The Impact of Short-Lived Halocarbons on Ozone and Climate (ISHOC): An International Multi-Model Intercomparison

在过去的40年里，平流层臭氧层的耗尽一直是环境问题的前沿。这一层保护地球表面免受某些波长的有害紫外线(UV)辐射的影响，否则这些辐射将对人类和植物的健康造成损害。臭氧还会吸收地面红外线(IR)辐射，这意味着它是一种温室气体，因此其丰度的变化可能会影响气候。臭氧消耗的主要原因是氯氟烃(CFCs)和哈龙等长寿命的人为化合物释放出卤素(氯和溴)。这些消耗臭氧的化合物的生产现在受到联合国蒙特利尔议定书的控制，但它们曾被广泛用于制冷和灭火装置等应用中。由于《议定书》的成功，平流层中氯和溴的丰度现在正在下降，尽管速度很慢，人们普遍预计臭氧层将在本世纪中下半叶“恢复”到1980年前的水平。然而，世界气象组织/环境署2014年对平流层臭氧消耗的评估强调了一个关键的不确定性，那就是不受控制的含氯极短寿命物质(氯-VSLS)的排放量不断增加，这些物质也可能到达平流层并造成臭氧损失。最丰富的氯-VSLS是二氯甲烷(CH2Cl2)，其对流层丰度在过去十年中增加了60%。CH2Cl2是人类产生的，在靠近工业来源的北半球，长期观察显示CH2Cl2的平均增长率为每年约8%。这些增长的确切原因尚不清楚。然而，众所周知，CH2Cl2(和其他氯-VSLS)的排放量在亚洲相对较大，在没有对生产进行政策控制的情况下，大气浓度预计在未来几年将继续增加。我们最近的模拟工作表明：(I)仅在过去十年里，氯-VSLS对平流层氯的贡献就已经翻了一番，(Ii)CH2Cl2的持续增长可能会将南极臭氧层空洞的恢复推迟长达数十年。这将大大抵消《蒙特利尔议定书》取得的一些成果，而且由于臭氧空洞在几个方面影响南半球的地表气候，可能会影响对气候变化的预测。该项目(ISHOC)建立了一个由世界领先的化学-气候模拟小组组成的新工作队。我们将对CH2Cl2增长对平流层臭氧构成的威胁进行第一次协调一致的多模式评估。兰开斯特大学将与剑桥大学和一个由9个合作伙伴组成的国际财团合作，领导模型相互比较。我们将开发一系列增长情景，描述大气中CH2Cl2未来可能的轨迹。我们联盟中的每个模型都将进行考虑到这些情景的正向模拟，并将对输出进行分析，以确定(A)由于CH2Cl2的增长而导致平流层不同区域臭氧恢复的预期延迟，以及(B)随后对气候和地表紫外线的影响。ISHOC的结果将对不受《蒙特利尔议定书》管制的化合物在臭氧消耗中的作用提供强有力的新见解，这将与未来对臭氧和气候变化的国际评估高度相关(例如，世界气象组织/环境署和气专委报告)。虽然ISHOC的重点是CH2Cl2，但工作队将在项目之外保持活跃，以便在其他未受控制的氯-VSLs(例如，CHCl3、C2H4Cl2)出现时审查它们对臭氧的未来威胁。事实上，我们正在进行的工作表明，这些氯-VSLS的排放量也在增加。

Depletion of the stratospheric ozone layer has been at the forefront of environmental concern over the last 40 years. The layer shields Earth's surface from certain wavelengths of harmful ultraviolet (UV) radiation that would otherwise be detrimental to human and plant health. Ozone also absorbs terrestrial infra-red (IR) radiation meaning it is a greenhouse gas, and changes in its abundance can therefore impact climate. The primary cause of ozone depletion is the release of halogens (chlorine and bromine) from long-lived anthropogenic compounds, such as chlorofluorocarbons (CFCs) and halons. Production of these ozone-depleting compounds is now controlled by the UN Montreal Protocol, but they were once widely used in refrigeration and fire suppression units, among other applications. Due to the success of the Protocol, the stratospheric abundance of chlorine and bromine is now declining, albeit slowly, and the ozone layer is widely expected to 'recover' to levels observed pre-1980 in the middle to latter half of this century. However, a key uncertainty, highlighted in the WMO/UNEP 2014 Assessment of Stratospheric Ozone Depletion, is the increasing emissions of uncontrolled chlorine-containing Very Short-Lived Substances (Cl-VSLS) which can also reach the stratosphere and cause ozone loss.The most abundant Cl-VSLS is dichloromethane (CH2Cl2), whose tropospheric abundance has increased by >60% over the last decade. CH2Cl2 is human-produced and in the Northern Hemisphere, close to industrial sources, long-term observations show a mean CH2Cl2 growth rate of ~8%/year. The precise cause of these increases is unknown. However, emissions of CH2Cl2 (and other Cl-VSLS) are known to be relatively large over Asia, and in the absence of policy controls on production, atmospheric concentrations are expected to continue to increase in coming years. Our recent modelling work has shown (i) that the contribution of Cl-VSLS to stratospheric chlorine has already doubled in the last decade alone, and (ii) that sustained CH2Cl2 growth could delay the recovery of the Antarctic Ozone Hole by up to several decades. This would significantly offset some of the gains achieved by the Montreal Protocol, and because the Ozone Hole influences surface climate of the Southern Hemisphere in several ways, could affect forward predictions of climate change.This project (ISHOC) establishes a new task force comprised of world-leading chemistry-climate modelling groups. We will perform the first concerted multi-model assessment of the threat posed to stratospheric ozone from CH2Cl2 growth. Lancaster University will lead the model intercomparison in collaboration with the University of Cambridge, and an international consortium of 9 partners. We will develop a series of growth scenarios describing possible future trajectories of CH2Cl2 in the atmosphere. Each of the models in our consortium will perform forward simulations considering these scenarios and the output will be analysed to determine (a) the expected delay to ozone recovery in different regions of the stratosphere due to CH2Cl2 growth and (b) the subsequent implications for climate and surface UV. The results from ISHOC will provide powerful new insight into the role of compounds not controlled by the Montreal Protocol in ozone depletion, which will be highly relevant to future international assessments of ozone and climate change (e.g. WMO/UNEP and IPCC reports). While the focus of ISHOC is on CH2Cl2, the task force will remain active beyond the project to examine future threats to ozone from other uncontrolled Cl-VSLS (e.g. CHCl3, C2H4Cl2) as they emerge. Indeed, our ongoing work suggests that emissions of these Cl-VSLS are also increasing.