There’s two times more carbon in the earth’s soil than in all of its vegetation and the atmosphere — combined. Biogeochemist Asmeret Asefaw Berhe dives into the science of soil and shares how we could use its awesome carbon-trapping power to offset climate change. “[Soil] represents the difference between life and lifelessness in the earth system, and it can also help us combat climate change — if we can only stop treating it like dirt,” she says.

地球の土壌には、すべての植物と大気を合わせた炭素の 2 倍の炭素が存在します。

生物地球化学者のアスメレット・アセファウ・バーヘ氏は、土壌の科学に深く入り込み、その驚異的な炭素捕捉力を気候変動を相殺するためにどのように利用できるかを共有します。 「(土壌は)地球システムにおける生命と無生命の違いを表しており、土壌を土のように扱うことをやめることさえできれば、気候変動と戦うのにも役立ちます」と彼女は言います。

A climate change solution that’s right under our feet
スピーカー アスメレット・アセファウ・バーヘ
アップロード 2019/09/27

「私たちの足元にある気候変動ソリューション(A climate change solution that’s right under our feet)」の文字起こし

So one of the most important solutions to the global challenge posed by climate change lies right under our foot every day. It’s soil. Soil’s just the thin veil that covers the surface of land, but it has the power to shape our planet’s destiny. See, a six-foot or so of soil, loose soil material that covers the earth’s surface, represents the difference between life and lifelessness in the earth system, and it can also help us combat climate change if we can only stop treating it like dirt.

Climate change is happening, the earth’s atmosphere is warming, because of the increasing amount of greenhouse gases we keep releasing into the atmosphere. You all know that. But what I assume you might not have heard is that one of the most important things our human society could do to address climate change lies right there in the soil. I’m a soil scientist who has been studying soil since I was 18, because I’m interested in unlocking the secrets of soil and helping people understand this really important climate change solution.

So here are the facts about climate. The concentration of carbon dioxide in the earth’s atmosphere has increased by 40 percent just in the last 150 years or so. Human actions are now releasing 9.4 billion metric tons of carbon into the atmosphere, from activities such as burning fossil fuels and intensive agricultural practices, and other ways we change the way we use land, including deforestation. But the concentration of carbon dioxide that stays in the atmosphere is only increasing by about half of that, and that’s because half of the carbon we keep releasing into the atmosphere is currently being taken up by land and the seas through a process we know as carbon sequestration.

So in essence, whatever consequence you think we’re facing from climate change right now, we’re only experiencing the consequence of 50 percent of our pollution, because the natural ecosystems are bailing us out. But don’t get too comfortable, because we have two major things working against us right now. One: unless we do something big, and then fast, emissions will continue to rise. And second: the ability of these natural ecosystems to take up carbon dioxide from the atmosphere and sequester it in the natural habitats is currently getting compromised, as they’re experiencing serious degradation because of human actions. So it’s not entirely clear that we will continue to get bailed out by these natural ecosystems if we continue on this business-as-usual path that we’ve been.

Here’s where the soil comes in: there is about three thousand billion metric tons of carbon in the soil. That’s roughly about 315 times the amount of carbon that we release into the atmosphere currently. And there’s twice more carbon in soil than there is in vegetation and air. Think about that for a second. There’s more carbon in soil than there is in all of the world’s vegetation, including the lush tropical rainforests and the giant sequoias, the expansive grasslands, all of the cultivated systems, and every kind of flora you can imagine on the face of the earth, plus all the carbon that’s currently up in the atmosphere, combined, and then twice over. Hence, a very small change in the amount of carbon stored in soil can make a big difference in maintenance of the earth’s atmosphere.

But soil’s not just simply a storage box for carbon, though. It operates more like a bank account, and the amount of carbon that’s in soil at any given time is a function of the amount of carbon coming in and out of the soil. Carbon comes into the soil through the process of photosynthesis, when green plants take carbon dioxide from the atmosphere and use it to make their bodies, and upon death, their bodies enter the soil. And carbon leaves the soil and goes right back up into the atmosphere when the bodies of those formerly living organisms decay in soil by the activity of microbes. See, decomposition releases carbon dioxide into the atmosphere, as well as other greenhouse gases such as methane and nitrous oxide, but it also releases all the nutrients we all need to survive.

