibit substantial emissions reductions by mid-century and beyond. Emissions of CH
4
and
N
2
O are converted to the CO
2
-equivalents using the 100-year global warming potential (GWP)
that includes indirect climate-carbon cycle feedbacks as reported in Myhre et al. (2013).
We examine five different scenarios (see Table 1) that are partly interlinked. The scenarios
are described in more detail in sections 2.22.5.
2.2 Methane and nitrous oxide emissions in the reference scenario
N
2
O emissions associated with fertilizer and manure application is estimated by calculating the
required N input in crop and pasture production given the N content in the required biomass
output and the N utilization efficiency of the applied N. The N efficiency is here defined as the
proportion of applied N that ends up in the above-ground biomass. Regional estimates of N
efficiencies range from 28 % for rice in China (Fan et al. 2009) to about 75 % in forages (Smil
2001; Lenssen et al. 2010). We use data from Cassman et al. (2002), Ladha et al. (2005), and
Smil (2001) to estimate current N efficiencies for different crops in Europe. We then rescale the
values for other regions based on Bouwman et al. (2009). The N efficiency is assumed to
improve over time in most regions. In the reference (baseline) scenario, we assume that N
efficiency remains constant in Europe and gradually converges to the European level in North
America and Pacific OECD. In other regions, N efficiency is assumed to be 20 % lower than in
Europe by 2050. Direct N
2
O emissions related to fertilizer and manure application were
calculated using an emission factor of 1 % N
2
O-N per N applied (IPCC 2006).
CH
4
emissions from enteric fermentation is estimated as a fraction of feed intake in energy
terms (IPCC 2006), assumed to be 7 % for permanent pasture, 6.5 % for forages (silage/hay),
5 % for protein concentrates and 3.5 % for cereal grains in gross energy terms.
For N
2
OandCH
4
from manure management, we estimate the fractions of different manure
handling systems in different regions (based on IPCC 2006) and the respective emission
Climatic Change (2014) 124:7991 81
factors (based on temperature-averages, according to IPCC 2006). Further, from the assumed
feedto-product ratios (Table 2) we estimate the amount of manure produced, which in
combination with the assumptions on manure systems are used to estimate N
2
OandCH
4
emissions.
2.3 Livestock productivity scenarios
Feed requirements are calculated using feed-to-product ratios, here defined as the amount of
feed (in gross energy) required to produce one unit of product (in human-metabolizable
energy). In addition to the feed-to-product ratio, the feed ration is of great importance for
the overall land requirements for animal food production and the associated GHG emissions.
We use two scenarios that reflect different agricultural productivity developments. We
construct one Reference (baseline) scenario (REF) and one Increased Productivity scenario
(IP). In the REF scenario we assume moderate increases in livestock productivity, largely in
line with FAO projections. In the IP scenario we assume faster livestock productivity growth,
based on extrapolations from the Increased Livestock Productivityscenario in Wirsenius
et al. (2010). Table 2presents the corresponding estimates of feed-to-product efficiencies.
2.4 Technical mitigation scenario
We co n s tru c t a Technical Mitigation (TM) scenario, based on the IP scenario, where we assess
mitigation potentials of different technologies and management practices, and how fast these
technologies may be diffused in different regions.
We assume that the N efficiency in crop production gradually increases so that all regions
reach the efficiency level of Europe by 2050. At the global aggregate level, this gives a
Tab l e 1 Main characteristics of scenarios to 2070 of global agricultural greenhouse gas emissions
Scenario Acronym Food consumption Livestock productivity Dedicated technical
measures
Reference
(baseline)
REF Based on FAO
projections
Increases in line with
FAO projections
None
Increased
productivity
IP As in REF Global average feed-to-
product ratio compared
to REF:
Ruminant meat +20 %
Dairy + 50 %
Other meat + 25 %
None
Technical
mitigation
TM As in REF As in IP Improved N efficiency
Altered manure
management
Fat additives to ruminants
Reduced methane from
rice
Climate
carnivore
CC 75 % of ruminant meat
and dairy products are
replaced by other meat
(on kcal basis)
As in IP As in TM
Flexitarian FL 75 % of animal food is
replaced by pulses and
cereals (on kcal basis)
As in IP As in TM
82 Climatic Change (2014) 124:7991
reduction in N
2
O emissions of 12 % in 2070 compared to the IP scenario. No additional N
2
O
mitigation is assumed. There is a rather large potential for mitigation options for CH
4
from
paddy rice production. We assume that emissions per kg rice are gradually abated over time;
the abatement starts in 2030 and grows linearly to 80 % reduction by 2070 (Lucas et al. 2007).
