譯/李京倫
空汙讓颶風變多或變少 端看住處
Global warming can affect hurricanes, in part because a warmer ocean provides more energy to fuel them. But it is not the only factor in play: A study confirms that, for the frequency of hurricanes, the effects of particulate air pollution are even greater.
全球暖化能影響颶風，部分是因為更溫暖的海水提供更多能量促進颶風形成。不過這不是唯一起作用的因素：一份研究證實，就颶風頻率而言，微粒空氣汙染的效應甚至更大。
Over the past four decades, the new research shows, the decline in pollution in the form of tiny aerosol particles from transportation, energy production and industry in North America and Europe was responsible for the increased numbers of hurricanes and other tropical cyclones in the North Atlantic.
這份新研究顯示，過去四十年，北美和歐洲源自運輸、發電與工業並以迷你氣膠粒子形式呈現的汙染減少，是北大西洋颶風和其他熱帶氣旋數量增加的原因。
Over the same period, increasing pollution from the growing economies of India and China had the opposite effect, reducing hurricane activity in the western North Pacific, the study found.
研究顯示，同一期間，崛起經濟體印度和中國大陸日漸嚴重的汙染，起了相反作用，使北太平洋西部颶風活動減少。
A growing body of research has shown links between tropical cyclones and global warming.
愈來愈多研究顯示，熱帶氣旋與全球暖化有關。
The new study looked at the numbers, not the strength, of these kinds of storms. Its author, Hiroyuki Murakami, said it shows that reducing or increasing anthropogenic aerosols "is the most important component" affecting frequency.
這份新研究關注這幾種風暴的數量而非強度。作者村上裕之說，研究顯示，人類活動引起的氣膠減少或增加，是影響颶風頻率「最重要的部分」。
In recent decades, aerosol pollution has declined, perhaps by as much as 50%, in North America and Europe as a result of laws and regulations that reduce emissions from sources like vehicles and power plants. Hurricane seasons in the North Atlantic over roughly the same period have been more active, with a greater number of storms, than in previous decades.
最近幾十年，北美和歐洲氣膠汙染減少，可能少了一半之多，原因是法律及規範減少了車輛與電廠等來源的排放。大約同一時期的北大西洋颶風季比先前數十年更活躍，生成更多風暴。
In the North Atlantic, Murakami found, the decline in aerosols led to warming that had two effects on tropical cyclones. First, less pollution resulted in more ocean warming, which meant there was more energy for storms to form.
村上發現，在北大西洋，氣膠減少引起暖化，對熱帶氣旋產生兩個效應。其一，汙染變少致海水更暖，意味有更多能量可供風暴形成。
The pollution decline led to warming of the land as well, and the combined warming affected atmospheric circulation, weakening winds in the upper atmosphere. That in turn led to less wind shear, the changes in wind speed and direction that can affect how cyclonic storms develop. Less wind shear meant that storms formed more readily.
汙染減少也使陸地變暖，而兩種暖化結合影響了大氣環流，削弱高層大氣的風，因此使風切變弱。風切就是能影響氣旋風暴如何發展的風速和風向。風切減弱意味風暴形成更迅速。
Murakami's simulations showed a different mechanism at work in the Pacific. There, he found, increasing aerosol pollution, largely from China and India, led to cooling of the land surface. This reduced the temperature difference between the land and ocean, weakening the monsoonal winds that develop there. That, in turn, led to fewer tropical cyclones, including typhoons, the Pacific equivalent of hurricanes.
村上的模擬顯示，在太平洋有一種不同的機制在運作。他發現，主要來自大陸和印度日漸增加的氣膠汙染，使地表降溫。這減少了陸地與海洋的溫差，削弱了在當地發展的季風，因此導致熱帶氣旋變少，包括在太平洋地區等同於颶風的颱風。
