研究人員透過宏偉的冰島火山爆發來解開氣膠在氣候變遷中扮演的腳色

原文網址：www.sciencedaily.com/releases/2017/06/170621133451.htm

研究人員透過宏偉的冰島火山爆發來解開氣膠在氣候變遷中扮演的腳色

研究顯示雲系可以「大量緩衝」氣膠在大氣中造成的變化

發生在冰島長達6個月的火山噴發形成了整片熔岩原野，它提供科學家關鍵線索來解開氣膠透過跟雲層的互動，在氣候變遷中扮演了何種腳色。

由艾克斯特大學主導的氣候學家國際團隊，詳盡研究了2014-15年發生在冰島Holuhraun的火山噴發對周遭地區成雲作用造成的影響。

2014-15年Holuhraun的裂隙噴發是冰島自1783-84年長達8個月的Laki噴發之後最大的火山噴發。研究人員發現此次噴發排放二氧化硫的速率比歐洲28個國家合起來的速率還快，並在北大西洋上空形成了由硫酸氣膠粒子組成的巨大氣柱。

如同預料的一樣，氣膠分子減小了雲滴的大小；但跟預期相反的是，它們並未增加雲層含有的水量。

研究人員相信這些驚人結果可以讓我們更精確地描繪出人類工業活動產生的氣膠分子，會對氣候變遷產生什麼樣的影響，而大幅降低未來氣候預估中的不確定性。

研究主要作者，艾克斯特大學數學系的Florent Malavelle博士表示：「大型火山噴發提供了絕佳的天然試驗，讓我們可以計算氣膠和雲層的交互作用。」

「我們知道氣膠有能力對氣候造成巨大影響，尤其是透過跟雲層的交互作用。然而，此效應的規模大小至今仍然無法確定。這項研究不僅讓我們有望能消除此不確定性，更重要的是提供機會給我們來否決一些現存的氣候模型，意謂我們預測未來氣候變遷的精確度可以大幅提升至前所未有的程度。」

在工業產生的大氣氣膠分子中，長久以來都認為硫酸氣膠是其中最重要的一員。但是自然界中也存在著其他硫酸氣膠的來源，包括火山噴發釋出的二氧化硫所形成的硫酸氣膠。

科學家認為2014-15的Holuhraun火山噴發在其噴發階段每一天都釋放出40000至100000噸的二氧化硫。結合最尖端的氣候系統模型，與NASA和布魯塞爾自由大學提供由衛星取回的詳盡數據，研究團隊得以探討火山噴發形成的雲量的繁複性質。

他們發現氣膠使產生的水滴尺寸縮小了，繼而使形成的雲朵更加明亮――這會導致照射進地球的陽光中有更多比例被反射回太空，最終對氣候造成冷化作用。

但重要的是，在雲的許多其他性質上，像是雲含有的液態水量，卻未能辨識出氣膠造成的影響。

團隊認為研究顯示雲系可以「大量緩衝」氣膠在大氣中造成的變化。

共同作者，同樣來自艾克斯特大學的教授Jim Haywood補充：「爆裂式和溢流式火山噴發有相當大的不同。1991年皮納土波火山發生相當劇烈的爆裂式火山噴發，將氣膠注入到25公里高以上的平流層中。此代表性事件已經被納入我們的模型當中，以增進爆裂性火山噴發對氣候造成影響的模擬結果。」

「火山現在又提供了新線索來解答這則氣候問題：若氣膠釋放到跟人為排放所能達到的類似高度，會對氣候造成什麼樣的影響？毫無疑問地，Holuhraun發生的溢流式火山噴發會成為研究此方面時的首要對象。」

Role aerosols play in climate change unlocked by spectacular Icelandic volcanic eruption

Cloud systems 'well buffered' against aerosol changes in the atmosphere, research shows

A spectacular six-month Icelandic lava field eruption could provide the crucial key for scientists to unlock the role aerosols play in climate change, through their interactions with clouds.

An international team of climate scientists, led by the University of Exeter, have meticulously studied the effects that the 2014-15 eruption at Holuhraun, in Iceland had on cloud formations in the surrounding region.

