The drastic melting of Arctic sea ice has finally ended for the year, scientists announced  Wednesday, but not before demolishing the previous record — and setting off new warnings about the rapid pace of change in the region.

The apparent low point for 2012 was reached Sunday, according to the National Snow and Ice Data Center, which said that sea ice that day covered about 1.32 million square miles, or 24 percent, of the surface of the Arctic Ocean. The previous low, set in 2007, was 29 percent.
 

When satellite tracking began in the late 1970s, sea ice at its lowest point in the summer typically covered about half the Arctic Ocean, but it has been declining in fits and starts over the decades.

“The Arctic is the earth’s air-conditioner,” said Walt Meier, a research scientist at the snow and ice center, an agency sponsored by the government. “We’re losing that. It’s not just that polar bears might go extinct, or that native communities might have to adapt, which we’re already seeing — there are larger climate effects.”

His agency waited a few days before announcing the low to be sure sea ice had started to refreeze, as it usually does at this time of year, when winter closes in rapidly in the high Arctic. A shell of ice will cover much of the Arctic Ocean in coming months, but it is likely to be thin and prone to melting when summer returns.

Scientists consider the rapid warming of the region to be a consequence of the human release of greenhouse gases, and they see the melting as an early warning of big changes to come in the rest of the world.

Some of them also think the collapse of Arctic sea ice has already started to alter atmospheric patterns in the Northern Hemisphere, contributing to greater extremes of weather in the United States and other countries, but that case is not considered proven.

The sea ice is declining much faster than had been predicted in the last big United Nations report on the state of the climate, published in 2007. The most sophisticated computer analyses for that report suggested that the ice would not disappear before the middle of this century, if then.

Now, some scientists think the Arctic Ocean could be largely free of summer ice as soon as 2020. But governments have not responded to the change with any greater urgency about limiting greenhouse emissions. To the contrary, their main response has been to plan for exploitation of newly accessible minerals in the Arctic, including drilling for more oil.

Scientists said Wednesday that the Arctic has become a prime example of the built-in conservatism of their climate forecasts. As dire as their warnings about the long-term consequences of heat-trapping emissions have been, many of them fear they may still be underestimating the speed and severity of the impending changes.

In a panel discussion on Wednesday in New York sponsored by Greenpeace, the environmental group, James E. Hansen, a prominent NASA climate scientist, said the Arctic melting should serve as a warning to the public of the risks that society is running by failing to limit emissions.

“The scientific community realizes that we have a planetary emergency,” Dr. Hansen said. “It’s hard for the public to recognize this because they stick their head out the window and don’t see that much going on.”

A prime concern is the potential for a large rise in the level of the world’s oceans. The decline of Arctic sea ice does not contribute directly to that problem, since the ice is already floating and therefore displacing its weight in water.

But the disappearance of summer ice cover replaces a white, reflective surface with a much darker ocean surface, allowing the region to trap more of the sun’s heat, which in turn melts more ice. The extra heat in the ocean appears to be contributing to an accelerating melt of the nearby Greenland ice sheet, which does contribute to the rise in sea level.

At one point this summer, surface melt was occurring across 97 percent of the Greenland ice sheet, a development not seen before in the era of satellite measurements, although geological research suggests that it has happened in the past.

The sea is now rising at a rate of about a foot per century, but scientists like Dr. Hansen expect this rate to increase as the planet warms, putting coastal settlements at risk.

A scientist at the snow and ice center, Julienne C. Stroeve, took a ride on a Greenpeace ship recently to inspect the Arctic Ocean for herself. Interviewed this week after pulling into port at the island of Spitsbergen, she said one of her goals had been to debark on ice floes and measure them, but that it had been difficult to find any large enough to support her weight.

Ice floes were numerous in spots, she said, but “when we got further into the ice pack, there were just large expanses of open water.”

