地球温暖化緩和対策
Climate change mitigation/ja
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気候変動緩和策（または脱炭素化）は、温室効果ガスを大気中で制限し、気候変動の原因を排除するための行動である。気候変動緩和策には、エネルギーを節約することや、化石燃料を持続可能なエネルギーに置き換えることが含まれる。二次的な緩和戦略には、土地利用の変化や、大気からの二酸化炭素（CO2）除去がある。現在の気候変動緩和政策は不十分であり、2100年までに約2.7℃の地球温暖化をもたらすだろう。これは、2015年のパリ協定が目標とする2℃未満への地球温暖化抑制を大幅に上回る。
太陽エネルギーと風力発電は、他の再生可能エネルギーの選択肢と比較して、最も低いコストで化石燃料を代替できる。日照と風の利用可能性は変動するため、電力網のアップグレードが必要となる場合がある。例えば、様々な電源をまとめるための長距離送電の利用が挙げられる。エネルギー貯蔵も電力出力を均等化するために使用でき、電力生成が低い場合にはデマンドマネジメントによって電力使用を制限できる。クリーンに生成された電力は、通常、輸送機関の動力源、建物の暖房、産業プロセスの稼働において化石燃料を代替できる。航空やセメント生産など、特定のプロセスは脱炭素化がより困難である。このような状況では、炭素回収・貯留（CCS）が正味排出量を削減する選択肢となりうるが、CCS技術を備えた化石燃料発電所は、現在のところ高コストの気候変動緩和策である。
農業や森林破壊といった人間の土地利用の変化は、気候変動の約4分の1を引き起こしている。これらの変化は、植物によって吸収されるCO
2の量や、有機物が腐敗または燃焼して CO
2を放出する量に影響を与える。これらの変化は速い炭素循環の一部である一方、化石燃料は遅い炭素循環の一部として地中に埋蔵されていたCO2を放出する。メタンは、腐敗する有機物や家畜、化石燃料の採掘によって生成される短命の温室効果ガスである。土地利用の変化は、降水パターンや地表の反射率にも影響を与える可能性がある。農業からの排出量を削減するには、食品ロスと廃棄物を減らすこと、より植物ベースの食事（低炭素食とも呼ばれる）に切り替えること、そして農業プロセスを改善することが可能である。
様々な政策が気候変動緩和を促進しうる。炭素価格設定システムが導入されており、それはCO2排出量を課税するか、あるいは総排出量に上限を設け、排出枠を取引するかのいずれかである。化石燃料補助金は、クリーンエネルギー補助金を有利にする形で廃止でき、エネルギー効率対策の導入や電力源への転換に対するインセンティブが提供される。もう一つの問題は、新しいクリーンエネルギー源の建設や送電網の変更を行う際に生じる環境上の異論を克服することである。温室効果ガス排出量の削減または大気からの温室効果ガスの除去による気候変動の抑制は、日射管理（または太陽地球工学）のような気候技術によって補完されうる。補完的な気候変動対策には、気候変動活動が含まれ、政治的および文化的側面に焦点が当てられている。
定義と範囲
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気候変動緩和は、生態系を維持して人間文明を保全することを目指している。これには温室効果ガス排出量の大幅な削減が必要である。気候変動に関する政府間パネル（IPCC）は、（気候変動の）「緩和」を「排出を削減するか、温室効果ガスの吸収源を強化するための人間による介入」と定義している。
様々な気候変動緩和策を並行して進めることが可能である。これは、地球温暖化を1.5度または2度に制限するための単一の道筋が存在しないためである。対策には以下の4つの種類がある。
- 持続可能なエネルギーおよび持続可能な輸送
- エネルギー保全（効率的なエネルギー利用を含む）
- 持続可能な農業およびグリーン産業政策
- 炭素吸収源の強化と二酸化炭素除去（CDR）（炭素隔離を含む）
IPCCは、二酸化炭素除去を「大気中のCO
2を人為的に除去し、地質学的、陸域、または海洋の貯留層、あるいは製品中に永続的に貯蔵する人間活動。既存および潜在的な生物学的または地球化学的CO
2吸収源の人為的強化、および直接空気炭素回収・貯蔵（DACCS）を含むが、人間の活動によって直接引き起こされない自然なCO
2吸収は含まない」と定義している。
排出の傾向と誓約
GHG排出量 2020年 ガスタイプ別
土地利用変化を含まず
100年GWPを使用
合計: 49.8 GtCO
2e
2 主に化石燃料による (72%)
2O 亜酸化窒素 (6%)
CO
2排出量 燃料種別
人間活動による温室効果ガス排出は温室効果を強め、これが気候変動の一因となっている。そのほとんどは、化石燃料（石炭、石油、天然ガス）の燃焼による二酸化炭素だ。人間が引き起こした排出により、大気中の二酸化炭素は産業革命以前のレベルから約50%増加した。2010年代の排出量は年間平均560億トン（Gt）と過去最高を記録した。2016年には、電力、熱、輸送のためのエネルギーが温室効果ガス排出量の73.2%を占めた。直接的な産業プロセスが5.2%、廃棄物が3.2%、農業、林業、土地利用が18.4%を占めている。
発電と輸送は主要な排出源である。最大の単一排出源は石炭火力発電所で、温室効果ガス排出量の20%を占める。森林破壊やその他の土地利用の変化も二酸化炭素やメタンを排出する。人為的なメタン排出の最大の発生源は農業、そしてガス抜きや化石燃料産業からの漏出である。農業における最大のメタン発生源は家畜である。農地の土壌は、一部は肥料が原因で亜酸化窒素を排出する。現在、冷媒からのフッ素化ガスの問題には政治的な解決策がある。これは、多くの国がキガリ改正を批准したためである。
二酸化炭素（CO2）は、排出される温室効果ガスの中で最も優勢である。メタン（CH4）排出量は、短期的にはほぼ同じ影響力を持つ。亜酸化窒素（N2O）とフッ素化ガス（F-ガス）は軽微な役割を果たす。家畜とその排泄物は、全温室効果ガス排出量の5.8%を占める。しかし、この割合は、それぞれのガスの地球温暖化係数を計算するために使用される時間枠によって異なる。
温室効果ガス（GHG）排出量は、CO2換算量で測定される。科学者は、それぞれの地球温暖化係数（GWP）からCO2換算量を決定する。これは大気中での寿命によって決まる。メタン、亜酸化窒素、その他の温室効果ガスの量を二酸化炭素換算量に変換する温室効果ガス会計手法が広く用いられている。推定値は、海洋や陸上吸収源がこれらのガスを吸収する能力に大きく依存する。短寿命気候汚染物質（SLCP）は、数日から15年の期間、大気中に残留する。二酸化炭素は何千年もの間、大気中に留まることがある。短寿命気候汚染物質には、メタン、ハイドロフルオロカーボン（HFC）、対流圏オゾン、ブラックカーボンが含まれる。
科学者たちは、人工衛星を用いて温室効果ガス排出量と森林破壊の位置特定と測定を行うことが増えています。以前は、科学者たちは主に温室効果ガス排出量の推定値や政府の自己申告データに頼っていましたが、現在は衛星による直接的な観測が進んでいる。
必要な排出削減量
UNEPによる年次「排出量ギャップ報告書」は、2022年に排出量をほぼ半減させる必要があると述べた。地球温暖化を1.5℃に制限する軌道に乗るためには、世界の年間GHG排出量を、現在実施されている政策の下での排出予測と比較して、わずか8年間で45%削減しなければならない。そして、残された限られた大気中の炭素予算を使い果たさないために、2030年以降も急速に減少し続けなければならない。」同報告書は、世界は漸進的な変化ではなく、広範な経済全体の変革に焦点を当てるべきだとコメントした。
2022年、気候変動に関する政府間パネル（IPCC）は第6次評価報告書を公表した。同報告書は、地球温暖化を1.5℃（2.7°F）に抑える良い機会を得るためには、温室効果ガス排出量が遅くとも2025年までにピークを迎え、2030年までに43%減少する必要があると警告している。あるいは、国際連合事務総長のアントニオ・グテーレスの言葉を借りれば、「主要な排出国は、今年から排出量を劇的に削減しなければならない」。
有力な気候科学者らによる2023年の統合報告書は、政策に重大な影響を与える気候科学における10の重要な分野を強調した。これらには、1.5℃の温暖化限界を一時的に超えることのほぼ不可避性、化石燃料の迅速かつ管理された段階的廃止の緊急の必要性、二酸化炭素除去技術の規模拡大における課題、自然の炭素吸収源の将来の寄与に関する不確実性、および生物多様性損失と気候変動の相互に関連する危機が含まれる。これらの洞察は、気候変動の多面的な課題に対処するための、即時かつ包括的な緩和戦略の必要性を強調している。
誓約
クライメート・アクション・トラッカーは、2021年11月9日時点の状況を次のように記述した。現在の政策では、今世紀末までに世界の気温は2.7℃上昇し、各国が採択した政策では2.9℃上昇するだろう。各国が2030年の誓約のみを実施した場合、気温は2.4℃上昇する。長期目標も達成された場合、上昇は2.1℃となるだろう。発表されたすべての目標が完全に達成された場合、世界の気温上昇は1.9℃でピークに達し、2100年までに1.8℃に低下するだろう。
