農業からの温室効果ガス排出量
Greenhouse gas emissions from agriculture/ja
農業からの温室効果ガス排出量は著しい。農業、林業、土地利用部門は、世界の温室効果ガス排出量の13%から21%を占める。排出は直接的な温室効果ガス排出（例えば、稲作や畜産から）と間接的な排出から生じる。直接排出に関しては、亜酸化窒素とメタンが農業からの総温室効果ガス排出量の半分以上を占める。 2023年のレビューでは、農地の排出量が土壌の種類、気候、管理慣行などの要因によって形成されることが強調されている。また、不耕起栽培、精密農業、水利用の改善、バイオ炭の施用など、排出量を削減し、土壌炭素貯蔵を強化できるいくつかの緩和戦略も強調されている。一方、間接排出は、森林などの非農業用地を農地に転換することから生じる。さらに、輸送や肥料生産のための化石燃料消費もある。例えば、窒素肥料の製造と使用は、世界の温室効果ガス排出量全体の約5%を占める。畜産は温室効果ガス排出の主要な発生源である。同時に、畜産は気候変動の影響を受ける。
家畜の消化器系は、単胃動物と反芻動物の2つのカテゴリーに分けられる。肉牛や乳牛などの反芻動物は温室効果ガス排出量が上位に位置する。それに比べて、単胃動物、つまり豚や家禽関連の食品は排出量が少ない。単胃動物の消費は排出量を減らす可能性がある。単胃動物は飼料転換効率が高く、メタンもあまり生産しない。家禽などの非反芻動物は、温室効果ガスの排出量がはるかに少ない。
農業からの温室効果ガス排出量を削減するための戦略は多く存在する（これは気候スマート農業の目標の一つである）。フードシステムにおける緩和策は、需要側の変化、生態系保護、農場での緩和、サプライチェーンにおける緩和の4つのカテゴリーに分けられる。需要側では、食品廃棄を制限することが食品排出量を削減する効果的な方法である。植物性食品への依存度を低くする食事への変更も効果的である。これには、牛乳代替品や肉代替品が含まれる。また、畜産からの温室効果ガス排出量を削減するために、遺伝子選抜、メタン酸化細菌のルーメンへの導入、飼料の変更、放牧管理など、いくつかの方法が研究されている。
世界的な推定
2022年のアグリフードシステムからの総排出量は162億トンの二酸化炭素換算（Gt CO2eq）の温室効果ガスが大気中に放出され、これは2000年と比較して10%（1.5 Gt CO2eq）の増加である。
2020年には、食料システム全体で温室効果ガス総排出量の37%を占め、この数字は人口増加と食生活の変化により2050年までに30〜40%増加すると推定された。
2010年から2019年の間に、農業、林業、土地利用は世界の温室効果ガス排出量の13%から21%を占めた。亜酸化窒素とメタンは、農業からの温室効果ガス総排出量の半分以上を占める。
過去の推定
2010年には、農業、林業、土地利用変化が世界の年間排出量の20〜25%を占めると推定された。
活動の種類別の排出量
土地利用の変化
農業は、主に4つの方法で土地利用を通じて温室効果ガス増加に寄与している。
これらの農業プロセスは合わせて、メタン排出量の54%、亜酸化窒素排出量の約80%、そして土地利用に関連するほとんどすべての二酸化炭素排出量を占める。
1750年以降、人間が温帯地域を森林破壊したため、土地被覆は大きく変化した。森林や森林地帯が畑や牧草地に道を譲るために開墾されると、影響を受けた地域のアルベドが増加し、現地の状況に応じて温暖化または冷却効果をもたらす可能性がある。森林破壊はまた、地域の炭素再吸収にも影響を与え、その結果、主要な温室効果ガスであるCO2の濃度が増加する可能性がある。焼畑のような土地開墾方法は、バイオマスの燃焼が温室効果ガスやすすのような粒子状物質を直接大気中に放出するため、これらの影響をさらに悪化させる。土地開墾は土壌炭素スポンジを破壊する可能性がある。
畜産
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畜産は、農業からの温室効果ガス排出量の大部分を占め、農業における淡水需要の約30%を必要とする一方で、世界のカロリー摂取量のわずか18%しか供給していない。動物由来の食品は人間のタンパク質需要を満たす上でより大きな役割を果たすが、それでも供給量の39%と少数派であり、残りは作物が供給している。
Out of the Shared Socioeconomic Pathways used by the Intergovernmental Panel on Climate Change, only SSP1 offers any realistic possibility of meeting the 1.5 °C (2.7 °F) target. Together with measures like a massive deployment of green technology, this pathway assumes animal-derived food will play a lower role in global diets relative to now. As a result, there have been calls for phasing out subsidies currently offered to livestock farmers in many places worldwide, and net zero transition plans now involve limits on total livestock headcounts, including substantial reductions of existing stocks in some countries with extensive animal agriculture sectors like Ireland. Yet, an outright end to human consumption of meat and/or animal products is not currently considered a realistic goal.Therefore, any comprehensive plan of adaptation to the effects of climate change, particularly the present and future effects of climate change on agriculture, must also consider livestock.
Livestock activities also contribute disproportionately to land-use effects, since crops such as corn and alfalfa are cultivated to feed the animals.
