Pernafrost | 谢尔盖·齐莫夫：“永久冻土融化将对气候构成直接威胁”

在北极圈内的西伯利亚北部腹地，俄罗斯科学家谢尔盖·齐莫夫（Sergey Zimov）创办了一家“更新世公园”——他将大型食草动物重新引入土壤严重枯竭的猛犸草原，希望以此重建冰河时代的生态系统。恢复这些生态系统还有助于遏制永久冻土的融化。永久冻土是一片浩瀚的冰川冰层，其中储存着数10亿吨有机碳。一旦融化，生活在冻土层中的微生物将把曾经冻结的有机物迅速转化为二氧化碳和甲烷。这些因气候变化而释放出的温室气体是长期以来被忽视的威胁。

俄罗斯地球物理学家和生态学家谢尔盖·齐莫夫在西伯利亚北部的切尔斯基创建了东北科学站（现为露天实验室）。他还是俄罗斯科学院远东分院太平洋地理研究所的研究主管之一。

齐莫夫在1988年率先尝试将哺乳动物重新引入科雷马河流域，并在1996年创办了更新世公园。园区的使命是恢复类似于猛犸草原的生态系统。在距今258万至11700年的更新世晚期，这种草原曾经遍布亚欧大陆。

问：永久冻土为什么会开始融化？

答：排放到大气中的温室气体正在使地球变暖。截至目前，地球的温度已经上升了不止1℃。在北半球，例如俄罗斯等地，观测到的升温幅度甚至超过了3℃。这是由于陆地比海洋升温速度更快，而北半球的陆地面积更大。

不过，永久冻土的温度不单是由空气温度决定的，还要看积雪的厚度。当积雪很多时，地表和永久冻土的温度在入冬之后便难以下降，而如今，积雪层的覆盖面积比几十年前提高了1.5倍。随着全球不断变暖，从海洋中蒸发的水分增多，云产生的降雪也比以前多了。结果就造成俄罗斯的地表温度上升了5–7℃。

西伯利亚北部永久冻土的温度过去是在零下6–8℃之间，南部则保持在零下2–3℃。如今，几乎一半的西伯利亚土地正在融化，其中既包括西伯利亚南部，也包括靠近北冰洋的连续多年冻土区——科雷马低地。我家附近一些区域的永久冻土层已经融化了四米多。在广袤的科雷马沿海平原上，这种现象从三年前就开始了。

永久冻土融化会带来哪些危险？

永久冻土覆盖着1100万平方公里的俄罗斯国土。这些肥沃的土壤富含有机物和休眠的细菌，一旦土壤解冻，古老的微生物会苏醒过来，分解当初没来得及分解吸收的一切，在土壤干燥时释放二氧化碳，在土壤水分饱和时释放甲烷。

永久冻土中的有机物数量比地球上所有植物群的有机物总数还要多一倍，其中大部分（相当于一万亿吨）集中在地表下三米以内。这个深度的冻土层融化速度非常快，因此只需三到五年的时间。永久冻土融化便会对当今全球气候构成直接威胁。冻土融化会产生温室气体，而由此导致的全球变暖又会进一步加速永久冻土的融化。这是一个难解的循环。

虽然2015年通过了旨在推动全球向低碳经济转型的《巴黎协定》，但其设定的目标在这种情况下显得苍白无力。与永久冻土的巨大排放量相比，通过落实《巴黎协定》和使用可再生能源可以实现的温室气体减排量可以说是九牛一毛。

永久冻土释放的甲烷比二氧化碳要危险得多……

假如冻土融化之后只释放二氧化碳，那么排放量尚能与人类产生的排放量大致相当。但实际上因冻土融化而进入大气的气体有10%–20%是甲烷。由于甲烷的温室效应在短期内比二氧化碳高80倍，释放甲烷给气候造成的影响比排放二氧化碳要严重四倍。

在新冠疫情暴发后的一年半里，人为温室气体排放量增幅明显放缓，人们甚至可以预见大气中的温室气体总体含量下降。但是，甲烷的浓度增幅却达到前所未有的峰值。我时常看到新形成的泉眼里正在咕嘟咕嘟地冒出甲烷。我想，除了永久冻土融化，再没有别的什么原因可以解释这一情况。