We as s ume th at CH
4
from enteric fermentation may be reduced by 20 %, either by fat
additives or other additives in non-pasture feed. We assume that this mitigation option is
applied to all ruminant systems in Europe, Pacific Oceania and North America by 2070, and to
50 % of ruminant systems in the rest of the world. We assume a linear interpolation from 2030.
This assumption is in line with previous estimates of the mitigation potential for ruminants
(DeAngelo et al. 2006; Beach et al. 2008).
Mitigation of manure emissions is arguably most effectively done by using either anaerobic
digester or coverage and flaring of methane in slurry systems. Both options reduce CH
4
and
N
2
O by around 70 % (Montes et al. 2013). There is an even greater N
2
O mitigation potential if
solid manure systems are converted to slurry systems. We assume a gradual transition first
from solid systems towards slurry systems, and thereafter to slurry systems with flaring. This
means that regional aggregate emission factors of CH
4
and N
2
O from manure management
drop by 3070%to2070comparedtoyear2000.
2.5 Dietary change scenarios
We design two scenarios to explore the potential GHG mitigation from dietary changes. In
both scenarios we use the TM scenario as a basis. The rationale for this is that substantial
deviations from current dietary preferences are unlikely and would probably occur only as a
result of policy interventions. However, policy-driven dietary changes are contentious and
would almost certainly emerge only after productivity improvements and technical measures
largely have been exhausted.
We construct a Climate Carnivore (CC) scenario where 75 % of the ruminant meat and
dairy consumption is replaced by other meat (in kcal terms). This scenario represents an
increase in total meat by consumption per capita by 45 % compared to the baseline, but the
Tab l e 2 Feed-to-product ratios in year 2000 and 2050 for the reference (REF) and increased productivity (IP)
scenarios (region acronyms are explained in the supplementary material)
MJ GE feed/ MJ ME product AFR CPA EUR FSU LAM MEA NAM PAS SAS
Ruminant meat 2000 237 99 60 65 155 161 60 148 440
2050 REF 189 58 51 53 117 71 51 87 191
2050 IP 148 51 51 51 52 51 51 57 153
Dairy bulls 2000 100 54 23 25 77 52 25 81 198
2050 REF 67 32 21 21 48 21 21 48 89
2050 IP 63 21 20 20 26 20 20 31 69
Dairy (whole milk) 2000 47 11 8 12 20 20 7 18 23
2050 REF 38 8 7 9 17 10 7 13 20
2050 IP 16 6 5 6 7 7 5 10 8
Other meat 2000 16 11 7 9 13 10 7 11 14
2050 REF 12 8 7 8 9 8 7 8 9
2050 IP 11 6 6 6 6 6 6 6 7
Data is extrapolated to 2070 by assuming the same relative change as between 2030 and 2050, but with the
European level in the 2050 IP scenario as a floor level
Climatic Change (2014) 124:7991 83
average GHG intensity of this meat consumption is lower. We also make a Flexitarian (FL)
scenario, where 75 % of all meat and dairy products are replaced by cereals and pulses (in kcal
terms). Flexitarianis a term assigned to people that often, but not always, choose to eat
vegetarian food (Forestell et al. 2012). It should be noted that the degree of substitution of
75 % is not based on any systematic analysis of limiting factors, and should therefore be
considered as a tentative basis for an estimate of the upper-end mitigation potential from
dietary changes. In addition to the generally conservative nature of food preferences, factors
that might limit a far-reaching shift away from meat and dairy consumption include, for
example, preservation of landscapes and cultures related to grazing and pastoralism.