2020年までに設定されたほとんどの目標について、決定的または詳細な評価は行われていない。しかし、世界はその年に設定された国際目標のほとんど、あるいは全てを達成できなかったようである。
グラスゴーで開催された2021年国連気候変動会議で一つの進展があった。「クライメート・アクション・トラッカー」を運営する研究者グループは、温室効果ガス排出量の85%を占める国々を調査した。その結果、EU、英国、チリ、コスタリカの4つの国または政治的実体のみが、2030年の緩和目標を達成するための具体的な手順を記述した詳細な公式政策計画を発表していることが判明した。これらの4つの政治体は、世界の温室効果ガス排出量の6%を担っている。
2021年、米国とEUは、2030年までにメタン排出量を30%削減するグローバル・メタン・プレッジを立ち上げた。英国、アルゼンチン、インドネシア、イタリア、メキシコがこのイニシアチブに参加した。ガーナとイラクは参加への関心を示した。ホワイトハウスの会議要約によると、これらの国々は世界のメタン排出量上位15カ国のうち6カ国を占めている。
低炭素エネルギー
エネルギーシステムには、エネルギーの供給と利用が含まれる。これはCO
2の主要な排出源である。地球温暖化を2℃を十分に下回るように制限するためには、エネルギー部門からの二酸化炭素およびその他の温室効果ガス排出量の迅速かつ大幅な削減が必要である。IPCCの提言には、化石燃料消費の削減、低炭素およびゼロ炭素エネルギー源からの生産増加、そして電力と代替エネルギーキャリアの使用増加が含まれる。
ほぼ全てのシナリオと戦略は、再生可能エネルギーの利用を大幅に増加させるとともに、エネルギー効率対策の強化を伴っている。地球温暖化を2℃未満に抑えるためには、再生可能エネルギーの導入を2015年の年間成長率0.25%から1.5%へと6倍に加速する必要があるだろう。
再生可能エネルギーの競争力は、その迅速な導入の鍵となります。2020年には、多くの地域で陸上風力発電と太陽光発電が、新規の大規模電力生産において最も安価な電源となった。再生可能エネルギーは貯蔵コストが高い場合があるが、非再生可能エネルギーは環境浄化コストが高くなる可能性がある。炭素価格は、再生可能エネルギーの競争力を高めることができる。
太陽エネルギーと風力エネルギー
風力と太陽光は、競争力のある生産コストで大量の低炭素エネルギーを供給できる。IPCCは、これら2つの緩和策が2030年までに低コストで排出量を削減する最大の可能性を秘めていると推定しる。
太陽光発電（PV）は、世界の多くの地域で最も安価な発電方法となっている。太陽光発電の成長はほぼ指数関数的であり、1990年代以降、約3年ごとに倍増している。異なる技術として集光型太陽熱発電（CSP）がある。これは、鏡やレンズを用いて広い範囲の太陽光を受光器に集中させる。CSPでは、エネルギーを数時間貯蔵できるため、夕方に供給が可能となる。太陽熱温水器は2010年から2019年の間に倍増した。
高緯度の北半球および南半球の地域は、風力発電の最大の可能性を秘めている。洋上風力発電所はより高価だが、洋上設備は設備容量あたりの発電量が多く、変動が少ない。ほとんどの地域で、風力発電はPV出力が低い冬に高くなる。このため、風力と太陽光発電の組み合わせは、よりバランスの取れたシステムにつながる。
その他の再生可能エネルギー
その他の確立された再生可能エネルギー源には、水力発電、バイオエネルギー、地熱エネルギーがある。
- 水力発電は水力によって発電され、ブラジル、ノルウェー、中国などの国々で主導的な役割を果たしている。しかし、地理的な限界や環境問題も存在する。潮力発電は沿岸地域で利用できる。
- バイオエネルギーは、電力、熱、輸送のためのエネルギーを供給できる。バイオエネルギー、特にバイオガスは、ディスパッチャブル発電を提供できる。植物由来のバイオマスを燃焼させるとCO
2が放出されるが、植物は成長中に大気からCO
2を吸収する。燃料の生産、輸送、加工の技術は、その燃料のライフサイクル排出量に大きな影響を与える。例えば、航空業界は再生可能なバイオ燃料の使用を開始している。 - 地熱発電は、地熱エネルギーから発電される電力である。地熱発電は現在26カ国で利用されている。地熱暖房は70カ国で利用されている。
変動する再生可能エネルギーの統合
風力発電と太陽光発電の電力生産は、常に需要と一致するわけではない。風力や太陽光のような変動性再生可能エネルギー源から信頼性の高い電力を供給するためには、電力システムが柔軟でなければならない。ほとんどの電力網は、石炭火力発電所のような非間欠的なエネルギー源のために構築された。より大量の太陽光および風力エネルギーを系統に統合するには、エネルギーシステムの変更が必要である。これは、電力供給が需要と一致することを確実にするために必要である。
電力システムをより柔軟にするための様々な方法がある。多くの場所で、風力発電と太陽光発電は日ごとおよび季節ごとの規模で補完的である。太陽エネルギー生産が低い夜間や冬には、より多くの風が吹く。異なる地理的地域を長距離送電線で結ぶことも、変動性を減らすことを可能にする。エネルギー需要を時間的にシフトさせることも可能である。エネルギー需要管理とスマートグリッドの利用は、変動性エネルギー生産が最も高い時間帯に合わせることを可能にする。セクターカップリングはさらなる柔軟性を提供できる。これには、パワー・トゥ・ヒートシステムと電気自動車を介して、電力部門を熱およびモビリティ部門に結合させることが含まれる。
エネルギー貯蔵は、間欠性再生可能エネルギーの障壁を克服するのに役立つ。最も一般的に使用され、利用可能な貯蔵方法は揚水発電である。これには、大きな高低差と水へのアクセスが必要である。これらは通常、短期間の電力を貯蔵する。バッテリーはエネルギー密度が低い。このこととコストのために、エネルギー生産の季節間の変動を均衡させるために必要な大規模なエネルギー貯蔵には実用的ではない。一部の場所では、数ヶ月間の使用が可能な揚水式水力貯蔵が実施されている。
原子力
原子力は電力のために再生可能エネルギーを補完しうる。一方で、環境上および安全保障上のリスクがその利益を上回る可能性もある。これらの環境リスクの例としては、放射性水の近隣生態系への排出や、放射性ガスの日常的な放出が挙げられる。
新しい原子炉の建設には現在約10年かかり、これは風力発電や太陽光発電の導入拡大よりもはるかに長い。そして、この期間は信用リスクを生じさせる。しかし、中国では原子力がはるかに安価である可能性がある。中国は相当数の新しい発電所を建設している。2019年時点で、原子力の廃棄物処分の長期費用を計算から除外した場合、原子力発電所の寿命延長費用は他の発電技術と競争力がある。また、原子力事故に対する十分な財政保険も存在しない。
石炭を天然ガスに置き換える
需要削減
温室効果ガス排出を引き起こす製品やサービスへの需要を減らすことは、気候変動の緩和に役立つ。一つは、行動や文化の変化によって需要を減らすことである。例えば、食事を変えること、特に食肉消費を減らすという決定は、個人が気候変動と闘うための効果的な行動である。もう一つは、公共交通機関ネットワークの整備など、インフラを改善することによって需要を減らすことである。最後に、最終用途技術の変化によってエネルギー需要を減らすことができる。例えば、断熱性の良い家は、断熱性の悪い家よりも排出量が少ない。
製品やサービスへの需要を減らす緩和策は、人々が自身の炭素排出量を削減するための個人的な選択をするのに役立つ。これは交通手段や食品の選択において当てはまる。したがって、これらの緩和策には需要削減に焦点を当てた多くの社会側面があり、それゆえ「需要側」の「緩和行動」である。例えば、社会経済的地位が高い人々は、低い人々よりも多くの温室効果ガス排出量を引き起こすことが多い。もし彼らが排出量を削減し、グリーン政策を推進すれば、これらの人々は低炭素ライフスタイルのロールモデルになる可能性がある。しかし、消費者には意識や知覚されたリスクなど、多くの心理的変数が影響を与える。
政府の政策は、需要側の気候変動緩和策を支援することも、妨げることもあり得ます。例えば、公共政策は、循環経済の概念を推進することで気候変動緩和を支援できます。温室効果ガス排出量の削減は、シェアリングエコノミーとも関連している。
経済成長と排出量の相関関係については議論がある。経済成長が必ずしも排出量の増加を意味するわけではなくなってきているようだ。
2024年に『ネイチャー・クライメート・チェンジ』誌に掲載された論文は、気候変動緩和戦略に行動科学を統合することの重要性を強調している。 この論文では、温室効果ガス排出量削減に向けた個人および集団の行動を改善するための6つの主要な提言が示されている。これには、研究における障壁の克服、学際的な協力の促進、実践的な行動志向型ソリューションの推進などが含まれる。 これらの洞察は、気候変動への対処において、行動科学が技術的・政策的措置と並んで極めて重要な役割を果たすことを示唆している。