政府间气候变化专门委员会（IPCC）的分析和预测为什么没有考虑到永久冻土融化问题？

30年前，科学界自认为对碳循环了如指掌，没人想过永久冻土的问题。当我将冻土融化产生的影响摆上桌面时，所有的方程式显然都需要推翻重写。早年间，曾有许多科学家对我的结论提出质疑，他们不肯相信永久冻土正在融化。长期以来，这一问题在科学界就像是一个被遗弃的孩子。而最新估算显示，冻土层的面积到本世纪末将缩小10%–20%。由于观测到全球温室气体的浓度显著增加，人们可能在年内就会意识到永久冻土问题的严重性。

你在20年前创办更新世公园的时候，并没想到公园有朝一日会为保护气候作出贡献，能给我们讲讲这座公园吗？

我创办更新世公园的目的，是观察动物能用多快的速度将苔原转化为肥沃多产的草原。我当时主要是想解开一个科学谜题——曾经孕育了大片草原以及大批野马、野牛和猛犸象的自然环境为什么会变得如此贫瘠荒芜？

这座公园地处植被丰富的科雷马河流域，附近的丘陵上随处可见茂密的落叶松、灌木林、沼泽和苔藓。我们在园区四周架设了围栏，引进了许多种动物，包括美洲野牛、麝牛、驯鹿、牦牛和雅库特马。如今，园区的动物主要生活在一片200公顷的区域内，而另外2000公顷的土地则相对荒僻。20年来，动物们不断踩踏这里的泥炭沼泽，摧折灌木丛，使可以排干沼泽的草地面积明显增加。

我们的公园位于相当典型的西伯利亚地区，这说明在干燥质密的土地上，丰饶的草原完全可以取代随处可见的泥炭沼泽和稀疏的林木。生活在这里的大多数动物都能够适应新的环境。

猛犸草原的生态系统为什么会消失？

主要是人为原因。丰富多姿的草原生态系统需要悉心呵护。青苔和树木肆意生长，草地难以招架。要维护草原，就需要大量“园丁”。人类在西伯利亚和美洲定居之后，各地的动物数量随之减少。当然，人类并没有杀光所有动物，但即使动物数量仅减少一半，也会导致树木、灌木和苔藓侵入草原。由于人类过度狩猎，这些大型生态系统遭到了破坏。

你在更新世公园开展的研究表明，草原生态系统可以减缓永久冻土的融化。

我们的测算结果丝毫没有超出大家的认知范围。以反射率为例，科学家很久以前就已经知道，颜色较深的森林会吸收太阳光，而冰雪覆盖的浅色草原在冬季会反射太阳光。

积雪的厚度对于地表和永久冻土的温度有很大影响，这也是各方公认的。专业文献资料表明，积雪厚度每增加10厘米，地表和永久冻土的温度会升高 1℃以上。

在草原生态系统中，夏天生长的万物到了冬天都会被消耗掉。动物在寒冷的季节要想吃草，唯一的办法就是挖雪。我们的动物整个冬天都在不停地挖雪，这非常有助于土壤降温。

要维持这样一个生态系统，需要多少动物？

要对气候形成影响，一个生态系统需要数千万只动物，平均每平方公里10吨左右；考虑到全球变暖，甚至可能需要15吨。一匹马重400公斤，一头野牛重500公斤，平均每平方公里10吨动物是个什么概念呢？大约相当于10只动物吧。

在人类到来之前，野马和野牛在动物种群数量中占到60%–70%，驯鹿和猛犸象各占10%，其他动物占剩余的10%。事实上，在非洲大草原也观测到了类似的比例。近年来，非洲动物种群仍以斑马、角马、大象和羚羊为主。我们的目标是为所有幸存下来的物种提供一个机会，在初期施以一定援助，然后放手让它们独自面对彼此之间的关系以及与捕食者之间的关系。

去哪儿找这些动物呢？

首先，大型动物的繁殖速度相对较快，它们的数量在25年里可以成百倍地增加。如今俄罗斯拥有几百万头驯鹿、50万匹雅库特马、数千头麝牛和雪羊。5到10年内将几千万只动物引进西伯利亚，并不算什么难事；25年或30年后，它们的数量可能会上亿。换言之，用不了10年，我们就能让食草动物在西伯利亚的生态系统中繁衍生息，并对气候产生重大影响。

我们的家族在没有得到国家资助的情况下创建了两处小规模的公园（另一处位于莫斯科以南约250公里的图拉地区，名为“荒野”）。我们要使用昂贵的交通工具来运输动物，还要应对卫生法规和海关检查等各种行政问题，不过，公园还是成功建成了。像这样的工作，致力于科学事业的家族能够做到，想必大国通过国际合作更是可以轻松办到。