2.6 Emission pathways consistent with 2 °C stabilization
To estimate multi-gas emission pathways compatible with the 2 °C target, we use an integrated
climate-economy model based on Johansson (2011). The emission pathways generated in the
model represent the least-cost solutions for meeting the temperature target. The emissions of the
three most important greenhouse gases CO
2
,CH
4
and N
2
O are determined endogenously in the
model. Abatement of these emissions is modeled by marginal abatement cost functions and
with constraints on how rapidly emissions may fall over time. We generate two emission
pathways, using two different assumptions on climate sensitivity (being either 3 or 4 °C for a
doubling of the CO
2
concentration). Climate sensitivity is defined as the equilibriumincrease in
the global annual average surface temperature from a doubling of the atmospheric CO
2
concentration with respect to pre-industrial levels and its value is likely between 1.5 and
4.5 °C (IPCC 2013). Climate sensitivity is critical for the emission space compatible with a
given climate target. The emission pathway generated with a climate sensitivity of 4 °C will
have a larger chance of keeping the global temperature increase below 2 °C as compared to the
emission pathways generated using a climate sensitivity of 3 °C, see supplementary information
for further details.
3Results
3.1 Emissions per unit of product
Figure 1a and bpresent GHG emissions per kg of product across scenarios and selected
regions for ruminant meat and dairy products. The assumed increases in feed efficiency in the
baseline scenario explain most of the difference between 2000 and the REF scenario. As can
be seen, a substantial reduction in emissions per unit of output can be expected by 2050 in
todays low and mid-income regions. However, by assuming faster livestock productivity
improvements than in REF, emissions can be reduced even more in IP. In the TM scenario,
emissions are reduced even further, but the mitigation effect is less than that achieved by
increased livestock productivity.
There are a number of counteracting factors that exert upward pressure on emission levels
as feed-to-product efficiencies increase. First, to achieve higher livestock productivity, larger
shares of the rations have to consist of energy and protein-rich feed, such as cereals or
soybeans. For ruminant systems, the fractions of permanent pasture and crop residues in the
ration therefore typically decline as productivity increases. This means that a larger share of the
land used for feed production has higher N fertilizer application rates, which contributes to
higher overall N
2
O emissions. The smaller share of pasture-based feed in the rations also
means that more manure is produced in stables, which implies higher CH
4
emissions from
84 Climatic Change (2014) 124:7991
manure storage facilities. Second, as milk yield in the dairy sector improves, less cattle meat
(from surplus dairy calves and culled cows) per unit of milk is produced as a by-product. Most
of the GHG emissions from dairy systems may be allocated to the milk output, since it
represents roughly 90 % of the milk and meat output in sales value. Therefore, cattle meat from
the dairy system may be considered to have lower GHG emissions per unit of meat compared
to cattle meat from suckler cow beef systems. Given constant beef demand, this means that the
higher the dairy cow milk yield, the higher the average GHG emissions per unit of total beef
supply. Similarly, in regions where dairy consumption increases at a slower rate than beef
consumption, the share in total beef supply of co-product meat from the dairy sector decreases,
and therefore the average emission intensity of ruminant meat increases. These factors are
behind the increase in Europe in emissions per unit of ruminant meat in 2050 compared to
2000, see Fig. 2.
3.2 Emissions from the global food system
Emissions in the REF scenario amount to 12 Gton CO
2
eq in 2050; ruminant meat is
responsible for around two thirds and animal products in total for about 80 %, see Fig. 2.