エネルギー保全と効率
2018年には、世界の一次エネルギー需要が161,000テラワット時（TWh）を超えた。これは、電力、輸送、暖房の全てを含み、その際の損失も考慮した数値である。 特に輸送と電力生産において、化石燃料の利用効率は50%未満と低い。発電所や車両のモーターからは大量の熱が無駄に排出されている。実際に消費されたエネルギー量は大幅に少なく、116,000 TWhにとどまっている。
エネルギー保全とは、エネルギー消費量を減らすために、エネルギーサービスの使用量を減らす努力である。一つの方法は、エネルギーをより効率的に利用することである。これは、同じサービスを生産するために、以前よりも少ないエネルギーを使用することを意味する。もう一つの方法は、使用するサービスの量を減らすことである。これの例としては、運転を減らすことなどが挙げられる。エネルギー保全は、持続可能なエネルギーのヒエラルキーの頂点に位置する。消費者が無駄や損失を減らすことで、エネルギーを保全できる。技術のアップグレード、および運用と保守の改善は、全体的な効率改善につながる。
エネルギー効率（または「エネルギー効率化」）とは、製品やサービスを提供するために必要なエネルギー量を削減するプロセスである。建物のエネルギー効率（「グリーンビルディング」）、工業プロセス、および輸送の改善は、2050年までに世界のエネルギー需要を3分の1削減できる可能性がある。これは、温室効果ガスの世界的排出量を削減するのに役立つだろう。例えば、建物を断熱することで、熱的快適性を達成および維持するために必要な暖房および冷房エネルギーの使用量を減らすことができる。エネルギー効率の改善は、一般により効率的な技術や生産プロセスを採用することによって達成される。もう一つの方法は、一般的に受け入れられている方法を用いてエネルギー損失を減らすことである。
ライフスタイルの変化
気候変動に関する個人の行動は、多くの分野における個人的な選択を含みうる。これらには、食事、旅行、家庭でのエネルギー使用、商品やサービスの消費、家族構成などが含まれる。自身の炭素排出量を削減したいと願う人々は、頻繁な飛行機利用やガソリン車の回避、主に植物ベースの食事を摂ること、子どもの数を減らすこと、衣類や電気製品を長く使うことなど、影響の大きい行動をとることができる。これらのアプローチは、高消費型のライフスタイルを持つ高所得国の国民にとってより現実的である。当然ながら、電気自動車のような選択肢が利用できないため、低所得層の人々がこれらの変化を起こすことはより困難である。気候変動の原因としては、人口増加よりも過剰消費の方がより大きな責任がある。高消費型のライフスタイルは環境への影響が大きく、最も裕福な10%の人々がライフスタイルに起因する総排出量の約半分を排出している。
Dietary change
Some scientists say that avoiding meat and dairy foods is the single biggest way an individual can reduce their environmental impact. The widespread adoption of a vegetarian diet could cut food-related greenhouse gas emissions by 63% by 2050. China introduced new dietary guidelines in 2016 which aim to cut meat consumption by 50% and thereby reduce greenhouse gas emissions by 1 Gt per year by 2030. Overall, food accounts for the largest share of consumption-based greenhouse gas emissions. It is responsible for nearly 20% of the global carbon footprint. Almost 15% of all anthropogenic greenhouse gas emissions have been attributed to the livestock sector.
A shift towards plant-based diets would help to mitigate climate change. In particular, reducing meat consumption would help to reduce methane emissions. If high-income nations switched to a plant-based diet, vast amounts of land used for animal agriculture could be allowed to return to their natural state. This in turn has the potential to sequester 100 billion tonnes of CO
2 by the end of the century. A comprehensive analysis found that plant based diets reduce emissions, water pollution and land use significantly (by 75%), while reducing the destruction of wildlife and usage of water.
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Preserving and enhancing carbon sinks
2 emissions have been absorbed by carbon sinks, including plant growth, soil uptake, and ocean uptake (2020 Global Carbon Budget).
An important mitigation measure is "preserving and enhancing carbon sinks". This refers to the management of Earth's natural carbon sinks in a way that preserves or increases their capability to remove CO2 from the atmosphere and to store it durably. Scientists call this process also carbon sequestration. In the context of climate change mitigation, the IPCC defines a sink as "Any process, activity or mechanism which removes a greenhouse gas, an aerosol or a precursor of a greenhouse gas from the atmosphere". Globally, the two most important carbon sinks are vegetation and the ocean.
To enhance the ability of ecosystems to sequester carbon, changes are necessary in agriculture and forestry. Examples are preventing deforestation and restoring natural ecosystems by reforestation. Scenarios that limit global warming to 1.5 °C typically project the large-scale use of carbon dioxide removal methods over the 21st century. There are concerns about over-reliance on these technologies, and their environmental impacts. But ecosystem restoration and reduced conversion are among the mitigation tools that can yield the most emissions reductions before 2030.
