Earth system response to atmospheric carbon dioxide removal

鉴于国际社会努力将温室气体(GHGs)排放量减少到与巴黎目标一致的水平，巴黎的目标是将全球变暖限制在“比工业化前水平低得多的2摄氏度以下，并努力将气温升幅进一步限制在1.5摄氏度以下”，因此，有关人为地从大气中消除二氧化碳的技术的提议正受到越来越多的关注。这类二氧化碳清除(CDR)技术包括陆基方法，如重新造林、植树造林和利用碳捕获和封存(BECCS)生产生物能源、直接从环境空气中捕获二氧化碳的技术、增强海洋碳吸收的方法、增强风化和生物炭的方法。CDR是温室气体排放情景的一个组成部分，符合将升温限制在工业化前的1.5摄氏度和2摄氏度以内，尽管我们对CDR的有效性及其对气候的影响知之甚少。 拟议的研究试图通过追求以下目标来填补这一知识空白： 增进我们对CDR对气候-碳耦合循环系统影响的理解，并量化CDR作为一种二氧化碳减排措施的有效性。 确定我们关于耦合气候碳循环对正二氧化碳排放的响应的知识在多大程度上适用于对CDR的响应。 探索在涉及暂时超额完成变暖目标的情景下，人类引起的气候变化的可逆性。 这些目标将使用地球系统模型来解决，这些模型是代表不同地球系统组成部分(例如大气、海洋、陆地表面、生物圈)以及它们之间相互作用的复杂数值模型。更好地了解气候和碳循环对CDR的反应是设计有效的气候政策的关键，这些政策将使加拿大和国际社会走上实现巴黎气候目标的轨道。例如，大多数与将变暖限制在远低于2摄氏度的情景一致，都假设目前温室气体减排的延迟可以通过未来大规模部署CDR来弥补。如果事实证明CDR没有想象的那么有效，或者气候影响在暂时超额完成变暖目标后是不可逆转的，这样的情景可能不可行。

Given the delay in international efforts to reduce emissions of greenhouse gases (GHGs) to a level consistent with the Paris goal of limiting global warming “to well below 2C above pre-industrial levels and to pursue efforts to limit the temperature increase even further to 1.5C”, proposals for technologies to artificially remove CO2 from the atmosphere are receiving increasing attention. Such carbon dioxide removal (CDR) technologies include land-based methods such as reforestation, afforestation and bio-energy production with carbon capture and storage (BECCS), technologies that capture CO2 directly from ambient air, methods to enhance ocean carbon uptake, enhanced weathering and biochar. CDR is an integral part of GHG emission scenarios consistent with limiting warming to 1.5C and 2C above pre-industrial, although we have only limited knowledge of the effectiveness of CDR and its effects on climate. The proposed research seeks to fill this knowledge gap by pursuing the following objectives: Advance our understanding of the effects of CDR on the coupled climate-carbon cycle system and quantify the effectiveness of CDR as a CO2 mitigation measure. Establish the extent to which our knowledge of the response of coupled-climate carbon cycle to positive CO2 emissions is applicable to the response to CDR. Explore the reversibility of human-induced climate changes under scenarios that involve a temporary overshoot of the warming goal. These objectives will be addressed using Earth System Models, which are complex numerical models hat represent the different Earth System components (e.g. atmosphere, ocean, land surface, biosphere) and the interactions among them. A better understanding of the climate and carbon cycle response to CDR is key to designing effective climate policies that put Canada and the international community on track to meeting the Paris climate goals. Most scenarios that are consistent with limiting warming to well below 2C, for instance, assume that current delays in GHG emission reductions can be compensated by large-scale deployment of CDR in the future. If it turns out that CDR is less effective than assumed, or climate impacts are irreversible following a temporary overshoot of the warming target, such scenarios may not be viable.