根據美國賓州地球系統科學中心與其他三所大學專家學者研究發現，近10年來北半球地表的暖化程度已打破過去1300年內任何一段時期的記錄。甚至，若氣候學家們引用頗具爭議的樹木年輪推論數據，則會呈現出1998年至2008年之間的暖化活動，要較過去1700年以來還要劇烈。
專長氣象學與地球科學且兼任地球系統科學中心主任的助理教授麥可（Michael Mann）表示，「過去曾有先進認為年輪數據不適用於這類的氣候研究。」「如今我們可以透過所謂的『替代數據』（proxies），在持續充分取得北半球溫度變化記錄資料的條件下，取代過去具爭議性的年輪數據」。
研究人員所使用的替代數據來源包括海洋與湖泊沈積物的樣本型心、冰核、珊瑚核與樹木年輪等。在不列入年輪數據下的研究結果顯示，過去10年來所呈現的暖化記錄，並不如90年代相關文獻中所指，僅為1000年內的不尋常現象，而是至少須再回溯300年至西元前700年，共計有1300年之久。而若研究人員加入年輪數據，則結果將再追溯至西元前300年。
樹木年輪數據在這些研究中所引起的廣泛爭議之處在於所謂的「時間片段長度瓶頸」（segment length curse）。由於樹木年輪每年增加，然而樹齡愈高者，其新生年輪則愈密集。當年輪研究人員欲藉由拼湊兩顆不同樹齡的樹木年輪來推斷時間時，他們必須考慮如何將年長樹木的後期年輪和年輕樹木的年輪接合在一起。在這個過程之中，常常會遺漏一些關於長期趨勢的資料，此情況即所謂的「瓶頸」。
即使不採用年輪數據，最近的研究仍指證近期不尋常的地表暖化現象是具體事實，且大多數的科學家皆相信，這是人類活動對氣候造成影響的結果。
研究人員於9月2日刊載於「國家科學研究學院報」（PNAS）線上版表示，「由於南半球有信度的替代數據稀少且不完整，增加其不確定性，使得南半球和全球性研究結果較不確鑿」。
麥可教授指出，儘管過去10年暖化情形加速惡化，他仍對未來抱有希望。他相信，全球社群在全球暖化的肇端後面會密切聯繫與整合，將致力減緩氣候變遷的種種效應。
Surface temperatures in the Northern Hemisphere were warmer over the last 10 years than any time during the last 1300 years, according to researchers at Penn State's Earth System Science Center and three other U.S. universities. And, if the climate scientists include the somewhat controversial data derived from tree-ring records, they say that warming is greater from 1998 to the present than at any time for at least 1700 years.
"Some have argued that tree-ring data is unacceptable for this type of study," says Michael Mann, associate professor of meteorology and geosciences and director of the Earth System Science Center. "Now we can eliminate tree rings and still have enough data from other so-called proxies to derive a long-term Northern Hemisphere temperature record."
The proxies used by the researchers included information from marine and lake sediment cores, ice cores, coral cores and tree rings. Results of this study without tree-ring data show that for the Northern Hemisphere, the last 10 years are likely unusually warm for not just the past 1,000 as reported in the 1990s paper and others, but for at least another 300 years going back to about A.D. 700 without using tree-ring data. The same conclusion holds back to A.D. 300 if the researchers include tree-ring data.
One of the reasons that including tree-ring data in these studies raises possible concerns is something called the "segment length curse." This "curse" occurs because trees put on rings every year, but older trees put on narrower rings. When tree ring researchers piece together tree-ring series from two trees, they must account for this factor in how they combine the later rings on one tree with the earlier rings on a younger tree. In the process, some information regarding long-term trends can be lost.
The new study shows that even without including tree ring data, it is clear that the unusual nature of recent warmth, which most scientists believe to be a result of human impacts on climate, is a reality.
In today's online edition of the "Proceedings of the National Academy of Sciences," the researchers note, "Conclusions are less definitive for the Southern Hemisphere and globe, which we attribute to larger uncertainties arising from the sparser available proxy data in the Southern Hemisphere."
Dr. Mann says he is hopeful about the future despite the past 10 years of rapid warming. He believes that the world will rally behind the cause of global warming and start to mitigate the effects of climate change.
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