Land-based mitigation options are referred to as "AFOLU mitigation options" in the 2022 IPCC report on mitigation. The abbreviation stands for "agriculture, forestry and other land use" The report described the economic mitigation potential from relevant activities around forests and ecosystems as follows: "the conservation, improved management, and restoration of forests and other ecosystems (coastal wetlands, peatlands, savannas and grasslands)". A high mitigation potential is found for reducing deforestation in tropical regions. The economic potential of these activities has been estimated to be 4.2 to 7.4 gigatonnes of carbon dioxide equivalent (GtCO2 -eq) per year.
Forests
Conservation
The Stern Review on the economics of climate change stated in 2007 that curbing deforestation was a highly cost-effective way of reducing greenhouse gas emissions. About 95% of deforestation occurs in the tropics, where clearing of land for agriculture is one of the main causes. One forest conservation strategy is to transfer rights over land from public ownership to its indigenous inhabitants. Land concessions often go to powerful extractive companies. Conservation strategies that exclude and even evict humans, called fortress conservation, often lead to more exploitation of the land. This is because the native inhabitants turn to work for extractive companies to survive.
Proforestation is promoting forests to capture their full ecological potential. This is a mitigation strategy as secondary forests that have regrown in abandoned farmland are found to have less biodiversity than the original old-growth forests. Original forests store 60% more carbon than these new forests. Strategies include rewilding and establishing wildlife corridors.
Afforestation, reforestation and preventing desertification
2) until 2100. But they are not a viable alternative to aggressive emissions reduction. This is because the plantations would need to be so large they would eliminate most natural ecosystems or reduce food production. One example is the Trillion Tree Campaign. However, preserving biodiversity is also important and for example not all grasslands are suitable for conversion into forests. Grasslands can even turn from carbon sinks to carbon sources.
Reforestation is the restocking of existing depleted forests or in places where there were recently forests. Reforestation could save at least 1 GtCO2 per year, at an estimated cost of $5–15 per tonne of carbon dioxide (tCO2). Restoring all degraded forests all over the world could capture about 205 GtC (750 GtCO
2). With increased intensive agriculture and urbanisation, there is an increase in the amount of abandoned farmland. By some estimates, for every acre of original old-growth forest cut down, more than 50 acres of new secondary forests are growing. In some countries, promoting regrowth on abandoned farmland could offset years of emissions.
Planting new trees can be expensive and a risky investment. For example, about 80 per cent of planted trees in the Sahel die within two years. Reforestation has higher carbon storage potential than afforestation. Even long-deforested areas still contain an "underground forest" of living roots and tree stumps. Helping native species sprout naturally is cheaper than planting new trees and they are more likely to survive. This could include pruning and coppicing to accelerate growth. This also provides woodfuel, which is otherwise a major source of deforestation. Such practices, called farmer-managed natural regeneration, are centuries old but the biggest obstacle towards implementation is ownership of the trees by the state. The state often sells timber rights to businesses which leads to locals uprooting seedlings because they see them as a liability. Legal aid for locals and changes to property law such as in Mali and Niger have led to significant changes. Scientists describe them as the largest positive environmental transformation in Africa. It is possible to discern from space the border between Niger and the more barren land in Nigeria, where the law has not changed.
Rangelands account for more half the world’s land and could sequester 35% of terrestrial carbon. Pastoralists are those who move with their herds that feed and migrate over often unenclosed grazelands. Such land is usually unable to grow any other kind of food. Rangelands coevolved with large wild herds, many of which have decreased or gone extinct, and pastoralists’ herds replace such wild herds and thus help maintain the ecosystem. However, the movement of herds grazing on large areas over vast distances is increasingly restricted by governments, who often grant exclusive title to lands for more profitable uses which restricts pastoralists to more enclose spaces. This has led to the overgrazing of the land and desertification, as well as conflict.
Soils
There are many measures to increase soil carbon. This makes it complex and hard to measure and account for. One advantage is that there are fewer trade-offs for these measures than for BECCS or afforestation, for example.
Globally, protecting healthy soils and restoring the soil carbon sponge could remove 7.6 billion tonnes of carbon dioxide from the atmosphere annually. This is more than the annual emissions of the US. Trees capture CO
2 while growing above ground and exuding larger amounts of carbon below ground. Trees contribute to the building of a soil carbon sponge. Carbon formed above ground is released as CO
2 immediately when wood is burned. If dead wood remains untouched, only some of the carbon returns to the atmosphere as decomposition proceeds.
Farming can deplete soil carbon and render soil incapable of supporting life. However, conservation farming can protect carbon in soils, and repair damage over time. The farming practice of cover crops is a form of carbon farming. Methods that enhance carbon sequestration in soil include no-till farming, residue mulching and crop rotation. Scientists have described the best management practices for European soils to increase soil organic carbon. These are conversion of arable land to grassland, straw incorporation, reduced tillage, straw incorporation combined with reduced tillage, ley cropping system and cover crops.
Another mitigation option is the production of biochar and its storage in soils This is the solid material that remains after the pyrolysis of biomass. Biochar production releases half of the carbon from the biomass—either released into the atmosphere or captured with CCS—and retains the other half in the stable biochar. It can endure in soil for thousands of years. Biochar may increase the soil fertility of acidic soils and increase agricultural productivity. During production of biochar, heat is released which may be used as bioenergy.
Wetlands
Wetland restoration is an important mitigation measure. It has moderate to great mitigation potential on a limited land area with low trade-offs and costs. Wetlands perform two important functions in relation to climate change. They can sequester carbon, converting carbon dioxide to solid plant material through photosynthesis. They also store and regulate water. Wetlands store about 45 million tonnes of carbon per year globally.
Some wetlands are a significant source of methane emissions. Some also emit nitrous oxide. Peatland globally covers just 3% of the land's surface. But it stores up to 550 gigatonnes (Gt) of carbon. This represents 42% of all soil carbon and exceeds the carbon stored in all other vegetation types, including the world's forests. The threat to peatlands includes draining the areas for agriculture. Another threat is cutting down trees for lumber, as the trees help hold and fix the peatland. Additionally, peat is often sold for compost. It is possible to restore degraded peatlands by blocking drainage channels in the peatland, and allowing natural vegetation to recover.
Mangroves, salt marshes and seagrasses make up the majority of the ocean's vegetated habitats. They only equal 0.05% of the plant biomass on land. But they store carbon 40 times faster than tropical forests. Bottom trawling, dredging for coastal development and fertiliser runoff have damaged coastal habitats. Notably, 85% of oyster reefs globally have been removed in the last two centuries. Oyster reefs clean the water and help other species thrive. This increases biomass in that area. In addition, oyster reefs mitigate the effects of climate change by reducing the force of waves from hurricanes. They also reduce the erosion from rising sea levels. Restoration of coastal wetlands is thought to be more cost-effective than restoration of inland wetlands.
Deep ocean
These options focus on the carbon which ocean reservoirs can store. They include ocean fertilization, ocean alkalinity enhancement or enhanced weathering.The IPCC found in 2022 ocean-based mitigation options currently have only limited deployment potential. But it assessed that their future mitigation potential is large. It found that in total, ocean-based methods could remove 1–100 Gt of CO
2 per year. Their costs are in the order of US$40–500 per tonne of CO
2. Most of these options could also help to reduce ocean acidification. This is the drop in pH value caused by increased atmospheric CO2 concentrations.
The recovery of whale populations can play a role in mitigation as whales play a significant part in nutrient recycling in the ocean. This occurs through what is referred to as the whale pump, where whales’ liquid feces stay at the surface of the ocean. Phytoplankton live near the surface of the ocean in order use sunlight to photosynthesize and rely on much of the carbon, nitrogen and iron of the feces. As the phytoplankton form the base of the marine food chain this increases ocean biomass and thus the amount of carbon sequestrated in it.
