原文網址：https://news.wisc.edu/mapping-methane-emissions-from-rivers-around-globe-reveals-surprising-sources/
By Adam
Hinterthuer
甲烷是會導致全球暖化的強力溫室氣體。在全世界的甲烷排放源中，淡水生態系占了一半——特別是河川溪流，科學家認為它們排放了非常大量的甲烷，但是從全球來看它們的排放速率以及模式為何，大部分還是沒有文獻探討。
研究發現河川溪流產生的甲烷多寡跟它們所在的緯度或溫度沒有關係，而主要受控於周圍與它們連通的棲地類型。圖片來源：Jerry87/iStock
威斯康辛大學麥迪遜分校的淡水生態學家參與的國際研究團隊改變了上述情形，他們對於全世界流動水域排放甲烷的速率、模式以及成因進行了全新的敘述。今日發表於期刊《自然》(Nature)的這項發現，可以讓未來的研究人員更加精準地估算甲烷並改良氣候變遷模型，也指出我們可以透過改變土地管理方式與復育機會，減少逸散到大氣中的甲烷量。
新研究證實河川溪流的確產生很多甲烷，因此在氣候變遷動力學中具有很重要的地位。但研究對於甲烷的產生過程與地點也得出了某些驚人的結果。
這篇發表在《自然》的報告的共同作者之一，威斯康辛大學麥迪遜分校湖泊學中心的教授Emily
Stanley表示：「我們預期會發現甲烷排放量最高的地方在熱帶，因為生物產生甲烷的過程深受溫度影響。」她說，他們的團隊反而發現熱帶的甲烷排放量，比起寒冷許多、流經北方針葉林(北半球面積廣大以松樹為主的森林)以及極地苔原棲地的河川溪流差不多。
結果顯示溫度並非造成水體排放甲烷的主要變數。Stanley表示研究發現其實「河川溪流會產生多少甲烷與它們的緯度和溫度沒有關係，主要受控於周遭與之相連的棲地。」
北方針葉林與極區的河川溪流經常連到泥炭地與溼地，而亞馬遜河與剛果河流域的茂密森林也提供了流經其中的河流富含有機質的土壤。這兩種系統都能產生非常大量的甲烷，因為它們經常會形成低氧的環境，對於把有機質全數分解的同時產生甲烷的微生物來說，這是相當有利的條件。
然而，並非所有排放許多甲烷的河川溪流都是由自然因素所產生。在世上某些地區，淡水甲烷的排放量主要受控於周遭都市與鄉村的人類活動。
此報告的主要作者Gerard
Rocher表示：「人類相當積極地改造世界各地的河川網路，一般而言，這些改變似乎都有利於甲烷排放。」Rocher是瑞典農業科學大學與西班牙布拉內斯高級研究中心的博士後研究員。
被人類高度改造過的棲地，像是為了引水至農田而開鑿溝渠的溪流、汙水處理廠下方的河川、水泥分洪道……等，常常會形成富含有機物、低氧的環境條件而促使大量甲烷產生。
Rocher表示人為介入的重要性可以視作一道好消息。
「這項發現帶來了一項啟發，如果我們盡量去保護並復育淡水環境，或許可以讓甲烷排放量減少，」他說。
減緩肥料、人類活動與畜牧業的廢水等污染物以及過多的表土進到河川溪流，或許有助於限制淡水系統當中產生大量甲烷所需的原料。
「從氣候變遷的觀點來看，相較於因為自然循環而產生甲烷的系統，我們更需要擔心因為人類創造出的條件而利於甲烷產生的系統，」Stanley表示。
這項研究也顯示出在瞭解氣候變遷的範圍時，科學家同心協力彙整巨量資料並加以探討是非常重要的。這項成果有賴於瑞典農業科學大學、優密歐大學、威斯康辛大學麥迪遜分校以及各國的機構合作多年。他們蒐集了幾個國家河川溪流的甲烷測量數據，再運用最先進的電腦模型與機器學習方法來大幅擴張甲烷的資料庫。該資料庫的建造得回溯至2015年，最初是由Stanley和她的研究生把資料彙整起來。
Stanley表示現在「我們對於甲烷估計值的信心比以前高出許多。」研究人員希望他們的結果可以讓我們更加瞭解每一種進到大氣的甲烷來源規模有多大以及它們的空間分布模式，接著這些新的數據可以改良大尺度的氣候模型，使我們可以更加瞭解全球氣候並預測未來的變化。
Mapping methane emissions from rivers
around globe reveals surprising sources
Freshwater ecosystems account for half of
global emissions of methane, a potent greenhouse gas that contributes to global
warming. Rivers and streams, especially, are thought to emit a substantial
amount of that methane, but the rates and patterns of these emissions at global
scales remain largely undocumented.