本周三，科学家宣布，本年度的北冰洋海冰大融化总算是停止了，但还是打破了纪录，也对本地区的急剧变化发出了新警告。

据美国国家冰雪数据中心(National Snow and Ice Data Center)统计，北冰洋的海冰覆盖率似乎于上周日达到2012年最低值。该中心称，那天北冰洋海冰面积为132万平方英里，覆盖了北冰洋洋面的24%。上一次最低点出现在2007年，当时的覆盖率为29%。
 

20世纪70年代末，卫星开始被用于纪录北冰洋海冰状况 。那时夏日海冰覆盖率最低时，也通常是50%左右 。但在过去十年，海冰覆盖率一直时有下降。

国家冰雪数据中心是一家由政府赞助的机构。该中心的研究员瓦尔特·迈耶(Walt Meier)说，“北极是地球的空调。我们正在失去这个空调。不仅仅是北极熊可能灭绝，或者是当地人需要适应新环境，这些我们都已经看到了——还有更大的气候影响。”

在宣布最低覆盖率之前，该中心等了几天，以确保海水开始重新结冰。每年此时，冬天迅速包围北极高纬度地区，海水重新结冰是常态。接下来的几个月里，一层冰壳会覆盖北冰洋大部分洋面。但这层冰壳通常很薄，夏季再来时，容易融化。

科学家认为，本地区迅速变暖，是因为人类排放温室气体。他们认为海冰融化是未来全球巨变的早期征兆。

一些科学家也认为，北极海冰融化已经开始改变北半球的气候模式，导致美国等国出现更加极端的气候现象。但这个观点并未得到证实。

联合国最新的一个主要气候报告发布于2007年。而海冰目前的减少速度比该报告预测的速度快得多。 该报告指出，在进行了最精密的电脑分析后，结果显示，即使北极冰川最终消失，这种情况也不会在本世纪中叶之前发生。

现在，一些科学家认为，可能到2020年，北冰洋夏天就没有冰块存在了。但是各国政府并未在遏制温室气体排放方面表现出更大的急迫性，以对这一变化做出反应。相反，他们的主要反应是计划开采北冰洋新近可采的资源，包括钻取更多石油。

科学家于周三表示，北极已经成了他们预测气候时固有的保守主义的最好例证。尽管很多人对温室气体排放的长期后果发出了严厉的警告，但他们还是担心，自己可能仍然低估了即将到来的变化的速度和严重性。

周三，在由环保组织绿色和平(Greenpeace)发起的讨论会上，美国国家航空航天局(NASA)的知名气象学家詹姆斯·E·汉森(James E. Hansen)表示，北极融化应该成为对公众的警告，让公众注意到，社会因未能限制排放而面临的风险。

“科学界意识到，我们面临着一个全球性的紧急情况，”汉森博士说。“要让公众意识到这一点很难，因为他们把头伸出窗外，却看不到有太多变化。”

在这方面，主要的担忧在于全球海平面可能大幅上升。北极海冰的减少不会直接导致海平面上升，因为它们已经是浮冰了，在水中占据了吻合自身的重量的空间。

然而，夏季冰层消失，白色的反光地表被颜色深得多的海洋表面所取代，该地区由此得以吸收更多来自太阳的热量，这些热量反过来会融化更多的冰层。北冰洋多余的热量似乎正在导致附近的格陵兰冰盖加速融化。而这确实会导致海平面上升。

今年夏天，格陵兰冰盖97%的表面都曾一度融化。尽管地质研究表明这样的现象在过去发生过，但自使用卫星测量以来，却是首次出现这样的情况。

尽管海平面正在以每年约30厘米的速度上升，但包括汉森博士在内的科学家预计，随着全球的变暖，海平面上升的速度会加快，使沿海的人类聚集区陷于危险之中。

最近，冰雪数据中心的科学家朱莉安娜·C·斯特勒夫(Julienne C. Stroeve)搭乘绿色和平组织的船前往北冰洋，进行实地考察。本周，她所乘的船驶入了斯匹次卑尔根岛的海港。在接受采访时，她表示，自己的目标之一是登上浮冰并对它们进行测量，但却发现，要找到大到足以能支撑她的重量的浮冰却很困难。

她说，偶尔会有很多浮冰，但“当我们进一步向浮冰划去时，只发现大片无冰的水域。”