Blue carbon management is another type of ocean-based biological carbon dioxide removal (CDR). It can involve land-based as well as ocean-based measures. The term usually refers to the role that tidal marshes, mangroves and seagrasses can play in carbon sequestration. Some of these efforts can also take place in deep ocean waters. This is where the vast majority of ocean carbon is held. These ecosystems can contribute to climate change mitigation and also to ecosystem-based adaptation. Conversely, when blue carbon ecosystems are degraded or lost they release carbon back to the atmosphere. There is increasing interest in developing blue carbon potential. Scientists have found that in some cases these types of ecosystems remove far more carbon per area than terrestrial forests. However, the long-term effectiveness of blue carbon as a carbon dioxide removal solution remains under discussion.
Enhanced weathering
Enhanced weathering could remove 2–4 Gt of CO
2 per year. This process aims to accelerate natural weathering by spreading finely ground silicate rock, such as basalt, onto surfaces. This speeds up chemical reactions between rocks, water, and air. It removes carbon dioxide from the atmosphere, permanently storing it in solid carbonate minerals or ocean alkalinity.
Other methods to capture and store CO2
In addition to traditional land-based methods to remove carbon dioxide (CO2) from the air, other technologies are under development. These could reduce CO2 emissions and lower existing atmospheric CO2 levels. Carbon capture and storage (CCS) is a method to mitigate climate change by capturing CO2 from large point sources, such as cement factories or biomass power plants. It then stores it away safely instead of releasing it into the atmosphere. The IPCC estimates that the costs of halting global warming would double without CCS.
Among the most viable carbon dioxide removal methods considered alongside solar radiation modification, biochar soil amendment is already being deployed commercially. Studies indicate that the carbon it contains remains stable in soils for centuries, giving it a durable potential of removing gigatonnes of CO2 per year. Expert assessments place the net cost of removing CO2 with biochar between US$30 and $120 per tonne. Market data show that biochar supplied 94% of all durable CDR credits delivered in 2023, demonstrating current scalability. Stratospheric aerosol injection (SAI), by comparison, could reduce global temperature quickly by dispersing sulfate aerosols in the stratosphere; however, deployment at climatically relevant scale would require the design and certification of a new fleet of high‑altitude aircraft, a process estimated to take a decade or more, and ongoing operating costs of about US$18 billion for each degree Celsius of cooling. While models confirm that SAI would lower global mean temperature, there are potential side effect including ozone depletion, altered regional precipitation patterns, and the risk of a sudden "termination shock" warming if the programme were interrupted. These systemic risks are absent from biochar deployment.
Bioenergy with carbon capture and storage (BECCS) expands on the potential of CCS and aims to lower atmospheric CO2 levels. This process uses biomass grown for bioenergy. The biomass yields energy in useful forms such as electricity, heat, biofuels, etc. through consumption of the biomass via combustion, fermentation, or pyrolysis. The process captures the CO2 that was extracted from the atmosphere when it grew. It then stores it underground or via land application as biochar. This effectively removes it from the atmosphere. This makes BECCS a negative emissions technology (NET).
Scientists estimated the potential range of negative emissions from BECCS in 2018 as 0–22 Gt per year. 2022年現在[update], BECCS was capturing approximately 2 million tonnes per year of CO2 annually. The cost and availability of biomass limits wide deployment of BECCS.:10 BECCS currently forms a big part of achieving climate targets beyond 2050 in modelling, such as by the Integrated Assessment Models (IAMs) associated with the IPCC process. But many scientists are sceptical due to the risk of loss of biodiversity.
Direct air capture is a process of capturing CO
2 directly from the ambient air. This is in contrast to CCS which captures carbon from point sources. It generates a concentrated stream of CO
2 for sequestration, utilisation or production of carbon-neutral fuel and windgas.
Mitigation by sector
2 equivalent quantities (GtCO
2e).
Buildings
The building sector accounts for 23% of global energy-related CO
2 emissions. About half of the energy is used for space and water heating. Building insulation can reduce the primary energy demand significantly. Heat pump loads may also provide a flexible resource that can participate in demand response to integrate variable renewable resources into the grid. Solar water heating uses thermal energy directly. Sufficiency measures include moving to smaller houses when the needs of households change, mixed use of spaces and the collective use of devices. Planners and civil engineers can construct new buildings using passive solar building design, low-energy building, or zero-energy building techniques. In addition, it is possible to design buildings that are more energy-efficient to cool by using lighter-coloured, more reflective materials in the development of urban areas.
Heat pumps efficiently heat buildings, and cool them by air conditioning. A modern heat pump typically transports around three to five times more thermal energy than electrical energy consumed. The amount depends on the coefficient of performance and the outside temperature.
Refrigeration and air conditioning account for about 10% of global CO
2 emissions caused by fossil fuel-based energy production and the use of fluorinated gases. Alternative cooling systems, such as passive cooling building design and passive daytime radiative cooling surfaces, can reduce air conditioning use. Suburbs and cities in hot and arid climates can significantly reduce energy consumption from cooling with daytime radiative cooling.
Energy consumption for cooling is likely to rise significantly due to increasing heat and availability of devices in poorer countries. Of the 2.8 billion people living in the hottest parts of the world, only 8% currently have air conditioners, compared with 90% of people in the US and Japan. By combining energy efficiency improvements and decarbonising electricity for air conditioning with the transition away from super-polluting refrigerants, the world could avoid cumulative greenhouse gas emissions of up to 210–460 GtCO
2-eq over the next four decades. A shift to renewable energy in the cooling sector comes with two advantages: Solar energy production with mid-day peaks corresponds with the load required for cooling and additionally, cooling has a large potential for load management in the electric grid.
Urban planning
Cities emitted 28 GtCO2-eq in 2020 of combined CO2 and CH
4 emissions. This was from producing and consuming goods and services. Climate-smart urban planning aims to reduce sprawl to reduce the distance travelled. This lowers emissions from transportation. Switching from cars by improving walkability and cycling infrastructure is beneficial to a country's economy as a whole.
Urban forestry, lakes and other blue and green infrastructure can reduce emissions directly and indirectly by reducing energy demand for cooling. Methane emissions from municipal solid waste can be reduced by segregation, composting, and recycling.
Transport
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Transportation accounts for 15% of emissions worldwide. Increasing the use of public transport, low-carbon freight transport and cycling are important components of transport decarbonisation.
Electric vehicles and environmentally friendly rail help to reduce the consumption of fossil fuels. In most cases, electric trains are more efficient than air transport and truck transport. Other efficiency means include improved public transport, smart mobility, carsharing and electric hybrids. Fossil-fuel for passenger cars can be included in emissions trading. Furthermore, moving away from a car-dominated transport system towards low-carbon advanced public transport system is important.
The World Bank is helping lower income countries buy electric buses. Their purchase price is higher than diesel buses. But lower running costs and health improvements due to cleaner air can offset this higher price.
Between one quarter and three quarters of cars on the road by 2050 are forecast to be electric vehicles. Hydrogen may be a solution for long-distance heavy freight trucks, if batteries alone are too heavy.
Shipping
In the shipping industry, the use of liquefied natural gas (LNG) as a marine bunker fuel is driven by emissions regulations. Ship operators must switch from heavy fuel oil to more expensive oil-based fuels, implement costly flue gas treatment technologies or switch to LNG engines. Methane slip, when gas leaks unburned through the engine, lowers the advantages of LNG. Maersk, the world's biggest container shipping line and vessel operator, warns of stranded assets when investing in transitional fuels like LNG. The company lists green ammonia as one of the preferred fuel types of the future. It has announced the first carbon-neutral vessel on the water by 2023, running on carbon-neutral methanol. Cruise operators are trialling partially hydrogen-powered ships.