Increased livestock productivity cuts the global emission by 2 Gton CO
2
eq in 2050 as seen in
the IP scenario. In the TM scenario, were technical mitigation options are added to the IP
scenario the emissions are reduced to about 8.3 Gton CO
2
eq/year in 2050. The potential for
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Centrally planned
Asia
Europe Latin America North America
kg CO2-eq/kg milk
Dairya
0
10
20
30
40
50
60
Centrally planned
Asia
Europe Latin America North America
kg CO2-eq/kg carcass
Ruminant meatb
2000
REF 2050
IP 2050
TM 2050
2000
REF 2050
IP 2050
TM 2050
Fig. 1 a Estimated GHG emissions per average unit of output in ruminant meat production in 2000 and
scenarios for 2050. (REF Reference, IP Increased productivity, TM Technical mitigation). bEstimated GHG
emissions per unit of output in dairy production in 2000 and scenarios for 2050. (REF Reference, IP Increased
productivity, TM Technical mitigation)
Climatic Change (2014) 124:7991 85
productivity improvements and mitigation measures is more restricted in the ruminant
sector than for dairy, other meat and vegetables; notice that ruminant meat constitutes a
larger fraction of the total emissions in TM than in REF. Changing diets cuts the emissions
further, in the CC scenario the annual emissions are 4.9 Gton CO
2
eq, while in the FL
scenario the total emissions are about 3.1 Gton CO
2
eq/year. Hence, a radical shift in diets
with substantial productivity improvements and technical mitigation measures may cut
baseline emissions by about 75 %. More detailed data and an uncertainty analysis for each
scenario are presented in the supplementary material.
3.3 Agricultural emissions compared to emission pathways compatible with the 2 °C target
The two emission pathways compatible with the 2 °C target, but based on different
assumptions on climate sensitivity, are shown in Fig. 3. The emission pathway gener-
ated using the lower climate sensitivity (3 °C for a CO
2
doubling) has about 50 %
chance of staying below the temperature target, while the emission pathway generated
with the higher sensitivity, gives a higher chance of staying within the limits of the
Ctarget.,
As can be seen in Fig. 3, food-related emissions, if they remain unabated, are on track to
take up most, if not all, of the long-term annual emission space allowable under the 2 °C target
irrespective of the two emission pathways included here. By 2070 in the REF scenario, food-
related GHG emissions alone are as high as, or higher than, the annual emissions in either one
of the two emission pathways. However, that does not necessarily mean that the 2 °C target is
out of reach; if overall emissions can be cut faster than in our illustrative pathways, this could
compensate for higher sustained agricultural GHG emissions in the long-run, see section 4.4
for further discussion.
4Discussion
4.1 Limitations of data and method
The approach in this study is bio-physical and does not explicitly include any economic
factors. The scenario results presented here should be seen as descriptive rather than predictive.
0
2
4
6
8
10
12
14
REF IP TM CC FL
Gton CO2-eq/yr
Global emissions in 2050
Vegetables
Other meat
Dairy
Ruminant meat
Fig. 2 Global food-related GHG emissions from agriculture in scenarios for 2050. (REF Reference, IP Increased
productivity, TM Technical mitigation, CC Climate Carnivore, FL Flexiterian)
86 Climatic Change (2014) 124:7991
This study excluded some GHG emission sources (see 2.1), most notably CO
2
from
changes in land use. Plausible levels of long-term CO
2
emissions from land-use change
span over a wide range, since they depend on several highly uncertain factors. Valin
et al. (2013) estimated these at 1.9 billion ton CO
2
/year in 2050 in their baseline
scenariothis corresponds to about 16 % of total emissions in the REF scenario in this
study.
4.2 Comparison with previous studies
To make valid comparisons with earlier studies, in this section we use the GWP values
reported in Forster et al. (2007) (which were used in earlier studies) instead of Myhre
et al. (2013), which were used in this study. Popp et al. (2010) estimated agricultural
CH
4
and N
2
O emissions in 2055 at about 15 Gton CO
2
eq/year in an increased-meat-
consumption scenario, and at 10 Gton CO
2
eq/year in a scenario with technical mitiga-
tion. Corresponding numbers in this study are 10 Gton CO
2
eq/year (REF scenario) and
8.3Gton CO
2
eq/year (TM scenario), respectively. Lack of transparency in Popp et al.