An international team of researchers, including
University of Wisconsin–Madison freshwater ecologists, has changed that with a
new description of the global rates, patterns and drivers of methane emissions
from running waters. Their findings, published today in the journal Nature, will improve methane estimates
and models of climate change, and point to land-management changes and
restoration opportunities that can reduce the amount of methane escaping into
the atmosphere.
The new study confirms that rivers and streams do,
indeed, produce a lot of methane and play a major role in climate change
dynamics. But the study also reveals some surprising results about how – and
where – that methane is produced.
“We expected to find the highest methane emissions at
the tropics, because the biological production of methane is highly sensitive
to temperature,” says Emily Stanley, a professor at UW–Madison’s Center for
Limnology and co-author of the Nature
report. Instead, she says, their team found that methane emissions in the
tropics were comparable to those in the much colder streams and rivers of
boreal forests — pine-dominant forests that stretch around the Northern
Hemisphere — and Arctic tundra habitats.
Temperature, it turns out, isn’t the primary variable
driving aquatic methane emissions. Instead, the study found, “the amount of
methane coming out of streams and rivers regardless of their latitude or
temperature was primarily controlled by the surrounding habitat connected to
them,” Stanley says.
Rivers and streams in boreal forests and polar
regions at high latitudes are often tied to peatlands and wetlands, while the
dense forests of the Amazon and Congo river basins also supply the waters
running through them with soils rich in organic matter. Both systems produce
substantial amounts of methane because they often result in low-oxygen
conditions preferred by microbes that produce methane while breaking down all
that organic matter.
However, not all high methane rivers and streams come
by these emissions naturally. In parts of the world, freshwater methane
emissions are primarily controlled by human activity in both urban and rural
communities.
“Humans are actively modifying river networks
worldwide and, in general, these changes seem to favor methane emissions,” says
Gerard Rocher, lead author of the report and a postdoctoral researcher with
both the Swedish University of Agricultural Sciences and the Blanes Centre of
Advanced Studies in Spain.
Habitats that have been highly modified by humans —
like ditched streams draining agricultural fields, rivers below wastewater
treatment plants, or concrete stormwater canals — also often result in the
organic-matter-rich, oxygen-poor conditions that promote high methane
production.
The significance of human involvement can be
considered good news, according to Rocher.
“One implication of this finding is that freshwater
conservation and restoration efforts could lead to a reduction in methane
emissions,” he says.
Slowing the flow of pollutants like fertilizer, human
and animal waste or excessive topsoil into rivers and streams would help limit
the ingredients that lead to high methane production in freshwater systems.
“From a climate change perspective, we need to worry
more about systems where humans are creating circumstances that produce methane
than the natural cycles of methane production,” Stanley says.
The study also demonstrates the importance of teams
of scientists working to compile and examine gigantic datasets in understanding
the scope of climate change. The results required a years-long collaboration
between the Swedish University of Agricultural Sciences, Umeå University,
UW–Madison and other institutions around the world. They collected methane measurements
on rivers and streams across several countries, employed state-of-the-art
computer modelling and machine learning to “massively expand” a dataset Stanley
first began to compile with her graduate students back in 2015.
Now, Stanley says, “we have a lot more confidence in
methane estimates.” The researchers hope their results lead to better
understanding of the magnitude and spatial patterns of all sources of methane
into Earth’s atmosphere, and that the new data improves large-scale models used
to understand global climate and predict its future.
原始論文：Gerard
Rocher-Ros, Emily H. Stanley, Luke C. Loken, Nora J. Casson, Peter A. Raymond,
Shaoda Liu, Giuseppe Amatulli, Ryan A. Sponseller. Global methane
emissions from rivers and streams. Nature, 2023; DOI: 10.1038/s41586-023-06344-6
引用自：University of Wisconsin-Madison. "Mapping
methane emissions from rivers around globe reveals surprising
sources."
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