美国哈佛医学院的遗传学家乔治·丘奇（George Church）率领了一支科研小组，致力于通过基因手段复活已灭绝数千年的长毛猛犸象。他们的项目与更新世公园有什么联系吗？

所有的草原生态系统都离不开大象，如非洲象、亚洲象、乳齿象、草原猛犸象或普通的猛犸象。我认为象群在猛犸草原上的主要作用是提供水源。溪流河湖在夏季常常干涸，动物需要到几十公里以外去寻找水源。而大象和猛犸象可以在沟渠中挖洞掘水。象群喝饱之后，其他动物也可以来沾沾光。

动物在冬天可以吃雪，因而不需要饮水。不过西伯利亚的秋天往往很冷，河流湖泊在初雪降临之前已经结冰了，此时动物有可能被渴死。猛犸象为了喝到水，可以打破任何冰面，从而也造福其他动物。我们计划引入的是毛发更长、脂肪储备更多的亚洲象。哈佛的科学家们正在从事这方面的研究。我打算今年开始试着让亚洲象逐步适应西伯利亚的气候。

猛犸草原生态系统应该占据地球上的哪些地区，才能阻止永久冻土融化？

应该占据永久冻土层覆盖的所有地区。有了动物的帮助，永久冻土的温度可以下降4℃。这给了人类机会去适应全球变暖。我们不仅要在永久冻土区域重建上述生态系统，在俄罗斯全境，凡是土地还没有开发的地区都必须进行生态重建。对气候造成威胁的不仅是永久冻土，还有北部所有富含有机物的土地。土壤中有机物的分解速度主要取决于温度。要将碳封存在土壤中，唯一的办法是降低土壤温度。

眼下，除了扩建现有园区，我们必须在西伯利亚多地创建公园，包括英迪吉尔卡河流域、雅库特中部、泰梅尔半岛南部以及乌拉尔北部等，并将动物引进公园。日后还应扩大这些园区的面积，将多种已经适应了当地气候、适应了与彼此共存的动物引入新的地区。

西伯利亚的森林大火和永久冻土的融化正在破坏生态系统。每年都有越来越多的土地可以生长出草丛，形成天然草原。假如我们在这些地区开拓出一片荒野，大自然会遵循自己的规律演变发展，动物可在没有人类干预的情况下帮助我们调节气候变化。

Sergey Zimov: “Thawing permafrost is a direct threat to the climate”

Beyond the Arctic Circle, in the heart of northern Siberia, Russian scientist Sergey Zimov has created Pleistocene Park – to reconstitute the ecosystem of the Ice Age by reintroducing large herbivore species into the former Mammoth steppe, where the soil had become severely depleted. Restoring these ecosystems could also help halt the thawing of the permafrost, a gigantic layer of glacial ice that traps billions of tons of organic carbon. As it melts, the microbes that dwell here quickly convert formerly frozen organic matter into carbon dioxide and methane. The release of these greenhouse gases as a result of climate change is a threat that has long been ignored.

Interview by Katerina Markelova

UNESCO

Why has the permafrost started to thaw?

The release of greenhouse gases into the atmosphere is warming the planet. To date, its temperature has risen by more than 1 °C. In the northern hemisphere, in Russia for example, the warming observed is even more than 3 °C. This is because the land is warming faster than the ocean. And the land area is larger in the northern hemisphere.

But the temperature of the permafrost is not just dictated by the air temperature – it also depends on the thickness of the snow cover. When there is a lot of snow, the ground and the permafrost do not get much colder in winter. Today, the snow cover is one and a half times greater than it was several decades ago. With global warming, more water is evaporating from the ocean, and clouds are producing more snow than before. As a result, the ground temperature in Russia has risen by 5 °C to 7 °C.

Permafrost temperatures used to range between -6 °C and -8 °C in the north of Siberia and between -2 °C and -3 °C in the south. Now, almost half of Siberia is melting  – the southern part of the region, but also the lower Kolyma, an area of continuous permafrost on the edge of the Arctic Ocean. Near my home, the permafrost has thawed by more than four metres in some places. In the large coastal plains of Kolyma, this phenomenon began three years ago.

What are the dangers associated with melting permafrost?

