適度的地球工程可以降低氣候變遷的風險
倫敦大學學院和哈佛大學的研究人員表示，透過人為方法把適量的二氧化硫注入地球的上層大氣，使得可以反光的氣膠微粒層厚度增加，整體而言能減緩氣候變遷帶來的影響。
先前的研究顯示太陽地球工程可以運用市面上的航空科技來達成，每年花費數十億美元把氣膠微粒散佈出去即能降低全球均溫。但是一項懸而未解的問題是：這種方法在區域層級上是否也能降低重要的氣候災害？也就是說，隨著區域而不同的可用水量或者極端溫度的變化，是否也能由它來解決？
倫敦大學學院和哈佛大學的研究結果認為即使是把二氧化硫注入平流層這種簡陋的方法，也可以減少許多重要的氣候災害，而且不會使任何區域的狀況出現明顯的惡化。
這項結果發表在《環境研究通訊》(Environmental
Research Letters)。作者利用精密的模型來模擬在平流層注入氣膠的地球工程，藉此評估氣候變遷對世界各地的影響是否會因此而抵銷或者變得更加嚴重。此外，他們也測試了在不同溫度的情況下這些影響的差異。
團隊發現透過把氣膠注入平流層使暖化程度減半時，幾乎所有地區重要的氣候災害都會減少。他們只有在非常少部份的陸域看到氣候變遷的影響變得更加劇烈。
倫敦大學學院的地球科學教授Peter
Irvine是主要作者，他說：「大多數研究的重點放在太陽地球工程把未來的暖化給完全抵消的情況。雖然整體來說氣候變遷的程度會大幅減少，但如同我們在那些模擬結果中看到的，某些方面的改變會太過劇烈，使得9%左右的陸域遭受更嚴重的氣候變遷，也就是氣候變遷的影響被放大了。」
「然而，如果目標只是抵銷未來暖化程度的一半，我們發現在平流層注入氣膠的地球工程還是可以減少地球整體的氣候變遷，而且只有1.3%的陸域會遭受更加嚴重的變化。」
團隊強調太陽地球工程只能對付氣候變遷帶來的症狀而非背後的原因——也就是大氣中持續累積的二氧化碳和其他溫室氣體。因此我們應該把它當作一種輔助方案，還是得減少碳排放才能解決氣候變遷。
此研究接續了去年發表在《自然—氣候變遷》(Nature Climate Change)得到了類似結果的論文。該論文單純把日照量減少來大略模擬太陽地球工程。這項前導研究也提出了一項問題：如果更實際地去模擬太陽地球工程裡最簡單的方法，也就是注入二氧化硫到平流層，是否也能得到一樣的結論？
「我們的結果顯示如果使用適當的劑量加上減少排放溫室氣體，在平流層注入氣膠的地球工程可以有效地控管氣候變遷的影響。但是，這種方法可能會產生什麼影響還是有很多不確定的地方，因此仍需進行更多研究才能判斷這種想法是否可行。」
團隊使用的數據來自於Geoengineering
Large Ensemble Study(地球工程大型整合研究)。這項計畫運用了繁複的氣候化學模型，模擬假如在平流層注入氣膠的地球工程實行的話，氣候會產生什麼樣的反應。研究進行模擬的時候在熱帶地區的不同高度釋放出二氧化硫來產生一層氣膠，目標是在全球暖化十分劇烈的情況下讓氣溫穩定下來。
研究人員把重點放在平均溫度、極端溫度、可用水量、極端降雨量的變化，這些都是判定氣候風險時的關鍵因子。
先前的研究指出在平流層注入氣膠的地球工程會讓季風大幅減弱，使得乾旱更加嚴重。但作者發現進行平流層注入氣膠的地球工程來讓暖化程度減半時，那些氣候變遷加劇的地區的可用水量會增加而非減少。代表不用擔心這種地球工程會導致沙漠化和乾旱。
哈佛工程與應用科學學院及甘迺迪政府學院的教授David
Keith是共同作者。他說：「先前運用氣候模型的研究皆顯示統一調節各地的日射量，同時減少碳排放可以大幅降低氣候風險。但我們應該相信這些模型嗎？裡面的不確定性還很多，因此任何一方片面之辭都是不值得相信的。不過這項研究確實讓我們的模擬對象從只是注入氣膠的結果，深入至更加實際的區域影響。」
團隊現在進行的研究是更加深入探討在平流層注入氣膠的地球工程對水循環的影響，目標是要了解這種方法對社會以及生態系的潛在利益與危害。
The right dose of
geoengineering could reduce climate change risks
Injecting the right dose of sulphur
dioxide into Earth’s upper atmosphere to thicken the layer of light reflecting
aerosol particles artificially could reduce the effects of climate change
overall, according to UCL and Harvard researchers.