Hybrid and all electric ferries are suitable for short distances. Norway's goal is an all electric fleet by 2025.
Air transport
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2 emissions.
Jet airliners contribute to climate change by emitting carbon dioxide, nitrogen oxides, contrails and particulates. Their radiative forcing is estimated at 1.3–1.4 that of CO
2 alone, excluding induced cirrus cloud. In 2018, global commercial operations generated 2.4% of all CO
2 emissions.
The aviation industry has become more fuel efficient. But overall emissions have risen as the volume of air travel has increased.By 2020, aviation emissions were 70% higher than in 2005 and they could grow by 300% by 2050.
It is possible to reduce aviation's environmental footprint by better fuel economy in aircraft. Optimising flight routes to lower non-CO
2 effects on climate from nitrogen oxides, particulates or contrails can also help. Aviation biofuel, carbon emission trading and carbon offsetting, part of the 191 nation ICAO's Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA), can lower CO
2 emissions. Short-haul flight bans, train connections, personal choices and taxation on flights can lead to fewer flights. Hybrid electric aircraft and electric aircraft or hydrogen-powered aircraft may replace fossil fuel-powered aircraft.
Experts expect emissions from aviation to rise in most projections, at least until 2040. They currently amount to 180 Mt of CO
2 or 11% of transport emissions. Aviation biofuel and hydrogen can only cover a small proportion of flights in the coming years. Experts expect hybrid-driven aircraft to start commercial regional scheduled flights after 2030. Battery-powered aircraft are likely to enter the market after 2035. Under CORSIA, flight operators can purchase carbon offsets to cover their emissions above 2019 levels. CORSIA will be compulsory from 2027.
Agriculture, forestry and land use
Almost 20% of greenhouse gas emissions come from the agriculture and forestry sector. To significantly reduce these emissions, annual investments in the agriculture sector need to increase to $260 billion by 2030. The potential benefits from these investments are estimated at $4.3 trillion by 2030, offering a substantial economic return of 16-to-1.
Mitigation measures in the food system can be divided into four categories. These are demand-side changes, ecosystem protections, mitigation on farms, and mitigation in supply chains. On the demand side, limiting food waste is an effective way to reduce food emissions. Changes to a diet less reliant on animal products such as plant-based diets are also effective.
With 21% of global methane emissions, cattle are a major driver of global warming. When rainforests are cut and the land is converted for grazing, the impact is even higher. In Brazil, producing 1 kg of beef can result in the emission of up to 335 kg CO2-eq. Other livestock, manure management and rice cultivation also emit greenhouse gases, in addition to fossil fuel combustion in agriculture.
Important mitigation options for reducing the greenhouse gas emissions from livestock include genetic selection, introduction of methanotrophic bacteria into the rumen, diet modification and grazing management. Other options are diet changes towards ruminant-free alternatives, such as milk substitutes and meat analogues. Non-ruminant livestock, such as poultry, emit far fewer GHGs.
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.
Reducing the usage of nitrogen fertilizers through nutrient management could avoid nitrous oxide emissions equal to 2.77 - 11.48 gigatons of carbon dioxide from 2020 to 2050.
Industry
Global carbon dioxide emissions by country in 2023:
Industry is the largest emitter of greenhouse gases when direct and indirect emissions are included. Electrification can reduce emissions from industry. Green hydrogen can play a major role in energy-intensive industries for which electricity is not an option. Further mitigation options involve the steel and cement industry, which can switch to a less polluting production process. Products can be made with less material to reduce emission-intensity and industrial processes can be made more efficient. Finally, circular economy measures reduce the need for new materials. This also saves on emissions that would have been released from the mining of collecting of those materials.
The decarbonisation of cement production requires new technologies, and therefore investment in innovation. But no technology for mitigation is yet mature. So CCS will be necessary at least in the short-term.
Another sector with a significant carbon footprint is the steel sector, which is responsible for about 7% of global emissions. Emissions can be reduced by using electric arc furnaces to melt and recycle scrap steel. To produce virgin steel without emissions, blast furnaces could be replaced by hydrogen direct reduced iron and electric arc furnaces. Alternatively, carbon capture and storage solutions can be used.
Coal, gas and oil production often come with significant methane leakage. In the early 2020s some governments recognised the scale of the problem and introduced regulations. Methane leaks at oil and gas wells and processing plants are cost-effective to fix in countries which can easily trade gas internationally. There are leaks in countries where gas is cheap; such as Iran, and Turkmenistan. Nearly all this can be stopped by replacing old components and preventing routine flaring. Coalbed methane may continue leaking even after the mine has been closed. But it can be captured by drainage and/or ventilation systems. Fossil fuel firms do not always have financial incentives to tackle methane leakage.
Co-benefits
Co-benefits of climate change mitigation, also often referred to as ancillary benefits, were firstly dominated in the scientific literature by studies that describe how lower GHG emissions lead to better air quality and consequently impact human health positively. The scope of co-benefits research expanded to its economic, social, ecological and political implications.
Positive secondary effects that occur from climate mitigation and adaptation measures have been mentioned in research since the 1990s. The IPCC first mentioned the role of co-benefits in 2001, followed by its fourth and fifth assessment cycle stressing improved working environment, reduced waste, health benefits and reduced capital expenditures.
The IPCC pointed out in 2007: "Co-benefits of GHG mitigation can be an important decision criteria in analyses carried out by policy-makers, but they are often neglected" and added that the co-benefits are "not quantified, monetised or even identified by businesses and decision-makers". Appropriate consideration of co-benefits can greatly "influence policy decisions concerning the timing and level of mitigation action", and there can be "significant advantages to the national economy and technical innovation".
An analysis of climate action in the UK found that public health benefits are a major component of the total benefits derived from climate action.
Employment and economic development
Co-benefits can positively impact employment, industrial development, states' energy independence and energy self-consumption. The deployment of renewable energies can foster job opportunities. Depending on the country and deployment scenario, replacing coal power plants with renewable energy can more than double the number of jobs per average MW capacity. Investments in renewable energies, especially in solar- and wind energy, can boost the value of production. Countries which rely on energy imports can enhance their energy independence and ensure supply security by deploying renewables. National energy generation from renewables lowers the demand for fossil fuel imports which scales up annual economic saving.
The European Commission forecasts a shortage of 180,000 skilled workers in hydrogen production and 66,000 in solar photovoltaic power by 2030.
Energy security
A higher share of renewables can additionally lead to more energy security.Socioeconomic co-benefits have been analysed such as energy access in rural areas and improved rural livelihoods. Rural areas which are not fully electrified can benefit from the deployment of renewable energies. Solar-powered mini-grids can remain economically viable, cost-competitive and reduce the number of power cuts. Energy reliability has additional social implications: stable electricity improves the quality of education.
The International Energy Agency (IEA) spelled out the "multiple benefits approach" of energy efficiency while the International Renewable Energy Agency (IRENA) operationalised the list of co-benefits of the renewable energy sector.
Health and well-being
The health benefits from climate change mitigation are significant. Potential measures can not only mitigate future health impacts from climate change but also improve health directly. Climate change mitigation is interconnected with various health co-benefits, such as those from reduced air pollution. Air pollution generated by fossil fuel combustion is both a major driver of global warming and the cause of a large number of annual deaths. Some estimates are as high as 8.7 million excess deaths during 2018. A 2023 study estimated that fossil fuels kill over 5 million people each year, as of 2019, by causing diseases such as heart attack, stroke and chronic obstructive pulmonary disease. Particulate air pollution kills by far the most, followed by ground-level ozone.
Mitigation policies can also promote healthier diets such as less red meat, more active lifestyles, and increased exposure to green urban spaces. Access to urban green spaces provides benefits to mental health as well. The increased use of green and blue infrastructure can reduce the urban heat island effect. This reduces heat stress on people.
