为实现全球净零排放的区域贡献
The Paris Agreement aims to strengthen the global response to climate change by holding the increase in global average temperature to well below 2°C above preindustrial levels and pursuing efforts to limit the temperature increase to 1.5°C (UNFCCC 2015, 1–25). To achieve this temperature goal, the Paris Agreement also sets a long-term mitigation goal. Parties will aim to reach global peaking of greenhouse gas emissions as soon as possible and reduce emissions rapidly thereafter in order to reach a balance between anthropogenic sources and sinks of greenhouse gas emissions in the second half of the century. Both the long-term temperature goal and the long-term mitigation goal leave room for interpretation. Understanding how different interpretations of these two aspects relate to each other thus becomes a critical issue which can be explored with global scenarios that provide trajectories to limit global mean temperature increase to a given temperature level. A wide variety of scenarios is available in the published literature. However, the Principles of United Nations Framework Convention on Climate Change (UNFCCC 1992, 1–25) indicate that policies and measures to deal with climate change should follow the precautionary principle and be cost-effective so as to ensure global benefits at the lowest possible cost. Using cost-effective scenarios to explore relationships between long-term temperature and long-term mitigation goals is hence a particularly appropriate choice.
政府间气候变化面板的第五次评估报告（IPCC 2013）发现，全球平均温度升高与发射到大气中的碳二氧化物排放量的累积量大致相必威官网是真的吗关。这意味着可以将给定的温度目标转化为有限的碳预算。为了阻止二氧化碳排放量在大气中积累并保持在预算范围内，在某个时候，全球二氧化碳排放几乎必须变为零。这种关系还意味着，如果二氧化碳在短期内排放较高，则它们之后必须更陡峭，更早地达到零水平。《巴黎协定的长期缓解目标》与所有温室气体有关。但是，累积排放和全球平均温度升高之间的近似线性关系主要对二氧化碳和其他长寿命的温室气体有效，并且不适用于短暂的气候燃料。必威官网是真的吗对于后者而言，确定其变暖作用的年度排放量。然而，由于二氧化碳是主要的人为温室气体，因此对温度目标的解释与达到全球零温室气体排放的时间之间存在明显的关系。在最近的一项研究（Rogelj等人，2015年）中，我们表明，在一系列具有成本效益的缓解措施中，将中位变暖限制为2100（在本世纪早些时候暂时超过该水平之后），全球零温室气体，排放量是在2080年左右实现的，在2060 - 85年的可用方案中有一个范围。 Carbon dioxide emissions reach global zero levels earlier, around midcentury, with a range of 2045–60. For cost-effective scenarios that aim to limit global mean temperature increase to 2°C relative to preindustrial levels with at least 66 percent probability but do not manage to return it below 1.5°C by 2100, the timing of these mitigation goals is later. For example, global zero greenhouse gas emissions are only achieved during the last quarter of the 21st century (after 2075), and global zero carbon-dioxide emissions are achieved roughly in the third quarter of the 21st century (approximately 2050–75).
Achieving global mitigation targets requires national contributions that (in line with the UNFCCC’s principles) should be cost-effective to ensure global benefits at the lowest possible cost. Because mitigation potentials and capabilities between countries and regions differ markedly, regional emission reduction contributions toward achieving a cost-effective global zero mitigation goal also vary. For example, scenario data from five integrated assessment models that were produced as part of the European Union’s LIMITS project1(Kriegler et al. 2013) illustrates this potential variation in the timing of reaching regional zero emissions in globally cost-effective mitigation scenarios for scenarios that hold warming to below 2°C relative to preindustrial levels with at least 66 percent probability. For some regions, the year in which regional zero GHG emissions are reached in such scenarios is roughly the same as the year in which global zero GHG emissions are achieved. For example, this is the case for the region centered on China, and the Middle East. Other regions achieve zero GHG emissions generally earlier. This is the case for Latin America (ca. 20 years earlier), North America (20–30 years earlier), Pacific OECD countries (ca. 20 years earlier), and reforming economies of Eastern Europe and the former Soviet Union (20–30 years earlier). Some regions show a generally later achievement of zero GHG emissions (like the region centered on India, the Rest of Asia region, and Europe). This delay in reaching zero GHG emissions at the regional scale is generally on the order of 10 years. Sub-Saharan Africa shows a very large range, and the available scenarios do not allow us to identify a clear trend here.
