遮擋陽光來冷卻地球的工程無法減少全球暖化對農作物造成的損失
有人提出為了冷卻地球並對抗氣候變遷造成的暖化，可以把粒子注入大氣當中。根據加州大學柏克萊分校的研究人員最近做出的分析，這完全無法彌補全球氣溫升高對作物造成的損害。
如果火山爆發噴到大氣中的大量氣膠可以冷卻地球，那我們是否可以刻意把氣膠排放到大氣中，藉此冷卻地球並減緩氣候變遷帶來的衝擊？(圖片來源：Solomon Hsiang和Jonathan Proctor)
研究團隊分析過往曾讓地球冷卻的火山爆發事件造成的影響，以及農作物對陽光變化的反應之後，得出的結論指出因為氣溫降低造成的收成量增加，會被陽光減弱造成的生產量下降給抵消掉。太陽地球工程(solar geoengineering)是科學家提出幫助人類應對全球暖化衝擊的方法之一，而此發現讓我們對太陽地球工程有了十分重要的瞭解。
「遮蔽陽光使地球冷卻下來可以讓作物長得更好，然而植物生長時還是需要陽光，因此這種作法也會影響它們成長。對農業來說，太陽地球工程無意之間帶來的衝擊規模可能跟產生的益處相差不多。」加州大學柏克萊分校農業與自然資源經濟學系的博士候選人Jonathan
Proctor表示，「這有點像是動過實驗外科手術之後，發現產生的副作用和病症本身差不多糟糕。」
「一如所料，在論及關乎全球的政策時，未知的未知(unknown unknowns)因素總會觸動人們的神經。」研究共同作者，加州大學柏克萊分校公共政策的校長副教授Solomon Hsiang表示，「要找出太陽地球工程帶來的後果，困難點在於實驗規模會擴及整個地球，因此要進行實驗就必須實際執行這項科技。這項研究的突破在於我們瞭解到探討地球工程試著去模仿的巨型火山爆發帶來的效應，可以從中學習到某些事物。」
Hsiang現為加州大學柏克萊分校全球政策實驗室的主任，而Proctor則是其中的博士研究員。
Proctor和Hsiang的這項發現於8月8日發表在期刊《自然》(Nature )的線上版。
自然界的氣候實驗：火山
有些人指出過往的大型火山噴發事件，像是1991年菲律賓的皮納土波火山噴出的氣體曾造成全球冷化一段時間，因此主張人類可以主動把硫酸氣膠注入到高層大氣，藉此人為方式來冷化地球以減緩二氧化碳含量日漸升高造成的暖化現象。氣膠――在此例中是指微小的硫酸液滴――可以把少量陽光反射回太空，使氣溫下降幾度。
「就像是在天氣很熱時撐傘一樣。」Proctor說，「如果你在地球上空放一把陽傘，就能減緩暖化。」
以皮納土波火山為例，它噴發時把約莫2000萬噸的二氧化硫噴到大氣當中，造成日照量減少了大概百分之2.5並讓全球平均溫度下降0.5℃左右。
團隊探討1979至2009年105個國家玉米、黃豆、稻米和小麥的產量，以及全球衛星對這些氣膠的觀測結果之間有何關聯，來研究氣膠對農業的影響。將結果配合全球氣侯模型，團隊算出運用硫酸的地球工程計畫減少的日照量，會抵消此作法帶來的預期效益，也就是保護作物不被極端高溫損害的目的。
Hsiang說：「這就像是辦張信用卡來償還另一張的卡債一樣――到頭來還是回到了原點，沒有解決任何問題。」
有些前人研究提出氣膠能讓陽光散射，使得通常被上方葉子遮住的下方葉片可以接受到更多陽光帶來的能量，或許也有助於提高作物產量。但散射帶來的效益似乎比過往認為的還少。
「我們是第一個結合實際實驗和觀測證據，來瞭解運用硫酸的地球工程對作物產量的可能整體影響。」Proctor表示，「在開始這項研究之前，我本來認為改變陽光帶來的影響整體來說是正面的，所以我很驚訝地發現把陽光散射掉其實會減少作物產量。」
擾動陽光
儘管研究得到上述結論，Proctor說：「我不認為我們應該要澈底刪除太陽地球工程這個選項。雖然它在農業上的表現可能不佳，但對於經濟的其他層面來說可能大有助益。」
雖然降溫可以提高產量，但陽光變弱則會降低產量，使得遮擋地球來減輕全球緩化帶來衝擊的效益降低。
Proctor和Hsiang指出他們的方法也可以用在探討地球工程對其他經濟層面，以及人體健康和自然系統運作方式的影響。
他們並沒有討論其他類型的地球工程，像是二氧化碳的捕集和儲存；以及圍繞地球工程的其他議題，像是此方法對保護地球的臭氧層有何影響，以及地球的恆溫系統應該由誰來設定。
「社會必須要客觀看待地球工程這項科技，而且要持續釐清其潛在效益、成本和風險。」Proctor表示，「目前，我們對於這些要素的不確定遠超過我們所知的。」
作者強調還需要更多研究來瞭解地球工程對人類和生態的影響，不論是好或壞。
Proctor說：「要減輕作物損失，進而保全人類財產和福祉的最有效方法，無疑是減少二氧化碳排放。」
「也許最重要的是我們得審慎考慮地球工程科技可能的影響規模、程度和風險。」Hsiang表示，「陽光是地表一切事物的能量來源，因此如果我們試著去管理陽光就必須瞭解這麼做可能帶來什麼後果。」
Blocking sunlight to cool Earth won’t reduce crop
damage from global warming
Injecting
particles into the atmosphere to cool the planet and counter the warming
effects of climate change would do nothing to offset the crop damage from
rising global temperatures, according to a new analysis by UC Berkeley
researchers.