Climate change adaptation
Some mitigation measures have co-benefits in the area of climate change adaptation. This is for example the case for many nature-based solutions. Examples in the urban context include urban green and blue infrastructure which provide mitigation as well as adaptation benefits. This can be in the form of urban forests and street trees, green roofs and walls, urban agriculture and so forth. The mitigation is achieved through the conservation and expansion of carbon sinks and reduced energy use of buildings. Adaptation benefits come for example through reduced heat stress and flooding risk.
Negative side effects
Mitigation measures can also have negative side effects and risks. In agriculture and forestry, mitigation measures can affect biodiversity and ecosystem functioning. In renewable energy, mining for metals and minerals can increase threats to conservation areas. There is some research into ways to recycle solar panels and electronic waste. This would create a source for materials so there is no need to mine them.
Scholars have found that discussions about risks and negative side effects of mitigation measures can lead to deadlock or the feeling that there are insuperable barriers to taking action.
Costs and funding
Several factors affect mitigation cost estimates. One is the baseline. This is a reference scenario that the alternative mitigation scenario is compared with. Others are the way costs are modelled, and assumptions about future government policy. Cost estimates for mitigation for specific regions depend on the quantity of emissions allowed for that region in future, as well as the timing of interventions.
Mitigation costs will vary according to how and when emissions are cut. Early, well-planned action will minimise the costs. Globally, the benefits of keeping warming under 2 °C exceed the costs, which according to The Economist are affordable.
Economists estimate the cost of climate change mitigation at between 1% and 2% of GDP. While this is a large sum, it is still far less than the subsidies governments provide to the ailing fossil fuel industry. The International Monetary Fund estimated this at more than $5 trillion per year.
Another estimate says that financial flows for climate mitigation and adaptation are going to be over $800 billion per year. These financial requirements are predicted to exceed $4 trillion per year by 2030.
Globally, limiting warming to 2 °C may result in higher economic benefits than economic costs. The economic repercussions of mitigation vary widely across regions and households, depending on policy design and level of international cooperation. Delayed global cooperation increases policy costs across regions, especially in those that are relatively carbon intensive at present. Pathways with uniform carbon values show higher mitigation costs in more carbon-intensive regions, in fossil-fuels exporting regions and in poorer regions. Aggregate quantifications expressed in GDP or monetary terms undervalue the economic effects on households in poorer countries. The actual effects on welfare and well-being are comparatively larger.
Cost–benefit analysis may be unsuitable for analysing climate change mitigation as a whole. But it is still useful for analysing the difference between a 1.5 °C target and 2 °C. One way of estimating the cost of reducing emissions is by considering the likely costs of potential technological and output changes. Policymakers can compare the marginal abatement costs of different methods to assess the cost and amount of possible abatement over time. The marginal abatement costs of the various measures will differ by country, by sector, and over time.
Eco-tariffs on only imports contribute to reduced global export competitiveness and to deindustrialisation.
Avoided costs of climate change effects
It is possible to avoid some of the costs of the effects of climate change by limiting climate change. According to the Stern Review, inaction can be as high as the equivalent of losing at least 5% of global gross domestic product (GDP) each year, now and forever. This can be up to 20% of GDP or more when including a wider range of risks and impacts. But mitigating climate change will only cost about 2% of GDP. Also it may not be a good idea from a financial perspective to delay significant reductions in greenhouse gas emissions.
Mitigation solutions are often evaluated in terms of costs and greenhouse gas reduction potentials. This fails to take into account the direct effects on human well-being.
Distributing emissions abatement costs
Mitigation at the speed and scale required to limit warming to 2 °C or below implies deep economic and structural changes. These raise multiple types of distributional concerns across regions, income classes and sectors.
There have been different proposals on how to allocate responsibility for cutting emissions. These include egalitarianism, basic needs according to a minimum level of consumption, proportionality and the polluter-pays principle. A specific proposal is "equal per capita entitlements". This approach has two categories. In the first category, emissions are allocated according to national population. In the second category, emissions are allocated in a way that attempts to account for historical or cumulative emissions.
Funding
In order to reconcile economic development with mitigating carbon emissions, developing countries need particular support. This would be both financial and technical. The IPCC found that accelerated support would also tackle inequities in financial and economic vulnerability to climate change. One way to achieve this is the Kyoto Protocol's Clean Development Mechanism (CDM).
Policies
National policies
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2 emissions in the world with the U.S. second, per capita the U.S. leads China by a fair margin (data from 2017).
An evaluation was conducted on 1,500 climate policy interventions made between 1998 and 2022. The interventions took place in 41 countries and across 6 continents, which together contributed 81% of the world's total emissions as of 2019. The evaluation found 63 successful interventions that resulted in significant emission reductions; the total CO
2 release averted by these interventions was between 0.6 and 1.8 billion metric tonnes. The study focused on interventions with at least 4.5% emission reductions, but the researchers noted that meeting the reductions required by the Paris Agreement would require 23 billion metric tonnes per year. Generally, carbon pricing was found to be most effective in developed countries, while regulation was most effective in the developing countries. Complementary policy mixes benefited from synergies, and were mostly found to be more effective interventions than the implementation of isolated policies.
The OECD recognise 48 distinct climate mitigation policies suitable for implementation at national level. Broadly, these can be categorised into three types: market based instruments, non market based instruments and other policies.
- Other policies include the Establishing an Independent climate advisory body.
- Non market based policies include the Implementing or tighening of Regulatory standards. These set technology or performance standards. They can be effective in addressing the market failure of informational barriers.
- Among market based policies, the carbon price has been found to be the most effective (at least for developed economies), and has its own section below. Additional market based policy instruments for climate change mitigation include:
Emissions taxes These often require domestic emitters to pay a fixed fee or tax for every tonne of CO2 emissions they release into the atmosphere. Methane emissions from fossil fuel extraction are also occasionally taxed. But methane and nitrous oxide from agriculture are typically not subject to tax.
Removing unhelpful subsidies: Many countries provide subsidies for activities that affect emissions. For example, significant fossil fuel subsidies are present in many countries. Phasing-out fossil fuel subsidies is crucial to address the climate crisis. It must however be done carefully to avoid protests and making poor people poorer.
Creating helpful subsidies: Creating subsidies and financial incentives. One example is energy subsidies to support clean generation which is not yet commercially viable such as tidal power.
Tradable permits: A permit system can limit emissions.
Carbon pricing
Imposing additional costs on greenhouse gas emissions can make fossil fuels less competitive and accelerate investments into low-carbon sources of energy. A growing number of countries raise a fixed carbon tax or participate in dynamic carbon emission trading (ETS) systems. In 2021, more than 21% of global greenhouse gas emissions were covered by a carbon price. This was a big increase from earlier due to the introduction of the Chinese national carbon trading scheme.
Trading schemes offer the possibility to limit emission allowances to certain reduction targets. However, an oversupply of allowances keeps most ETS at low price levels around $10 with a low impact. This includes the Chinese ETS which started with $7/tCO
2 in 2021. One exception is the European Union Emission Trading Scheme where prices began to rise in 2018. They reached about €80/tCO
2 in 2022. This results in additional costs of about €0.04/KWh for coal and €0.02/KWh for gas combustion for electricity, depending on the emission intensity. Industries which have high energy requirements and high emissions often pay only very low energy taxes, or even none at all.
While this is often part of national schemes, carbon offsets and credits can be part of a voluntary market as well such as on the international market. Notably, the company Blue Carbon of the UAE has bought ownership over an area equivalent to the United Kingdom to be preserved in return for carbon credits.
