The Weather Amplifier


By Michael E. Mann M. E. マン
English 日本語 日本語
Consider the following summer extremes: In 2003 Europe’s worst heat wave in history killed more than 30,000 citizens. In 2010 wildfires in Russia and floods in Pakistan caused unprecedented damage and death. The 2011 U.S. heat wave and drought caused ranchers in Oklahoma to lose a quarter of their cattle. The 2016 Alberta wildfires constituted the costliest disaster in Canadian history. And the summer of 2018 that the U.S. experienced was notorious: temperatures flared above 100 degrees Fahrenheit for days on end across the desert Southwest, heavy rains and floods inundated the mid-Atlantic states, and California had a shocking wildfire season. Extreme heat waves, floods and wildfires raged across Europe and Asia, too.  近年の異常な夏の数々を思い起こそう。2003年,欧州史上最悪の熱波によって3万人以上が死んだ。2010年,ロシアの山火事とパキスタンの洪水で前代未聞の損害と死者が出た。2011年に米国を襲った熱波と干ばつでオクラホマの牧場は牛のざっと1/4を失った。2016年,カナダ・アルバータ州の山火事はカナダ史上最悪の損害をもたらした。そして2018年夏の悪夢は記憶に新しい。米国南西部の砂漠では気温が100°F(約38℃)を超える日が何日も続き,中部大西洋岸の各州は豪雨と水害に,カリフォルニアは衝撃的な山火事に見舞われた。異常な熱波と水害,山火事は欧州とアジアにも襲いかかった。
Is it coincidence that the most devastating summer weather has occurred in recent decades? My colleagues and I do not think so. All these events had a striking feature in common: a very unusual pattern in the jet stream. The jet stream is a narrow band of strong wind that blows west to east around the Northern Hemisphere, generally along the U.S.-Canada border, continuing across the Atlantic Ocean, Europe and Asia. The band is sometimes fairly straight, but it can take on big bends—shaped like an S lying on its side. It typically curls northward from the Pacific Ocean into western Canada, then turns southward across the U.S. Midwest, then back up toward Nova Scotia. This shape usually proceeds west to east across the U.S. in a few days, bringing warm air north or cool air south and creating areas of rain or snow, especially near the bends. The jet stream controls our daily weather.  これら最悪レベルの夏の異常気象が近年に発生しているのは偶然だろうか? 私や共同研究者はそうではないと考えている。これらすべての事象には著しい共通点がある。ジェット気流が極めて異常なパターンを取っていたことだ。
During the extreme events I noted, the jet stream acted strangely. The bends went exceptionally far north and south, and they stalled—they did not progress eastward. The larger these bends, the more punishing the weather gets near the northern peak and southern trough. And when they stall—as they did over the U.S. in the summer of 2018—those regions can receive heavy rain day after day or get baked by the sun day after day. Record floods, droughts, heat waves and wildfires occur.  先述の極端気象の期間中,ジェット気流は奇妙な振る舞いを見せていた。南北に異常に大きく蛇行し,そのパターンのまま停滞して東に進まなかったのだ。蛇行が大きいほど,北にできる蛇行の山と南にできる谷の近くでの天気はひどくなる。そして2018年夏の米国でそうだったようにジェット気流のパターンが停滞すると,それら蛇行の山谷に近い地域で豪雨や熱射が何日も続き,記録的な洪水や干ばつ,熱波,山火事が発生する。
My collaborators and I have recently shown that these highly curved, stalled wave patterns have become more common because of global warming, boosting extreme weather.  最近,私は共同研究者とともに,ジェット気流が大きく蛇行して停滞するこうしたパターンが地球温暖化の影響で以前よりも頻繁に生じて極端気象を悪化させていることを示した。