By analyzing the past
effects of Earth-cooling volcanic eruptions, and the response of crops to
changes in sunlight, the team concluded that any improvements in yield from
cooler temperatures would be negated by lower productivity due to reduced
sunlight. The findings have important implications for our understanding of
solar geoengineering, one proposed method for helping humanity manage the
impacts of global warming.
“Shading the planet keeps
things cooler, which helps crops grow better. But plants also need sunlight to
grow, so blocking sunlight can affect growth. For agriculture, the unintended
impacts of solar geoengineering are equal in magnitude to the benefits,” said
lead author Jonathan Proctor, a UC Berkeley doctoral candidate in the Department of Agricultural
and Resource Economics. “It’s a bit like performing an experimental surgery;
the side-effects of treatment appear to be as bad as the illness.”
“Unknown unknowns make
everybody nervous when it comes to global policies, as they should,” said Solomon
Hsiang, co-lead author of the study and Chancellor’s Associate Professor of
Public Policy at UC Berkeley. “The problem in figuring out the consequences of
solar geoengineering is that we can’t do a planetary-scale experiment without
actually deploying the technology. The breakthrough here was realizing that we
could learn something by studying the effects of giant volcanic eruptions that
geoengineering tries to copy.”
Hsiang is director of UC
Berkeley’s Global Policy Laboratory, where Proctor is a doctoral fellow.
Proctor and Hsiang will
publish their findings online in the journal Nature on August 8.
Volcanoes
are natural climate experiments
Some people have pointed to
past episodes of global cooling caused by gases emitted during massive volcanic
eruptions, such as Mt. Pinatubo in the Philippines in 1991, and argued that
humans could purposely inject sulfate aerosols into the upper atmosphere to
artificially cool Earth and alleviate the greenhouse warming caused by
increased levels of carbon dioxide. Aerosols – in this case, minute droplets of
sulfuric acid – reflect a small percentage of sunlight back into space,
reducing the temperature a few degrees.
“It’s like putting an
umbrella over your head when you’re hot,” Proctor said. “If you put a global sunshade
up, it would slow warming.”
Pinatubo, for example,
injected about 20 million tons of sulfur dioxide into the atmosphere, reducing
sunlight by about 2.5 percent and lowering the average global temperature by
about half a degree Celsius (nearly 1 degree Fahrenheit).
The team linked maize, soy,
rice and wheat production from 105 countries from 1979-2009 to global satellite
observations of these aerosols to study their effect on agriculture. Pairing
these results with global climate models, the team calculated that the loss of
sunlight from a sulfate-based geoengineering program would cancel its intended
benefits of protecting crops from damaging extreme heat.
“It’s similar to using one
credit card to pay off another credit card: at the end of the day, you end up
where you started without having solved the problem,” Hsiang said.
Some earlier studies
suggested that aerosols might improve crop yields also by scattering sunlight
and allowing more of the sun’s energy to reach interior leaves typically shaded
by upper canopy leaves. This benefit of scattering appears to be weaker than
previously thought.
“We are the first to use
actual experimental and observational evidence to get at the total impacts that
sulfate-based geoengineering might have on yields,” Proctor said. “Before I
started the study, I thought the net impact of changes in sunlight would be
positive, so I was quite surprised by the finding that scattering light
decreases yields.”
Messing
with sunlight
Despite the study’s
conclusions, Proctor said, “I don’t think we should necessarily write off solar
geoengineering. For agriculture, it might not work that well, but there are
other sectors of the economy that could potentially benefit substantially.”
Proctor and Hsiang noted
that their methods could be used to investigate the impact of geoengineering on
other segments of the economy, human health and the functioning of natural
ecosystems.
They did not address other
types of geoengineering, such as capture and storage of carbon dioxide, or
issues surrounding geoengineering, such as its impact on Earth’s protective
ozone layer and who gets to set Earth’s thermostat.
“Society needs to be
objective about geoengineering technologies and develop a clear understanding
of the potential benefits, costs and risks,” Proctor said. “At present,
uncertainty about these factors dwarfs what we understand.”
The authors emphasize the
need for more research into the human and ecological consequences of
geoengineering, both good and bad.
“The most certain way to
reduce damages to crops and, in turn, people’s livelihood and well-being, is
reducing carbon emissions,” Proctor said.
“Perhaps what is most
important is that we have respect for the potential scale, power and risks of
geoengineering technologies,” Hsiang said. “Sunlight powers everything on the
planet, so we must understand the possible outcomes if we are going to try to
manage it.”
原始論文：Jonathan
Proctor, Solomon Hsiang, Jennifer Burney, Marshall Burke, Wolfram
Schlenker. Estimating global agricultural effects of geoengineering
using volcanic eruptions. Nature, 2018; DOI: 10.1038/s41586-018-0417-3
引用自：University of California - Berkeley.
"Blocking sunlight to cool Earth won't reduce crop damage from global
warming."
沒有留言:
張貼留言