想象一下，2030年，太陽地球工程還是流氓地球工程？

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.

《巴黎氣候協(xié)議》要簽署國承諾在未來幾十年內(nèi)減少引發(fā)氣候變化的溫室氣體排放。但哪怕各國信守承諾(有些國家可能不會)，這可能還不足以避免災(zāi)難。想象一下，到2030 年，全球氣溫仍在攀升，海平面上升了數(shù)十厘米，顯著加劇了風暴潮將海水推向低洼地區(qū)并腐蝕沿?；A(chǔ)設(shè)施的影響。在歐洲和美國，夏季熱浪和冬季洪澇愈發(fā)嚴重。在美國南部各州、加勒比海地區(qū)和東南亞地區(qū)，更強烈的熱帶氣旋不斷沖擊海岸線。南半球遭受更嚴重的干旱和更不規(guī)則的季風，破壞了脆弱的農(nóng)業(yè)系統(tǒng)，造成饑荒和內(nèi)亂。不論窮國富國，全球氣候變化的現(xiàn)實都已顯而易見。

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.

在這種情況下，很可能有些國家會提議使用一種名為“太陽地球工程”的技術(shù)來冷卻地球或減緩其變暖。具體方法之一是將微小的反射粒子注入平流層充當遮陽篷，將太陽的一部分能量反射回太空。類似的事情也會自然發(fā)生：過去，巨大的火山噴發(fā)將大量物質(zhì)拋入大氣層，使地球持續(xù)降溫數(shù)月或數(shù)年。例如，1991 年菲律賓皮納圖博火山爆發(fā)，使北半球的氣溫連續(xù)四年降低了多達0.5 攝氏度。其倡導(dǎo)者說，太陽地球工程將以更加可控的方式達到同樣的效果。

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.

讓我們想象這個想法開始獲得政治支持。關(guān)于這些方案的詳細的國際討論從2020 年代中期開始出現(xiàn)，充滿了焦慮。發(fā)展中國家受氣候變化的影響更大但缺乏應(yīng)對的能力，要求在聯(lián)合國展開討論。孟加拉國是一個在國際氣候談判中態(tài)度堅定的中型經(jīng)濟體，由它領(lǐng)導(dǎo)的一些“最不發(fā)達國家”提出了一項動議。最終，這個問題進入了大會議程。但是，與減少全球排放量的談判一樣，多年的討論和決議并沒有帶來多少具體行動。很少人覺得行星遮陽篷是個理想的解決方案。支持者認為，考慮到減排的速度不夠快，遮陽篷會贏得更多時間來減少溫室氣體排放。但反對者表示，這將降低減排的緊迫性。

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.

另一個反對意見是它有可能產(chǎn)生意想不到的后果，因為該技術(shù)以前從未被嘗試過，而且學術(shù)研究和小規(guī)模實地試驗一向資金不足——因為擔心這會給各國提供一種簡單的方法來避開減排涉及的困難選擇。此外還存在“終端激波”的危險：如果一個地球工程項目啟動，并且成功地冷卻了地球，那么遮陽篷的任何故障(比如技術(shù)問題或蓄意破壞)都可能導(dǎo)致溫度在短短幾年突然升高。鑒于上述所有原因，有人要求制定國際規(guī)則來管理這項技術(shù)的使用——因為如果沒有規(guī)則的話，就沒有什么能夠阻止一個或多個國家自己發(fā)起“流氓地球工程”計劃。

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.

但這恰恰是聯(lián)合國辯論未達成任何協(xié)議后，一些國家(可能在2030 年)開始考慮的問題。一小部分發(fā)展中國家受夠了長期的不作為，可能會選擇就是否“自行”使用遮陽方案來展開“少邊”討論。如果該方案奏效了，它就能在給地球降溫的同時驗證其原理，也許就能說服其他國家支持這一方案。

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.

最快的實施方法是建立一支專用機隊。耶魯大學的韋克·史密斯(Wake Smith)和紐約大學的格諾·瓦格納(Gernot Wagner)于2018 年發(fā)表的一項分析描繪了如何做到這一點。飛機需要在2 萬米或更高的高度飛行，這就排除了使用現(xiàn)有商用飛機的可能性。相反，它需要一個由幾十架飛機組成的定制機隊，四個噴氣發(fā)動機安裝在兩個巨大的滑翔機式機翼上，讓它們可以在平流層的稀薄空氣中飛行。在第一年，八架飛機可以進行4000 次每次持續(xù)五小時的飛行(四小時用于爬升和下降，一小時在平流層飛行)。到第五年將增加到34 架飛機，每年飛行20,000次;到第十年，71 架飛機每年飛行44,000 次。15 年后，飛機總數(shù)達到100 架。