They found that the 2014-15 Holuhraun fissure eruption, the largest since Laki which erupted for eight months in 1783-4, emitted sulphur dioxide at a higher rate than all 28 European countries added together causing a massive plume of sulphate aerosol particles over the North Atlantic.

As would be expected, these aerosols reduced the size of cloud droplets, but contrary to expectations did not increase the amount of water in the clouds.

The researchers believe these startling results could significantly reduce uncertainties in future climate projections by outlining the impact of sulphate aerosols formed from human industrial emissions on climate change.

The pioneering study is published in leading scientific journal, Nature.

這項前瞻性的研究刊登於頂尖科學期刊《自然》(Nature)。

Dr Florent Malavelle, lead author of the study and from the Mathematics department at the University of Exeter said: "The huge volcanic eruption provided the perfect natural experiment in which to calculate the interaction between aerosols and clouds.

"We know that aerosols potentially have a large effect on climate, and particularly through their interactions with clouds. However the magnitude of this effect has been uncertain. This study not only gives us the prospect of ending this uncertainty but, more crucially, offers us the chance to reject a number of existing climate models, which means we can predict future climate change far more accurately than ever before."

Aerosols play a pivotal role in determining the properties of clouds as they act as the nuclei on which water vapour in the atmosphere condenses to form clouds.

氣膠做為大氣中的水蒸氣凝結成雲時所需的凝結核，它在決定雲的性質時具有關鍵地位。

Sulphate aerosol has long been recognised as the most significant atmospheric aerosol from industrial sources, but other natural sources of sulphate aerosol also exist, including that formed from sulphur dioxide release as a result of volcanic eruptions.

The 2014-15 Holuhraun eruption is thought to have emitted between 40,000-100,000 tons of sulphur dioxide every single day during its eruptive phase. Using state-of-the-art climate system models, combined with detailed satellite retrievals supplied by NASA and the Université libre de Bruxelles, the research team were able to study the complex nature of the cloud cover formed as a result of the eruption.

They found that the size of the water droplets produce was reduced, which in turn led to cloud brightening -- which results in an increased fraction of incoming sunlight being reflected back into space and, ultimately, providing a cooling effect on the climate.

Crucially however, these aerosols had no discernible effect on many other cloud properties, including the amount of liquid water that the clouds hold and the cloud amount. The team believe the research shows that cloud systems are "well buffered" against aerosol changes in the atmosphere.

Professor Jim Haywood, co-author of the paper and also from the University of Exeter added: "Explosive and effusive volcanic eruptions are very different. The massive explosive eruption of Pinatubo in 1991, which injected aerosol to altitudes of 25km+ into the stratosphere, has been the go-to event for improving our model simulations of the impact of explosive volcanic eruptions on climate.

"Now volcanoes have provided a new clue in the climate problem: how aerosols emitted at altitudes similar to those from human emissions impact the climate. Without a doubt, the effusive eruption at Holuhraun will become the go-to study in this regard."

原始論文：Florent F. Malavelle, Jim M. Haywood, Andy Jones, Andrew Gettelman, Lieven Clarisse, Sophie Bauduin, Richard P. Allan, Inger Helene H. Karset, Jón Egill Kristjánsson, Lazaros Oreopoulos, Nayeong Cho, Dongmin Lee, Nicolas Bellouin, Olivier Boucher, Daniel P. Grosvenor, Ken S. Carslaw, Sandip Dhomse, Graham W. Mann, Anja Schmidt, Hugh Coe, Margaret E. Hartley, Mohit Dalvi, Adrian A. Hill, Ben T. Johnson, Colin E. Johnson, Jeff R. Knight, Fiona M. O’Connor, Daniel G. Partridge, Philip Stier, Gunnar Myhre, Steven Platnick, Graeme L. Stephens, Hanii Takahashi, Thorvaldur Thordarson. Strong constraints on aerosol–cloud interactions from volcanic eruptions. Nature, 2017; 546 (7659): 485 DOI: 10.1038/nature22974

引用自：University of Exeter. "Role aerosols play in climate change unlocked by spectacular Icelandic volcanic eruption: Cloud systems 'well buffered' against aerosol changes in the atmosphere, research shows." ScienceDaily. ScienceDaily, 21 June 2017.