北極海冰融化已經成了一個巨大的問題：海冰能反射陽光，有助於寒冷的極地地區維持低溫狀態；而在溫暖的天氣下，海冰變少，反射陽光的能力減弱，地球表面吸收了更多來自太陽的熱量，變得更溫暖，進一步加劇了冰的融化……最終，這一惡性迴圈導致海平面不斷上升。
要想從源頭上解決這個問題，最好的辦法是減少溫室氣體的排放。但目前我們在這方面並沒有取得很大的進展。與此同時，有人另闢蹊徑，大開腦洞：如果我們能為北極造出更多的冰呢？
這聽起來或許有些荒謬，但在極地地球工程（polar geoengineering）領域，有人正在嚴肅地推進這項計劃。
今年2月，加拿大北極群島的劍橋灣（Cambridge Bay）經歷了75年來最溫暖的冬天，北極的溫度甚至一度短暫升至冰點之上。科學家預測稱，最早到本世紀30年代，北極冰蓋可能就會在夏季開始完全融化，全球氣溫將因此進一步升高。
一家名為Real Ice（“真冰”）的英國科技公司的員工駐紮在這裡，試圖拯救全球冰川、冰蓋和海冰：他們希望透過人工凍結更多的海冰來阻止情況進一步惡化。一些科學家認為這是荒謬的，甚至是危險的，但Real Ice的聯合創始人奇安·舍溫（Cían Sherwin）稱：我們別無選擇，只能嘗試。
海冰從下方開始凍結，冰與水的介面正好在0℃左右。然而，一旦第一層冰形成，它就會在一定程度上將冰下的海水與冰上低至-50℃的寒冷空氣隔離開。隨著冰層不斷加厚，海冰生成的速度會越來越慢。Real Ice正試圖透過把冰下未結冰的海水引到海冰頂部，使其暴露在冷空氣中，加快結冰的速度。
在劍橋灣，Real Ice的團隊在距離城鎮約7千米的冰面上開始作業，這裡的冰超過一米厚。奇安會用一把電動螺旋長鑽在冰面上鑽洞，這也是冰釣常常用到的工具。接著，當地的因紐特人嚮導大衛·卡瓦納（David Kavanna）用冰鋸拓寬了洞口邊緣。奇安把連著工業泵的長軟管伸進洞裡，接通電源後，海水開始從軟管中湧出，在海冰上形成一汪熠熠閃光的藍色水池。
在水流速度不那麼快的地方，這些冰水從流到冰面上的那一刻起，就會像熔岩一樣開始變得粘稠，逐漸凍結。大約過了三個小時，團隊回來將泵取出。先前抽水形成的水池已經凝結成藍色的雪泥，就像旺旺碎冰冰一樣。奇安解釋道，如果到第二天的早晨再回來看，水池就已經完全凍上了。這是一層在原有海冰的基礎上成功形成的新冰層。
在地球工程領域，向空中釋放微小顆粒來阻擋陽光，可能是最常見的方案。但這種策略也會影響到空氣質量和天氣狀況，如改變降水模式等，因此極具爭議性。
2022年12月24日，在沒有任何公眾參與或科學審查的情況下，一家名為Make Sunsets的美國公司在墨西哥某地發射了含有二氧化硫的氣象氣球，他們預計，這些氣球會在上升到一定高度後發生爆炸，將反射性硫顆粒釋放到大氣平流層中，進而將更多陽光反射到太空。2023年1月13日，墨西哥政府在得知訊息後，迅速採取措施，禁止了在該國進行的一切太陽能地球工程（solar geoengineering）試驗。
2024年4月，美國華盛頓大學（University of Washington）海洋雲增亮（marine cloud brightening）研究計劃的研究人員還嘗試在加利福尼亞州阿拉米達市上空噴灑海鹽煙霧，但立刻就被當地市政府叫停，理由是擔心這項技術可能帶來的健康問題。
今年5月，據《自然》新聞（Nature News）訊息，英國高階研究和干預機構（ARIA）宣佈為地球工程領域的研究專案提供5680萬英鎊（約合人民幣5.58億元）的資助撥款，這也讓英國成為首批資助該領域戶外實驗的國家之一。最終有21個專案得到了資助，它們均涉及地球工程中最具爭議的領域，即為了驗證方案的有效性，須要進行對環境可能產生切實影響的戶外實驗。
Real Ice所在的研究小組得到了其中數額最大的一筆資助，約為990萬英鎊（約合人民幣9700萬元），這個小組中還有一家荷蘭初創公司Arctic Reflections（“北極反射”），致力於開發並測試一種巨型抽水平臺，增加挪威斯瓦爾巴特群島和加拿大紐芬蘭地區的海冰厚度。
極地地球工程試驗也在其他方面取得了進展。一家美國非營利組織一直在播撒微小的白色粘土顆粒，以反射更多的陽光，減緩冰島和喜馬拉雅山冰川的融化程序。在斯堪的納維亞半島上，一個團隊正試圖測試一種可以用於製造巨大水下幕布的材料，阻止溫暖的海水流進南極冰川底部，防止其進一步融化和坍塌。
北極海冰就像一面巨大的鏡子，當被積雪覆蓋時，能將高達90%的太陽輻射反射回太空。海水的情況則正好相反，它可以吸收90%的陽光。因此，冰融化得越多，海水變暖得越多，這不僅會加熱地球，還會進一步融化更多的海冰。
過去四十年裡，永凍厚海冰已縮減約40%。一旦這些海冰開始在夏季完全融化，到2050年，全球氣溫可能會再上升0.19℃。因此，諸如海冰增厚這樣的極地地球工程如果可行，將會對整個地球的氣候產生影響。
去年冬天，Real Ice為地球增厚了約25萬平方米的海冰。2027-2028年的冬季，該公司計劃增厚100平方千米的海冰。如果這一方案可行，團隊希望可以擴大規模，最終阻止北極海冰在夏季完全融化。奇安表示，如果能為整個北極地區人工凍結約100萬平方千米的冰，就足以防止海冰的流失。這個數字說小也小，僅佔目前北極夏季剩餘海冰的五分之一；說大也大，約等於四川省面積的兩倍。
Real Ice的願景並非天方夜譚，“只”需要50萬臺水下無人機就可以完成…… [檢視全文]