Stratospheric aerosol geoengineering is the idea that
adding a layer of aerosol particles to the upper atmosphere can reduce climate
changes caused by greenhouse gases such as carbon dioxide.
Previous research shows that solar geoengineering
could be achieved using commercially available aircraft technologies to deliver
the particles at a cost of a few billion dollars per year and would reduce
global average temperatures. However, the question remains whether this
approach could reduce important climate hazards at a regional level. That is,
could it reduce region-by-region changes in water availability or extreme
temperatures?
Results from a new study by UCL and Harvard
researchers suggest that even a crude method like injecting sulphur dioxide in
the stratosphere could reduce many important climate hazards without making any
region obviously worse off.
The findings, published today in Environmental Research Letters, used results from a sophisticated
simulation of stratospheric aerosol geoengineering to evaluate whether the
approach could offset or worsen the effects of climate change around the world.
How these effects differed under different temperature scenarios was also
tested.
The team found that halving warming by adding
aerosols to the stratosphere could moderate important climate hazards in almost
all regions. They saw an exacerbation of the effects of climate change in only
a very small fraction of land areas.
Lead author, Professor Peter Irvine (UCL Earth
Sciences), said: “Most studies focus on a scenario where solar geoengineering
offsets all future warming. While this reduces overall climate change substantially,
we show that in these simulations, it goes too far in some respects leading to
about 9% of the land area experiencing greater climate change, i.e. seeing the
effects of climate change exacerbated.
“However, if instead only half the warming is offset,
then we find that stratospheric aerosol geoengineering could still reduce
climate change overall but would only exacerbate change over 1.3% of the land
area.”
The team emphasise that solar geoengineering only
treats the symptoms of climate change and not the underlying cause, which is
the build-up of CO2 and other greenhouse gases in the atmosphere. It
should therefore be considered as a complementary approach to emissions cuts as
a way to address climate change.
The study is a follow-up to a paper published last
year in Nature Climate Change* showed
similar results when solar geoengineering was approximated by simply turning
down the sun. That prior study begged the question: would the results hold up
with a more realistic simulation using injection of sulphur dioxide, the
simplest known method of solar geoengineering.
“Our results suggest that when used at the right dose
and alongside reductions in greenhouse gas emissions, stratospheric aerosol
geoengineering could be useful for managing the impacts of climate change.
However, there are still many uncertainties about the potential effects of
stratospheric aerosol geoengineering and more research is needed to know if
this idea is truly viable,” added Dr Irvine.
The team used data from the Geoengineering Large
Ensemble Study, which used a sophisticated climate-chemistry model to simulate
the climate response to a hypothetical deployment of stratospheric aerosol
geoengineering. In this model study, sulphur dioxide was released at different
latitudes in the Tropics to produce a layer of aerosols tuned to keep
temperatures steady under an extreme global warming scenario.
The researchers focused on changes in mean and
extreme temperature, changes in water availability and changes in extreme
precipitation, i.e. climate variables that determine key climate risks.
Previous work suggested that stratospheric aerosol
geoengineering could lead to a substantial weakening of monsoons and an
intensification of drought. However, the authors found that in those regions
where halving warming with stratospheric aerosol geoengineering exacerbated
change, it increased water availability rather than reduced it. This suggests
that concerns that stratospheric aerosol geoengineering could lead to
aridification and drought could be misplaced.
Co-author, Professor David Keith (Harvard’s
Engineering and Applied Sciences and Kennedy school), said: “Early research
with climate models consistently shows that spatially uniform solar radiation
modification could significantly reduce climate risks when combined with
emissions cuts. But, should we trust the models? Uncertainties are deep and no
single result is trustworthy, but this paper is a step towards more realistic
modelling from injection to regional impacts.”
The team are now researching the projected effects of
stratospheric aerosol geoengineering on the water cycle in more depth to try to
understand the potential benefits and risks to society and ecosystems.
原始論文：
1. Peter J Irvine, David W Keith. Halving warming
with stratospheric aerosol geoengineering moderates policy-relevant climate
hazards. Environmental Research Letters, 2020; 15 (4): 044011
DOI: 10.1088/1748-9326/ab76de
2.
Peter Irvine, Kerry Emanuel, Jie He,
Larry W. Horowitz, Gabriel Vecchi, David Keith. Halving warming with
idealized solar geoengineering moderates key climate hazards. Nature
Climate Change, 2019; 9 (4): 295 DOI: 10.1038/s41558-019-0398-8
引用自：University College London. "The right dose
of geoengineering could reduce climate change risks."
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