(2010) prevents a closer investigation of these differences. The generally lower num-
bers in our study may be due to lower increases in per-capita meat consumption and/or
higher productivity growth in the livestock sector.
In contrast, Valin et al. (2013) produced estimates far lower than those in this study.
In their baseline scenario, agricultural CH
4
and N
2
O emissions reach about 4.5 Gton
CO
2
eq/year in 2050, which is less than half of our baseline (REF) scenario, and less
than a third of the corresponding scenario in Popp et al. (2010). Different approaches in
the modeling of the livestock sectorsfeedintakeandGHGemissionsarelikelytobe
the main explanation to these diverging results. This is illustrated by the lower base-
year estimate in Valin et al., which at 3.5 Gton CO
2
eq/year (in year 2000) is far lower
not only than in this study (5.9 Gton CO
2
eq/year), but also in comparison with other
studies, including the recent comprehensive FAO study (Gerber et al. 2013), which
estimated livestock-related emissions at 5.2 Gton CO
2
eq/year.
0
10
20
30
40
50
60
2000 2010 2020 2030 2040 2050 2060 2070 2080
Gton CO2-eq/yr
Reference (REF)
Increased productivity (IP)
Technical mitigation (TM)
Climate Carnivor (CC)
Flexitarian (FL)
Climate sensitivity: 3°C
Climate sensitivity: 4°C
Fig. 3 Emission pathways consistent with the 2 °C target under different climate sensitivities (lines), and
agricultural emissions in the different scenarios for 2030, 2050 and 2070 (bars)
Climatic Change (2014) 124:7991 87
4.3 Constraints in long-term emission reductions in the energy and food systems
To meet the 2 °C target with a probability larger than 50 %, global GHG emissions have to
drop to about 10 Gton CO
2
eq/year or less by the second half of this century. The prospects for
achieving such very deep emission cuts vary across sectors. As indicated in this study, deep
cuts in emissions from food and agriculture do not seem plausible without large changes in
consumption towards less GHG intensive food, in particular less ruminant meat and dairy.
Stronger advancement of mitigation technology than that assumed in this study could avoid
the assertion that dietary changes may be needed for meeting the 2 °C target. Our estimates of
the potential for technical mitigation of CH
4
from ruminants and N
2
O from soils are conser-
vative by design; currently known mitigation technology does not seem to promise large
reductions. Moreover, these sources constitute microbe-controlled pathways in the carbon and
nitrogen cycles in ruminants and soils, and any use of technology to block such pathways will
face the likelihood that microorganisms over time evolve to circumvent blocked pathways. For
instance, nitrification (which produces N
2
O as a by-flow) is an exothermic reaction, and
therefore represents an energetic potential for soil microorganisms to exploit. It therefore
cannot be ruled out that microorganisms would evolve to bypass any pathway blocked through
the use of technology such as nitrification inhibitors. On balance, the prospects for deep emission
cuts in agriculture through technology seem unfavorable.
Irrespective of the development in the food system, other sectors will need to achieve very
large GHG reductions, of the order of 90 % or more, if the 2 °C target is to be met with a large
chance. However, given the substantial constraints for emission reductions in the food system,
it might be cost-effective to seek even deeper cuts in other sectors. This applies particularly to
energy and transport, for which the prospects for deep reductions by technology are more
favorable compared to the food system. However, also in these sectors, technology has
limitations, and totally CO
2
-free systems in 4050 years from now are imaginable only under
optimistic assumptions of technological advancements. More importantly, even if CO
2
-neutral
technologies exist, their diffusion may be a relatively slow process (Wilson 2010).