Could Freezing Arctic Sea Ice Combat Climate Change?
Rachel Feltman: For Scientific American’s Science Quickly, I’m Rachel Feltman.
You don’t have to pay much attention to the news to know that climate change is causing Arctic sea ice to melt—and to understand that this is a huge problem. Ice reflects sunlight, which helps keep cold places cold. Warmer weather means less ice, but less ice means more heat from the sun, which means it gets warmer, which means there’s less ice—and the sea level keeps rising and rising.
It would be great if we could cut this problem off at the source by dropping our greenhouse gas emissions, but we’re not exactly making great progress on that front. In the meantime what if we could just make more ice?
It might sound silly, but some folks in the polar geoengineering space are making a very serious attempt to do just that.
To get the inside scoop I’m handing the reins over to Pulitzer Center ocean reporting fellow Alec Luhn. He’s the author of a feature on the subject in Scientific American’s June issue, and today he’s going to take us along on a trip to the Arctic.
[CLIP: Snowmobile engine starting.]
Alec Luhn: I’m snowmobiling out onto the sea ice from the Inuit village of Cambridge Bay in Canada’s Arctic Archipelago. It’s –26 degrees Celsius. That’s –15 degrees in Fahrenheit. The blasting wind makes it feel far colder. My goggles are freezing over, and my thumb is getting numb on the throttle. But this is actually warm for Cambridge Bay in February. It’s been the warmest winter in 75 years, and the temperature at the North Pole even briefly went above freezing.
In front of me a local Inuit guide is towing a sled full of team members from the U.K. company Real Ice to a point about seven kilometers [roughly 4.3 miles] from town.
Scientists say as early as the 2030s the Arctic ice cap could start melting away completely in the summertime, raising temperatures around the globe. Real Ice hopes to stop that by artificially freezing more sea ice. It’s one of several geoengineering projects trying to save the world’s glaciers, ice sheets and sea ice.
Some scientists think it’s ridiculous or even dangerous, but Real Ice co-founder Cían Sherwin says we no longer have any option but to try.