Figure 1: Variation of timing of reaching zero emissions levels globally (top panel) and regional variation (bottom panels). Variations are provided for ten reported regions.2Lines in the top panel show the minimum-maximum range across all models (bold) and per model. Circles show the median across all scenarios and all models. Diamonds are shown in case all scenarios from a single model provide the same value. Blue bars in the bottom panels are histograms of the 15 available scenarios; grey ranges are the full range across all available scenarios. Data from the EU’s FP7 LIMITS project, hosted by the International Institute for Applied Systems Analysis (IIASA) and available online athttps://tntcat.iiasa.ac.at/limitspublicdb。全球温室气体（GHG）排放量汇集了IPCC的第四次评估报告中的100年全球变暖潜力。研究提供的区域定义（请参见上面的链接）：非洲=撒哈拉以南非洲国家；中国+ =集中计划的亚洲国家，主要是中国；欧洲=东欧和西欧国家（即EU27）；印度+ =南亚国家，主要是印度；拉丁美洲=拉丁美洲和加勒比海国家；Midder_east =中东国家；North_am =北美国家，主要是美国和加拿大；PAC_OECD =太平洋经合组织国家； REF_ECON = countries from the reforming economies of Eastern Europe and the former Soviet Union; REST_ASIA = other countries of Asia.
It is important to note that this variation in timing of reaching regional zero emissions is purely informed by considerations of cost-effectiveness. The analyzed pathways represent a regional distribution of mitigation actions that are projected to achieve the global temperature target with the lowest overall mitigation costs. The UNFCCC also includes other important principles that refer to national circumstances, reflect equity principles, or support the principle of common but differentiated responsibilities and respective capabilities. To follow a global cost-effective path toward achieving the Paris Agreement’s long-term temperature goal, actions at the national level should consider these together with the limitations posed by a global carbon budget and a global cost-effective path. To be consistent with the requirement of cost-effectiveness, equity considerations should be dealt with by trading of emission reduction outcomes, supporting capacity building, transferring technology, and so on, rather than through the distribution of physical emissions between countries. However, cost-effective scenarios are not unchangeable. They can change with societal choices about the stringency of climate outcomes and the desirability of specific solutions. For example, if certain technology options, like nuclear power or large-scale bioenergy plantations, are undesirable, these preferences can be taken into account in cost-effective scenarios during their design phase. Such preferences are societal choices, which should be made by decision makers who base their assessment on the risks, trade-offs, and opportunities of specific mitigation options. Cost-effectiveness subsequently provides a concept to ensure that all desired outcomes are achieved in the most efficient way. All the potential variations in cost-effective pathways toward global zero emissions aside, the scientific scenario literature as assessed by the IPCC (2014) shows that achieving the stringent temperature goals set by the Paris Agreement is not possible without mitigation efforts in all regions.
参考
IPCC (Intergovernmental Panel on Climate Change). 2013.必威官网是真的吗气候变化2013：物理科学基础。工作组对政府间气候变化的第五次评估报告的贡献必威官网是真的吗。剑桥：剑桥大学出版社。
IPCC. 2014.缓解气候变化。必威官网是真的吗工作组III对政府间气候变化第五评估报告的贡献必威官网是真的吗。剑桥：剑桥大学出版社。
Kriegler, E., M. Tavoni, T. Aboumahboub, G. Luderer, K. Calvin, G. Demaere, V. Krey, K. Riahi, H. Rösler, M. Schaeffer, and D.P. Van Vuuren. 2013. “What Does the 2°C Target Imply for a Global Climate Agreement in 2020? The LIMITS Study on Durban Platform Scenarios.”必威官网是真的吗气候变化经济学04, 1340008.
Rogelj, J., M. Schaeffer, M. Meinshausen, R. Knutti, J. Alcamo, K. Riahi, and W. Hare. 2015. “Zero Emission Targets as Long-Term Global Goals for Climate Protection.”Environmental Research Letters10，105007。
UNFCCC（联合国气候变化框架公约）。必威官网是真的吗1992。United Nations Framework Convention on Climate Change。Rio de Janeiro: UNFCCC.
UNDCCC。2015。Paris Agreement。Paris: UNFCCC.
1As part of the European Union’s Seventh Framework Programme (FP7); see https://www.feem-project.net/limits/index.html. The scenarios used here are called LIMITS-450, LIMITS-RefPol-450, and LIMITS-StrPol-450. See online database hosted by the International Institute for Applied Systems Analysis (IIASA) (https://tntcat.iiasa.ac.at/LIMITSPUBLICDB) for more information.
2数据的区域定义是在https://tntcat.iiasa.ac.at/limitspublicdb/static/download/limits_10 +1_regions.pdf上提供的。
