Intervals（间隔，间歇） of regional warmth and cold in the past are linked to the El Niño（厄尔尼诺） phenomenon and the so-called "North Atlantic Oscillation（北大西洋振荡）" in the Northern hemisphere's jet stream（急流，喷气流）, according to a team of climate scientists. These linkages may be important in assessing the regional effects of future climate change. "Studying the past can potentially inform our understanding of what the future may hold," said Michael Mann, Professor of meteorology（气象学）, Penn State.

Mann stresses that an understanding of how past natural changes have influenced phenomena such as El Niño, can perhaps help to resolve current disparities（差距，不一致） between state-of the-art climate models regarding how human-caused climate change may impact this key climate pattern.

Mann and his team used a network of diverse climate proxies such as tree ring samples, ice cores, coral and sediments（沉淀物） to reconstruct spatial patterns（立体图，空间图案） of ocean and land surface temperature over the past 1500 years. They found that the patterns of temperature change show dynamic（动态的，有力的） connections to natural phenomena such as El Niño. They report their findings in today's issue (Nov. 27) of Science.

Mann and his colleagues reproduced the relatively cool interval from the 1400s to the 1800s known as the "Little Ice Age" and the relatively mild conditions of the 900s to 1300s sometimes termed the "Medieval Warm Period."

"However, these terms can be misleading," said Mann. "Though the medieval（中世纪的） period appears modestly（谨慎地，适当地） warmer globally in comparison with the later centuries of the Little Ice Age, some key regions were in fact colder. For this reason, we prefer to use 'Medieval Climate Anomaly' to underscore（强调，下划线） that, while there were significant climate anomalies（反常，异常） at the time, they were highly variable from region to region."

The researchers found that 1,000 years ago, regions such as southern Greenland may have been as warm as today. However, a very large area covering much of the tropical Pacific was unusually cold at the same time, suggesting the cold La Niña（拉尼娜） phase of the El Niño phenomenon.

This regional cooling offset relative warmth in other locations, helping to explain previous observations that the globe and Northern hemisphere on average were not as warm as they are today.