Permafrost covers 11 million square kilometres of Russian territory. These are rich soils that are full of organic matter and dormant bacteria. When the soil thaws, the old microorganisms wake up and attack what they have not had time to consume – releasing carbon dioxide when the soil is dry, and methane when it is saturated with water.

There is twice as much organic matter in our permafrost as there is in all the flora of the planet. Most of it, equivalent to 1,000 gigatons, is concentrated in the first three metres. And three metres thaw very quickly – it only takes three to five years. This is why the thawing of the permafrost is a direct threat to the global climate today. It produces greenhouse gases, and the resulting global warming in turn accelerates the thawing of the permafrost. It’s very difficult to stop this process.

Under these conditions, the goals of the Paris Agreement – adopted in 2015 to move to a low-carbon economy on a global scale – become meaningless. The reduction of greenhouse gas emissions that could be achieved through the Agreement and renewable energy represents only a small part of the permafrost emissions.

The methane released by permafrost is far more dangerous than carbon dioxide...

If it were only carbon dioxide that was released from the melting, permafrost emissions would be equivalent to those generated by humans. But some of the gas released into the atmosphere, about ten per cent to twenty per cent, is methane. And since the greenhouse effect of methane is eighty times more powerful than that of CO2 over short periods of time, the climatic consequences of the release of this gas can be up to four times greater than those resulting from the emission of carbon dioxide.

During the first year and a half of the Covid-19 epidemic, anthropogenic greenhouse gas emissions slowed down significantly. One might have expected the overall concentration of greenhouse gases in the atmosphere to decrease. Instead, there has been a historic increase in the concentration of methane. I regularly observe the appearance of new small springs where methane is bubbling up. I cannot see any other explanation for this high concentration.

Why is the thawing of the permafrost not taken into account in the analyses and forecasts of the Intergovernmental Panel on Climate Change (IPCC)?

Thirty years ago, the scientific community thought it knew the carbon cycle well. Nobody thought about permafrost. When I started talking about the effects of melting, it became clear that all the equations had to be rewritten. In the early years, many scientists disputed my conclusions, refusing to believe that the permafrost was melting. Permafrost has long been treated like an unwanted child in the scientific family. According to the latest estimates, it will have shrunk by ten per cent to twenty per cent by the end of the century. Awareness about it will probably come only this year, thanks to the observation of the significant increase in global greenhouse gas concentrations.

Twenty years ago, you created Pleistocene Park, not imagining that it would one day contribute to climate protection. Could you tell us about it?

I created Pleistocene Park to observe how quickly animals could transform the moss tundra into productive grassland. My main objective at the time was to solve a scientific mystery – why had the natural environment, which had known so many grasslands, horses, bison, mammoths, become so poor?

The park occupies part of the Kolyma River basin, where the vegetation is rich, and the surrounding hills are covered with larch and shrub forests, marshes and mosses. We have fenced off a portion of this land and introduced various animals: bison, musk oxen, reindeer, yaks, Yakutian horses. Today there is a 200-hectare area with a high density and another 2,000 hectares that have not yet been fully populated. In twenty years, the peat bogs have been trampled, the shrubs broken, and the amount of grass – which dries out the marshes – has increased significantly.

Our park is located in a fairly typical region of Siberia, and shows that it is possible to replace peat bogs and sparse forests everywhere with productive grasslands on dry, dense land. Most of the animals that used to live here can adapt to this environment again.

Why have the ecosystems of the Mammoth steppe disappeared?

Mainly because of humans. The preservation of diverse grassland ecosystems requires a lot of effort. Moss and trees grow everywhere, and it is very difficult for grasses to resist. The maintenance of grasslands requires a lot of “gardeners”. When humans began to colonize Siberia or America, they reduced the number of animals everywhere. They didn’t kill them all, of course, but if there are only half as many animals, trees, shrubs and mosses will invade the grassland. Through excessive hunting, humans have destroyed these large ecosystems.

Your research in Pleistocene Park has shown that grassland ecosystems can slow down the thawing of the permafrost.

Our measurements have not shown anything that is not already known. Take albedo, for example. Scientists have known for a long time that dark forests absorb the sun's rays, while the lighter, snow-covered grasslands reflect them in winter.

It is also well-established that the thickness of the snow cover has a significant influence on ground and permafrost temperatures. The specialist literature indicates that an additional ten centimetres of snow increases these temperatures by more than 1 °C.