4.4 Trade-offs between short and long-lived gases in long-term emission reductions
The temperature impact of short-lived GHGs (as CH
4
) and long-lived GHGs (as CO
2
) differ,
which has implications for the long-term emission spaces of the different gases. The temper-
ature impact of short-lived gases correlates well with the annual emissions, whereas that of
long-lived gases correlates well with the cumulative emissions (Smith et al. 2012). Hence, for a
given temperature stabilization level, any change in the cumulative amount of long-lived
GHGs emitted will affect the allowable sustained long-run level of emissions of short-lived
GHGs. This means that, for a given temperature-increase target, any lowering of the cumula-
tive CO
2
emissions will give room for higher sustained emissions of CH
4
.
This study indicates that, for baseline diets, long-term GHG emissions from the global food
system will be dominated by CH
4
, with a strong link to ruminant meat and dairy consumption
levels. Given the trade-off between CH
4
and CO
2
, in particular its asymmetrical nature with
respect to time, societys net influence on the cumulative CO
2
emissions will be decisive for
the long-term emission space of CH
4
, and hence also for the scope for continued dairy and
ruminant meat consumption. Thus, the more cumulative CO
2
emissions are depressed, the
larger room there will be for perpetual CH
4
emissions from ruminants.
In addition to deep cuts in CO
2
emissions from the energy and transport systems as
discussed above, atmospheric CO
2
levels could also be mitigated by technology and manage-
ment options that sequester atmospheric CO
2
. One of these is increased carbon sequestration in
88 Climatic Change (2014) 124:7991
vegetation and/or soils. However, using land for carbon sequestration is likely to compete with
food, fiber and bioenergy production. Another option is to use bioenergy together with carbon
capture and storage (BECCS), which could in theory offer net negative emissions from the
energy system. Such negative emissions could open up the possibility to have larger sustained
emissions of CH
4
(and N
2
O) for a given temperature target (Azar et al. 2013). However, due to
lack hitherto of operational experience, and the inertia in the diffusion of these capital-intensive
technologies, the prospect of a full-blown BECCS industry at the scale needed by the second
half of the century is highly uncertain.
5Conclusions
This paper indicates that, under current trends, food-related agricultural emissions of CH
4
and
N
2
O may increase to about 12.7 Gton CO
2
eq/year by the year 2070. This is likely to be larger
than the total CO
2
-equivalent emission level compatible with meeting the 2 °C limit at chance
larger than 50 % (on the order of 1013 Gton CO
2
eq/year or less in 2070). Under policies that
favor larger increases in livestock productivity as well as substantial implementation of
technical mitigation measures, we estimate that emissions can be kept closer to what is
required for the target (7.7 Gton CO
2
eq/year). However, only by also assuming reduced meat
and dairy consumption do we find agricultural emission levels that do not take more than half
of the total emissions space in 2070. We therefore conclude that dietary changes are crucial for
meeting the 2 °C target with high probability. This conclusion carries even more weight when
one considers that other GHG-emitting sectors, in particular energy, also face significant
constraints in achieving very large reductions.
Stronger advancement of technology than assumed in this study could significantly relax
the need for dietary changes. This applies to technology for mitigation of CH
4
from ruminants
and N
2
O from soils, for which assumed potentials in this study were conservative, on the basis
that known mitigation technology does not seem to promise large reductions. It also applies to
technologies such as BECCS which has the potential to sequester atmospheric CO
2
and
thereby make room for larger sustained agricultural GHG emissions. However, due to lack
of operational experience of BECCS, and the inertia in its diffusion, a full-blown BECCS
industry in 4050 years at the scale needed is unlikely.
Acknowledgments Financial support from E.ON, The Swedish Energy Agency and Carl Bennet AB is
gratefully acknowledged. The authors also wish to thank Christian Azar, David Bryngelsson, Christel Cederberg
and Paulina Essunger, as well as two anonymous reviewers, for valuable comments and suggestions.
Open Access This article is distributed under the terms of the Creative Commons Attribution License which
permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are
credited.
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