但是很少有人可以准确无误地计算出环流中的这些变化，因而很难揭示出臭氧层恢复的趋势。
But no one had been able to take those circulation changes into account accurately enough to reveal the underlying ozone recovery.
科学家们过去认为南极洲上大规模臭氧层年际变化会掩盖住微小的趋势。
Scientists thought large, wholly natural, year-to-year variations in Antarctic ozone would obscure any small upward trend.
从臭氧层的实际变化中除去正常情况下臭氧层的变化（对照），这个研究团队发现“自20世纪90年代以来臭氧层空洞缩小趋势”有一个系统性的机制。
Subtracting their estimate of the natural changes in ozone from actual changes, the group finds “a clear upward trend since the late 1990s” in the hole's ozone that represents “a systematic rebound.”
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