900-1300 AD，全球曾经经历过一次变暖，当时英国葡萄大丰收，其酿酒质量直逼法国葡萄酒，以至于法国不得不采取贸易手段抵制。当时的最高气温，比现在还要高。
紧接着1350-1850AD是历史上的一个小冰河期，上面历史频道的纪录片讲得很清楚。在欧洲，这个小冰河期直接造成了维京人的衰落。在北美，纽约港曾经冰冻5周，人们可以在冰面上，直接从Manhattan走到Staten Isand；新英格兰地区曾在6、7月份降雪2英尺——难道米锅也有“窦娥冤”？：）
而5000年人类文明以来，最暖的时期是3000多年前，接近我国商王武丁，以及埃及法老拉美西斯二世的时代。那时人类尚处于青铜时代。
由此可见，地球的气候变化是个非常复杂的问题，现在还没有明确的认识，也无法准确预测下一个小冰河期何时到来。我们人类，应该本着“知之为知之，不知为不知"的态度，积极探索，小心求证，而不要妄下结论。
相关阅读：http://www.longrangeweather.com/global_temperatures.htm
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Dr. Wheeler also discovered that approximately every 102 years, a much warmer and drier climatic cycle affects our planet. The last such "warm and dry" peak occurred in 1936, at the end of the infamous "Dust Bowl" period. During that time, extreme heat and dryness, combined with a multitude of problems during the “Great Depression,” made living conditions practically intolerable.
Assuming there is a brief cool down around 2020, the next “warm and dry” climatic phase is scheduled to arrive in the early 2030s, probably peaking around 2038. It is expected to produce even hotter and drier weather patterns than we saw during the late 1990s and early 2000s.
But, we should remember, that the Earth’s coldest periods have usually followed excessive warmth. Such was the case when our planet moved from the Medieval Warm Period between 900 and 1300 A.D. to the sudden “Little Ice Age,” which peaked in the 17th Century. Since 2,500 B.C., there have been at least 78 major climate changes worldwide, including two major changes in just the past 40 years.
By the end of this 21st Century, a cool down may occur that could ultimately lead to expanding glaciers worldwide, even in the mid-latitudes. Based on long-term climatic data, these major ice ages have recurred about every 11,500 years. The last extensive ice age was approximately 11,500 years ago, so we may be due again sometime soon. But, only time will tell.
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