In the grassland ecosystem, everything that grew during the summer must be consumed in the winter. The only way to access the grass during the cold season is to dig through the snow. Our animals dig in the snow all winter long. This contributes greatly to the cooling of the soil.

How many animals does it take to make an ecosystem like this work?

It takes tens of millions of animals to affect the climate, about ten tons per square kilometre. Maybe even fifteen tons, with global warming. What do ten tons of animals per square kilometre represent, when a horse weighs 400 kilos and a bison 500 kilos? That's about ten animals.

Before the arrival of humans, horses and bison accounted for sixty per cent to seventy per cent  of the zoo mass, and reindeer and mammoths, for ten per cent each. The other animals made up the remaining ten per cent. Similar proportions, in fact, are found in the African savannah. Until recently, the population there consisted mainly of zebras, wildebeest, elephants, and gazelles. Our goal is to give all surviving species a chance, to help them at the beginning. Then we will let them manage their own relationships – with each other, and with their predators.

Where will we find all these animals?

First of all, you should know that even large animals reproduce relatively quickly. They can multiply a hundred times over every twenty-five years. Today in Russia, there are several million reindeer, half a million Yakutian horses, thousands of musk oxen and snow sheep. It would therefore be easy to obtain tens of millions of animals in Siberia within five to ten years, and their number could reach several hundred million in twenty-five or thirty years. In other words, within a decade, we could populate the Siberian ecosystems with herbivores, and have a significant impact on the climate.

Our family has created two parks [the second, Wild Field, is located in the Tula region, about 250 kilometres south of Moscow] on a small scale, without any state funding. We use expensive means of transportation, we move a few animals, we face many administrative problems related to health regulations and customs – and yet we have succeeded. If one family of scientists could do it, large states could easily do it through international co-operation.

At Harvard Medical School in the United States, a team of scientists led by geneticist George Church is working on genetically resurrecting woolly mammoths thousands of years after they went extinct. What links do they have with Pleistocene Park?

Grassland ecosystems have all had elephants – the African elephant, the Asian elephant, mastodons, steppe mammoths, or mammoths in general. I believe that the main function of the elephant in the Mammoth steppe was to provide water sources. The streams and rivers often dry up in the summer, and the animals have to look for water several dozen kilometres away. Elephants and mammoths can dig holes to collect water in ditches. After they have drunk, other animals can also take advantage of this.

In winter, the animals eat snow and do not need water. But in Siberia, autumn is often cold, and the rivers and lakes freeze over before the first snow. At this time, animals risk dying of thirst. Mammoths could break through any ice, drink, and allow other animals to drink. In our case, we could imagine an Asian elephant with longer hair and increased fat reserves. This is what the Harvard scientists are working on. I plan to experiment this year with the adaptation of Asian elephants to our climate.

Which regions of the planet should be occupied by mammoth steppe ecosystems to stop the permafrost from melting?

The entire extent of the permafrost. With the help of animals, the permafrost could be cooled by 4 °C. This would give humanity a chance to adapt to global warming. These ecosystems must be reconstituted not only where there is permafrost, but more generally, everywhere in Russia, where the territory is not exploited. It is not just permafrost that threatens the climate, but also all the land in the north that is rich in organic matter. The rate of decomposition of organic matter in the soil depends mainly on its temperature. The only way to force the soil to retain carbon is to cool it down.

We must create parks right now in many parts of Siberia – besides expanding our own, we should open parks in the Indigirka River basin; in Central Yakutia; south of the Taimyr Peninsula; in the northern Urals – and introduce animals in these parks. Then it will be necessary to expand these parks and introduce animals accustomed to the climate, and to each other, into new territories.

Forest fires in Siberia and the melting permafrost are destroying ecosystems. Every year there are more and more places where grass can grow. These are all ready-made grasslands. If we created wildernesses in these areas, nature would continue to develop on its own. Animals could then, without human intervention, help regulate climate change.

Sergey Zimov

The Russian geophysicist and ecologist Sergey Zimov founded the Northeast Science Station in Cherskii, northern Siberia, which serves as an open-air laboratory today. He is also one of the research directors of the Pacific Geographical Institute of the Far Eastern Branch of the Russian Academy of Sciences.

Zimov initiated the first experiments to reintroduce mammals into the Kolyma River basin in 1988, and created Pleistocene Park in 1996. Its mission is to restore an ecosystem comparable to the Mammoth steppe which was dominant in Eurasia in the late Pleistocene epoch – between 2.58 million years and 11,700 years ago.

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