One of the things that makes soil such a fundamental component of any climate change mitigation strategy is because it represents a long-term storage of carbon. Carbon that would have lasted maybe a year or two in decaying residue if it was left on the surface can stay in soil for hundreds of years, even thousands and more. Soil biogeochemists like me study exactly how the soil system makes this possible, by locking away the carbon in physical association with minerals, inside aggregates of soil minerals, and formation of strong chemical bonds that bind the carbon to the surfaces of the minerals. See when carbon is entrapped in soil, in these kinds of associations with soil minerals, even the wiliest of the microbes can’t easily degrade it. And carbon that’s not degrading fast is carbon that’s not going back into the atmosphere as greenhouse gases.

But the benefit of carbon sequestration is not just limited to climate change mitigation. Soil that stores large amounts of carbon is healthy, fertile, soft. It’s malleable. It’s workable. It makes it like a sponge. It can hold on to a lot of water and nutrients. Healthy and fertile soils like this support the most dynamic, abundant and diverse habitat for living things that we know of anywhere on the earth system. It makes life possible for everything from the tiniest of the microbes, such as bacteria and fungi, all the way to higher plants, and fulfills the food, feed and fiber needs for all animals, including you and I.

So at this point, you would assume that we should be treating soil like the precious resource that it is. Unfortunately, that’s not the case. Soils around the world are experiencing unprecedented rates of degradation through a variety of human actions that include deforestation, intensive agricultural production systems, overgrazing, excessive application of agricultural chemicals, erosion and similar things. Half of the world’s soils are currently considered degraded. Soil degradation is bad for many reasons, but let me just tell you a couple. One: degraded soils have diminished potential to support plant productivity. And hence, by degrading soil, we’re compromising our own abilities to provide the food and other resources that we need for us and every member of living things on the face of the earth. And second: soil use and degradation, just in the last 200 years or so, has released 12 times more carbon into the atmosphere compared to the rate at which we’re releasing carbon into the atmosphere right now.

I’m afraid there’s even more bad news. This is a story of soils at high latitudes. Peatlands in polar environments store about a third of the global soil carbon reserves. These peatlands have a permanently frozen ground underneath, the permafrost, and the carbon was able to build up in these soils over long periods of time because even though plants are able to photosynthesize during the short, warm summer months, the environment quickly turns cold and dark, and then microbes are not able to efficiently break down the residue. So the soil carbon bank in these polar environments built up over hundreds of thousands of years. But right now, with atmospheric warming, the permafrost is thawing and draining. And when permafrost thaws and drains, it makes it possible for microbes to come in and rather quickly decompose all this carbon, with the potential to release hundreds of billions of metric tons of carbon into the atmosphere in the form of greenhouse gases.

And this release of additional greenhouse gases into the atmosphere will only contribute to further warming that makes this predicament even worse, as it starts a self-reinforcing positive feedback loop that could go on and on and on, dramatically changing our climate future. Fortunately, I can also tell you that there is a solution for these two wicked problems of soil degradation and climate change. Just like we created these problems, we do know the solution, and the solution lies in simultaneously working to address these two things together, through what we call climate-smart land management practices.

What do I mean here? I mean managing land in a way that’s smart about maximizing how much carbon we store in soil. And we can accomplish this by putting in place deep-rooted perennial plants, putting back forests whenever possible, reducing tillage and other disturbances from agricultural practices, including optimizing the use of agricultural chemicals and grazing, and even adding carbon to soil, whenever possible, from recycled resources such as compost and even human waste. This kind of land stewardship is not a radical idea. It’s what made it possible for fertile soils to be able to support human civilizations since time immemorial.

In fact, some are doing it just right now. There’s a global effort underway to accomplish exactly this goal. This effort that started in France is known as the “4 per 1000” effort, and it sets an aspirational goal to increase the amount of carbon stored in soil by 0.4 percent annually, using the same kind of climate-smart land management practices I mentioned earlier. And if this effort’s fully successful, it can offset a third of the global emissions of fossil-fuel-derived carbon into the atmosphere. But even if this effort is not fully successful, but we just start heading in that direction, we still end up with soils that are healthier, more fertile, are able to produce all the food and resources that we need for human populations and more, and also soils that are better capable of sequestering carbon dioxide from the atmosphere and helping with climate change mitigation. I’m pretty sure that’s what politicians call a win-win solution.

And we all can have a role to play here. We can start by treating the soil with the respect that it deserves: respect for its ability as the basis of all life on earth, respect for its ability to serve as a carbon bank and respect for its ability to control our climate. And if we do so, we can then simultaneously address two of the most pressing global challenges of our time: climate change and soil degradation. And in the process, we would be able to provide food and nutritional security to our growing human family. Thank you.

「私たちの足元にある気候変動ソリューション(A climate change solution that’s right under our feet)」の和訳