In 2010, enteric fermentation accounted for 43% of the total greenhouse gas emissions from all agricultural activity in the world. The meat from ruminants has a higher carbon equivalent footprint than other meats or vegetarian sources of protein based on a global meta-analysis of lifecycle assessment studies. Small ruminants such as sheep and goats contribute approximately 475 million tons of carbon dioxide equivalent to GHG emissions, which constitutes around 6.5% of world agriculture sector emissions. Methane production by animals, principally ruminants, makes up an estimated 15-20% of global production of methane.
Worldwide, livestock production occupies 70% of all land used for agriculture or 30% of the land surface of the Earth. The global food system is responsible for one-third of the global anthropogenic GHG emissions, of which meat accounts for nearly 60%.
Cows, sheep and other ruminants digest their food by enteric fermentation, and their burps are the main methane emissions from land use, land-use change, and forestry: together with methane and nitrous oxide from manure, this makes livestock the main source of greenhouse gas emissions from agriculture. In addition, manure left on pasture, applied to soil, and treated in management systems together contribute to 7%–10% of global agricultural greenhouse gas emissions.
The IPCC Sixth Assessment Report in 2022 stated that: "Diets high in plant protein and low in meat and dairy are associated with lower GHG emissions. [...] Where appropriate, a shift to diets with a higher share of plant protein, moderate intake of animal-source foods and reduced intake of saturated fats could lead to substantial decreases in GHG emissions. Benefits would also include reduced land occupation and nutrient losses to the surrounding environment, while at the same time providing health benefits and reducing mortality from diet-related non-communicable diseases."
| Food Types | Greenhouse Gas Emissions (g CO2-Ceq per g protein) |
|---|---|
| Ruminant Meat | 62
|
| Recirculating Aquaculture | 30
|
| Trawling Fishery | 26
|
| Non-recirculating Aquaculture | 12
|
| Pork | 10
|
| Poultry | 10
|
| Dairy | 9.1
|
| Non-trawling Fishery | 8.6
|
| Eggs | 6.8
|
| Starchy Roots | 1.7
|
| Wheat | 1.2
|
| Maize | 1.2
|
| Legumes | 0.25
|
In the context of global GHG emissions, food production within the global food system accounts for approximately 26%. Breaking it down, livestock and fisheries contribute 31%, whereas crop production, land use, and supply chains add 27%, 24%, and 18% respectively to the emissions.
A 2023 study found that a vegan diet reduced emissions by 75%.
Research in New Zealand estimated that switching agricultural production towards a healthier diet while reducing greenhouse gas emissions would cost approximately 1% of the agricultural sector's export revenue for New Zealand, which is an order of magnitude less than the estimated health system savings from a healthier diet.
Research continues on the use of various seaweed species, in particular Asparegopsis armata, as a food additive that helps reduce methane production in ruminants.
Fertilizer
Crop management
Rice production
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In 2022, greenhouse gas emissions from rice cultivation were estimated at 5.7 billion tonnes CO2eq, representing 1.2% of total emissions. Within the agriculture sector, rice produces almost half the greenhouse gas emissions from croplands, Methane is released from rice fields subject to long-term flooding, as this inhibits the soil from absorbing atmospheric oxygen, resulting in anaerobic fermentation of organic matter in the soil. Emissions can be limited by planting new varieties, not flooding continuously, and removing straw.
It is possible to cut methane emissions in rice cultivation by improved water management, combining dry seeding and one drawdown, or executing a sequence of wetting and drying. This results in emission reductions of up to 90% compared to full flooding and even increased yields.Emissions by type of greenhouse gas
Agricultural activities emit the greenhouse gases carbon dioxide, methane and nitrous oxide.
Carbon dioxide emissions
Activities such as tilling of fields, planting of crops, and shipment of products cause carbon dioxide emissions. Agriculture-related emissions of carbon dioxide account for around 11% of global greenhouse gas emissions. Farm practices such as reducing tillage, decreasing empty land, returning biomass residue of crops to the soil, and increasing the use of cover crops can reduce carbon emissions.
Methane emissions
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Traditional rice cultivation is the second biggest agricultural methane source after livestock, with a near-term warming impact equivalent to the carbon dioxide emissions from all aviation. Government involvement in agricultural policy is limited due to the high demand for agricultural products like corn, wheat, and milk. The United States Agency for International Development's (USAID) global hunger and food security initiative, the Feed the Future project, is addressing food loss and waste. By addressing food loss and waste, greenhouse gas emission mitigation is also addressed. By only focusing on dairy systems of 20 value chains in 12 countries, food loss and waste could be reduced by 4-10%. These numbers are impactful and would mitigate greenhouse gas emissions while still feeding the population.
Nitrous oxide emissions
Reducing emissions
Agriculture is often not included in government emissions reduction plans. For example, the agricultural sector is exempt from the EU emissions trading scheme which covers around 40% of the EU greenhouse gas emissions.
関連項目
外部リンク
- Climate change on the Food and Agriculture Organization of the United Nations website.
- Report on the relationship between climate change, agriculture and food security by the International Food Policy Research Institute
- Climate Change, Rice and Asian Agriculture: 12 Things to Know Asian Development Bank
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 | この記事は、クリエイティブ・コモンズ・表示・継承ライセンス3.0のもとで公表されたウィキペディアの項目Greenhouse gas emissions from agriculture(12 July 2025, at 03:13編集記事参照)を翻訳して二次利用しています。 |