字幕表 動画を再生する 英語字幕をプリント NASA scientists have performed the first multi-year study using satellite data to measure the 3-D distribution of dust that travels from the Sahara Desert to the Amazon rainforest. The Sahara is the world's largest desert. At more than 3 million square miles, it's almost the size of the continental United States. Each year, Saharan dust is lifted from the ground, and transported by winds on a 3000-mile journey across the Atlantic Ocean. A portion of the dust collects in the Amazon basin —the largest rainforest on the planet. Although dust particles are small, no larger than around a tenth of the width of a human hair, they form massive plumes that can be seen from space. NASA's CALIPSO satellite was launched in 2006 to study the vertical structure of clouds and particles in Earth's atmosphere. CALIPSO observations from 2007 through 2013 show that on average, 182 million tons of dust leaves Africa each year. Of this amount, about 27 million tons is deposited in the Amazon basin. Sahara dust contains phosphorus, which is an important nutrient for plants. In the tropic region, the phosphorus is quite limited. So it's important to estimate how much dust from the desert is transported to the Amazon. The study shows Saharan dust adds phosphorous to the soils that help compensate for losses due to surface runoff and floods. However, the amount of dust transported to the Amazon changes from year to year. According to the study, this variation is closely associated with changes in rainfall in the Sahel, a belt of semi-arid land just south of the Sahara. When the Sahel was dry, the dust transport to the Amazon in the next year would increase. When it was wet, dust transport would decrease. Using satellites to get a clear picture of dust is important for understanding, and eventually using computers to model where that dust will go, now and in future climate scenarios.