International agreements
International cooperation is considered a critical enabler for climate action Almost all countries are parties to the United Nations Framework Convention on Climate Change (UNFCCC). The ultimate objective of the UNFCCC is to stabilise atmospheric concentrations of greenhouse gases at a level that would prevent dangerous human interference with the climate system.
Although not designed for this purpose, the Montreal Protocol has benefited climate change mitigation efforts. The Montreal Protocol is an international treaty that has successfully reduced emissions of ozone-depleting substances such as CFCs. These are also greenhouse gases.
History
Historically efforts to deal with climate change have taken place at a multinational level. They involve attempts to reach a consensus decision at the United Nations, under the United Nations Framework Convention on Climate Change (UNFCCC). This is the dominant approach historically of engaging as many international governments as possible in taking action on a worldwide public issue. The Montreal Protocol in 1987 is a precedent that this approach can work. But some critics say the top-down framework of only utilising the UNFCCC consensus approach is ineffective. They put forward counter-proposals of bottom-up governance. At this same time this would lessen the emphasis on the UNFCCC.
The Kyoto Protocol to the UNFCCC adopted in 1997 set out legally binding emission reduction commitments for the "Annex 1" countries. The Protocol defined three international policy instruments ("Flexibility Mechanisms") which could be used by the Annex 1 countries to meet their emission reduction commitments. According to Bashmakov, use of these instruments could significantly reduce the costs for Annex 1 countries in meeting their emission reduction commitments.
The Paris Agreement reached in 2015 succeeded the Kyoto Protocol which expired in 2020. Countries that ratified the Kyoto protocol committed to reduce their emissions of carbon dioxide and five other greenhouse gases, or engage in carbon emissions trading if they maintain or increase emissions of these gases.
In 2015, the UNFCCC's "structured expert dialogue" came to the conclusion that, "in some regions and vulnerable ecosystems, high risks are projected even for warming above 1.5 °C". Together with the strong diplomatic voice of the poorest countries and the island nations in the Pacific, this expert finding was the driving force leading to the decision of the 2015 Paris Climate Conference to lay down this 1.5 °C long-term target on top of the existing 2 °C goal.
Barriers
2 emissions from developed fossil fuel reserves
There are individual, institutional and market barriers to achieving climate change mitigation. They differ for all the different mitigation options, regions and societies.
二酸化炭素除去の会計に関する困難は、経済的障壁となりうる。これはBECCS（バイオエネルギーと炭素回収・貯留）に当てはまる。企業がとる戦略も障壁となりうるが、脱炭素化を加速させることもできる。
社会を脱炭素化するためには、国家が主導的な役割を果たす必要がある。これは、大規模な調整努力が必要だからである。この強力な政府の役割は、社会凝集性、政治的安定性、信頼がある場合にのみうまく機能する。
陸上ベースの緩和策にとって、金融は大きな障壁である。その他の障壁としては、文化的価値観、ガバナンス、説明責任、制度的能力が挙げられる。
発展途上国は、緩和に関してさらなる障壁に直面している。
- 2020年代初頭に資本コストが増加した。利用可能な資本と資金の不足は、発展途上国では一般的である。規制基準の欠如と相まって、この障壁は非効率な設備の拡散を助長している。
- これらの国の多くには、財政的および能力の障壁も存在する。
ある研究では、気候関連研究への総資金のわずか0.12%しか気候変動緩和の社会科学に向けられていないと推定されている。はるかに多くの資金が気候変動に関する自然科学研究に向けられている。また、気候変動の影響とその適応に関する研究にもかなりの金額が費やされている。
社会と文化
コミットメントを売却する
8兆米ドル相当の投資を持つ1000以上の組織が、化石燃料からの投資撤退をコミットしている。社会的責任投資ファンドは、投資家が高い環境・社会・企業統治（ESG）基準を満たすファンドに投資することを可能にする。
COVID-19パンデミックの影響
COVID-19パンデミックは、一部の政府が気候変動対策から、少なくとも一時的に焦点を移す原因となった。この環境政策への障害は、グリーンエネルギー技術への投資減速の一因となった可能性がある。COVID-19による経済活動の停滞も、この影響をさらに大きくした。
2020年には、世界の二酸化炭素排出量が6.4%、つまり23億トン減少した。しかし、パンデミック中に多くの国が制限を解除し始めると、温室効果ガス排出量はその後反発した。パンデミック政策の直接的な影響は、気候変動に対する長期的な影響としてはごくわずかであった。
国別の事例
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米国
アメリカ合衆国政府は、温室効果ガス排出量への対応に関して態度を変化させてきた。ジョージ・W・ブッシュ政権は京都議定書に署名しないことを選択したが、オバマ政権はパリ協定に加盟した。トランプ政権はパリ協定から脱退し、原油とガスの輸出を増加させ、米国を最大の生産国とした。
2021年、バイデン政権は、2030年までに排出量を2005年レベルの半分に削減することを約束した。2022年には、バイデン大統領がインフレ削減法に署名し、これは気候変動対策として10年間で約3750億ドルを提供すると推定されている。2022年現在、炭素の社会的費用は1トンあたり51ドルであるが、学術界では3倍以上であるべきだとされている。中国
中国は、2030年までに排出量をピークアウトさせ、2060年までにネットゼロを達成することを公約している。しかし、中国の石炭火力発電所が（炭素回収なしで）2045年以降も稼働し続ける場合、地球温暖化を1.5℃に抑えることは不可能であるとされている。このような状況の中、中国では中国全国炭素排出権取引スキームが2021年に開始された。
欧州連合
欧州委員会は、欧州連合がFit-for-55の脱炭素化目標を達成するためには、年間4億7,700万ユーロの追加投資が必要だと試算している。
欧州連合（EU）では、政府主導の政策、特に欧州グリーンディールが、グリーンテック分野をベンチャーキャピタル投資の重要な領域として位置づける上で大きな役割を果たしました。その結果、2023年までにEUのグリーンテック分野へのベンチャーキャピタル投資額は米国と同水準に達しました。これは、的を絞った資金援助を通じて、イノベーションを推進し、気候変動緩和に取り組むというEUの強い意志を反映しています。 欧州グリーンディールが推進する政策は、2021年から2023年の間に、EUのグリーンテック企業へのベンチャーキャピタル投資が30%増加するという結果に貢献しました。これは、同時期に他のセクターで景気後退が見られた中での特筆すべき成果です。
EUにおけるベンチャーキャピタル投資総額は、依然として米国よりも約6倍低いものの、グリーンテック分野ではこの差が大幅に縮まり、多額の資金が流入している。 特に投資が増加している主要分野は、エネルギー貯蔵、循環経済に関する取り組み、そして農業技術です。これは、EUが2030年までに温室効果ガス排出量を少なくとも55%削減するという野心的な目標を掲げていることにも後押しされている。
関連するアプローチ
日射管理（SRM）との関係
日射管理（SRM）は地表温度を低下させる可能性があるが、それは温室効果ガスという根本原因に対処するのではなく、一時的に気候変動を隠蔽するに過ぎない。SRMは、地球が吸収する太陽放射の量を変化させることで機能する。例としては、地表に到達する日光の量を減らすこと、雲の光学的厚さと寿命を減らすこと、および地表が放射を反射する能力を変えることなどが挙げられる。IPCCは、SRMを気候緩和の選択肢ではなく、気候リスク低減戦略または補完的な選択肢として記述している。
この分野の用語はまだ進化中である。専門家は、CDRまたはSRMの技術が地球規模で使用される場合、科学文献において「ジオエンジニアリング」または「気候工学」という用語を用いることがある。IPCCの報告書では、「ジオエンジニアリング」または「気候工学」という用語はもはや使用されていない。
関連項目
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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Climate change mitigation(11 July 2025, at 22:21編集記事参照)を翻訳して二次利用しています。 |