想象一下，2030年，太阳地球工程还是流氓地球工程？

The paris climate deal commits its signatories to cuts in climate-changing greenhouse-gas emissions over the coming decades. But even if countries stick to their promises (and some may not), that may not be enough to avert catastrophe. Imagine that by 2030 global temperatures are still creeping up, and sea levels are tens of centimetres higher—significantly worsening the impact of storm surges that push seawater over low-lying areas and corrode coastal infrastructures. In Europe and America, summer heatwaves and winter flooding have become more severe. In America’s southern states, the Caribbean and South-East Asia, coastlines are battered by stronger tropical cyclones. The global South suffers worse droughts and more irregular monsoons, undermining fragile agricultural systems and causing famines and civil unrest. The reality of global climate change becomes apparent to rich and poor countries alike.

《巴黎气候协议》要签署国承诺在未来几十年内减少引发气候变化的温室气体排放。但哪怕各国信守承诺(有些国家可能不会)，这可能还不足以避免灾难。想象一下，到2030 年，全球气温仍在攀升，海平面上升了数十厘米，显著加剧了风暴潮将海水推向低洼地区并腐蚀沿海基础设施的影响。在欧洲和美国，夏季热浪和冬季洪涝愈发严重。在美国南部各州、加勒比海地区和东南亚地区，更强烈的热带气旋不断冲击海岸线。南半球遭受更严重的干旱和更不规则的季风，破坏了脆弱的农业系统，造成饥荒和内乱。不论穷国富国，全球气候变化的现实都已显而易见。

Under these conditions, it seems likely that some countries will propose the use of a technique called “solar geoengineering” to cool the planet or slow its warming. One way to do this involves injecting tiny reflective particles into the stratosphere, where they would act as a sunshade by bouncing part of the sun’s energy back out into space. Something similar also happens naturally: big volcanic eruptions have, in the past, thrown large amounts of material into the atmosphere, cooling the planet for months or years. The eruption of Mount Pinatubo in the Philippines in 1991, for example, reduced temperatures in the northern hemisphere by as much as 0.5ºC for four years. Solar geoengineering would, its advocates say, do the same thing in a more controlled manner.

在这种情况下，很可能有些国家会提议使用一种名为“太阳地球工程”的技术来冷却地球或减缓其变暖。具体方法之一是将微小的反射粒子注入平流层充当遮阳篷，将太阳的一部分能量反射回太空。类似的事情也会自然发生：过去，巨大的火山喷发将大量物质抛入大气层，使地球持续降温数月或数年。例如，1991 年菲律宾皮纳图博火山爆发，使北半球的气温连续四年降低了多达0.5 摄氏度。其倡导者说，太阳地球工程将以更加可控的方式达到同样的效果。

Imagine that the idea starts to gain political support. The first detailed international discussions of the options, starting in the mid-2020s, are fraught. Developing countries, more exposed but less well equipped to cope with the impacts of climate change, call for discussions at the United Nations. A motion is proposed by a group of “least developed countries”, led by Bangladesh, a medium-sized economy with a strong voice in international climate talks. Eventually, the issue makes it onto the agenda in the General Assembly. But, as with negotiations to cut global emissions, years of discussions and resolutions lead to little concrete action. Few see a planetary sunshade as a desirable solution. Supporters observe that a sunshade would buy more time to reduce greenhouse-gas emissions, given that cuts are not happening fast enough. But opponents say it will reduce the urgency of cutting emissions.

让我们想象这个想法开始获得政治支持。关于这些方案的详细的国际讨论从2020 年代中期开始出现，充满了焦虑。发展中国家受气候变化的影响更大但缺乏应对的能力，要求在联合国展开讨论。孟加拉国是一个在国际气候谈判中态度坚定的中型经济体，由它领导的一些“最不发达国家”提出了一项动议。最终，这个问题进入了大会议程。但是，与减少全球排放量的谈判一样，多年的讨论和决议并没有带来多少具体行动。很少人觉得行星遮阳篷是个理想的解决方案。支持者认为，考虑到减排的速度不够快，遮阳篷会赢得更多时间来减少温室气体排放。但反对者表示，这将降低减排的紧迫性。

A further objection is the risk of unintended consequences, given that the technique has never been tried before, and academic studies and small-scale field trials have been underfunded, for fear of giving countries an easy way to avoid the difficult choices emissions cuts require. There is also the danger of “termination shock”: if a geoengineering project is launched, and it successfully cools the planet, then any failure of the sunshade (due to technical problems, say, or sabotage) could cause a sudden increase in temperatures in just a few years. For all these reasons, there are calls for international rules to govern the use of the technology—because without them, there is nothing to stop one or more countries launching a “rogue geoengineering” scheme on their own.

另一个反对意见是它有可能产生意想不到的后果，因为该技术以前从未被尝试过，而且学术研究和小规模实地试验一向资金不足——因为担心这会给各国提供一种简单的方法来避开减排涉及的困难选择。此外还存在“终端激波”的危险：如果一个地球工程项目启动，并且成功地冷却了地球，那么遮阳篷的任何故障(比如技术问题或蓄意破坏)都可能导致温度在短短几年突然升高。鉴于上述所有原因，有人要求制定国际规则来管理这项技术的使用——因为如果没有规则的话，就没有什么能够阻止一个或多个国家自己发起“流氓地球工程”计划。

But that is exactly what some countries might start to consider, perhaps in 2030, after the un debate fails to reach any agreement. Fed up with yet more inaction, a small group of developing countries might choose to engage in “minilateral” discussions over whether to “go it alone” with a sunshade scheme that would, if it worked, both cool the planet and provide a proof of principle that might persuade other countries to back the idea.

但这恰恰是联合国辩论未达成任何协议后，一些国家(可能在2030 年)开始考虑的问题。一小部分发展中国家受够了长期的不作为，可能会选择就是否“自行”使用遮阳方案来展开“少边”讨论。如果该方案奏效了，它就能在给地球降温的同时验证其原理，也许就能说服其他国家支持这一方案。

The fastest way to do this would be to build a fleet of specialised planes. An analysis published in 2018 by Wake Smith, at Yale University, and Gernot Wagner, at New York University, maps out how to do it. The planes need to fly at altitudes of 20km (66,000ft) or higher, ruling out the possibility of using existing commercial aircraft for the purpose. Instead, a custom fleet of several dozen aircraft would be needed, with four jet engines mounted on two huge, glider-like wings, which would allow them to stay aloft in the thin air of the stratosphere. In the first year, eight aircraft could carry out 4,000 five-hour flights (four spent in ascent and descent and one in the stratosphere). By year five, this would be ramped up to 34 aircraft making 20,000 flights a year; by year ten, 71 aircraft would be making 44,000 flights year. After 15 years the fleet would be 100 strong.

最快的实施方法是建立一支专用机队。耶鲁大学的韦克·史密斯(Wake Smith)和纽约大学的格诺·瓦格纳(Gernot Wagner)于2018 年发表的一项分析描绘了如何做到这一点。飞机需要在2 万米或更高的高度飞行，这就排除了使用现有商用飞机的可能性。相反，它需要一个由几十架飞机组成的定制机队，四个喷气发动机安装在两个巨大的滑翔机式机翼上，让它们可以在平流层的稀薄空气中飞行。在第一年，八架飞机可以进行4000 次每次持续五小时的飞行(四小时用于爬升和下降，一小时在平流层飞行)。到第五年将增加到34 架飞机，每年飞行20,000次;到第十年，71 架飞机每年飞行44,000 次。15 年后，飞机总数达到100 架。