Cían Sherwin: So right now we’re about to start drilling the—that 10-inch [25.4-centimeter] auger hole for the pump.
[CLIP: Cían Sherwin drills into the sea ice.]
Luhn: Cían was part of a student group at Bangor University in Wales that built a “reicing machine” after they saw a TV documentary about the melting Arctic. In 2022 he co-founded Real Ice to try it on a larger scale.
The ice outside Cambridge Bay is more than a meter [approximately 3.3 feet] thick. Cían drills a hole in it with a long battery-powered auger. If you’ve ever been ice fishing, you’ve seen this kind of tool. It looks kind of like a jackhammer, only with a giant rotating screw rather than a chisel at the end.
Inuit guide David Kavanna widens the edges of the hole with an ice saw, and the team puts a wooden box around it. Cían lowers an industrial pump with a long hose through the hole. He plugs a cable into a battery pack, and seawater starts pouring out of the hose, creating a brilliant blue pool on the sea ice.
Sherwin: Where that flow rate isn’t as strong, the ice—or the water acts almost like lava, becoming thicker in viscosity, and ice formation starts to begin almost instantly.
Luhn: Sea ice freezes from below, where there’s water that’s just under zero degrees C [32 degrees F]. But once the first layer of ice forms it partially insulates that water from the freezing air above, which can be as cold as –50 degrees C [–58 degrees F]. So the thicker the ice gets, the slower it grows. Real Ice is trying to bring the water up to the cold air by pumping it on top of the sea ice.
After about three hours the team comes back to take the pump out. The pool of water has congealed into an electric blue slush, like a gas station Slurpee.
Sherwin: So by the time we return here now, tomorrow morning, this will already be frozen.
Luhn (tape): New sea ice?
Sherwin: New sea ice—or a new layer on top of the sea ice.
Luhn: Releasing small particles to block sunlight is probably the most common geoengineering idea. It’s also highly controversial because it could affect weather, like rainfall. Mexico banned solar geoengineering after an American firm released balloons full of sulfur dioxide there. A city in California recently halted an experiment spraying sea-salt particles into the air.
In May the U.K. allocated about $75 million to geoengineering research, becoming one of the first countries to fund outdoor experiments in this field. One experiment will launch balloons to test mineral dust that could someday be released into the atmosphere to block sunlight. Another two will develop nozzles to spray sea-salt particles, including potentially over Australia’s Great Barrier Reef.
But the largest grant in the British program, about $13 million, went to a research group that includes Real Ice. It also includes the Dutch company Arctic Reflections, which has been testing giant pumping platforms to thicken sea ice in Svalbard [Norway] and Newfoundland, Canada.
Polar geoengineering trials have been moving forward in other places, too. A U.S. nonprofit has been scattering tiny white clay granules to reflect more sunlight away from glaciers in Iceland and the Himalayas. And a Scandinavian project has been testing materials for huge underwater curtains to try and stop warm water from reaching the underside of Antarctic glaciers and melting and collapsing them.
If it works, polar geoengineering like sea-ice thickening could affect the entire Earth. Arctic sea ice is like a big mirror, reflecting up to 90 percent of the sun’s radiation back into space when it’s covered in snow. But ocean water absorbs 90 percent of sunlight. The more ice melts, the more ocean water warms. That heats up the planet—and melts even more ice.
The thick sea ice that lasts year round has shrunk about 40 percent in the last four decades. If it starts melting away completely in the summertime, global temperatures could rise an extra 0.19 degrees C [roughly 0.34 degrees F] by 2050.
Last winter real ice thickened about 250,000 square meters [almost 2.7 million square feet] of sea ice. In the winter of 2027–28 the company plans to thicken 100 square kilometers [about 38.6 square miles] as a demonstration. If that works, the team hopes it could scale up to eventually keep Arctic sea ice from disappearing in the summer.
Sherwin: Targeting an area roughly a million square kilometers [about 386,100 square miles]across the entire Arctic region could be enough to help prevent the loss of sea ice.
Luhn: On the one hand that’s small: it’s one fifth of how much ice is currently left in the summertime. On the other hand it’s enormous: the size of Texas and New Mexico combined. Real Ice says it could be possible. All they’d need is half a million underwater drones…[full transcript]
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