The sun's activity is winding down, triggering fevered debate among scientists about how low it will go, and what it means for Earth’s climate. Nasa recorded no sunspots on 266 days in 2008 - a level of inactivity not seen since 1913 - and 2009 looks set to be even quieter. Solar wind pressure is at a 50-year low and our local star is ever so slightly dimmer than it was 10 years ago.

Sunspots are the most visible sign of an active sun - islands of magnetism on the sun's surface where convection is inhibited, making the gas cooler and darker when seen from Earth - and the fact that they're vanishing means we're heading into a period of solar lethargy.

Where will it all end? Solar activity varies over an 11-year cycle, but it experiences longer-term variations, highs and lows that can last around a century.

"A new 11-year cycle started a year or two ago, and so far it’s been extremely feeble," says Nigel Weiss of the University of Cambridge. With Jose Abreu of the Swiss Federal Institute of Aquatic Science and Technology in Dübendorf and others, Weiss recently predicted that the long-term solar high we've been enjoying since before the second world war is over, and the decline now under way will reach its lowest point around 2020. Their prediction is based on levels of rare isotopes that accumulate in the Earth’s crust when weak solar winds allow cosmic rays to penetrate the Earth's atmosphere.

There's even a chance, says Weiss, that we might be heading for a low as deep as the Maunder minimum of the 17th century. Either side of that trough, Europe shivered through the Little Ice Age, when frost fairs were held on the Thames and whole Swiss villages disappeared under glaciers. So should we expect another freeze?

Those who claim the rise in temperatures we've seen over the last century are predominantly the result of intense solar activity might argue that we should, but they’re in the minority. Most scientists believe humans are the main culprit when it comes to global warming, and Weiss is no exception. He points out that the ice remained in Europe long after solar activity picked up from the Maunder minimum. Even if we had another, similar low, he says, it would probably only cause temperatures on Earth to drop by the order of a tenth of a degree Celsius- peanuts compared to recent hikes.

太阳活动正逐渐下降，这引发了科学家之间关于太阳活动会下降到什么程度以及对地球气候影响的白热化争论。美国航天局记录2008年一共有266天没有监测出太阳黑子——这种休眠状态1913年以来从未见过——2009年似乎将变得更安静。太阳风的压力正处于50年来的最低水平，我们的太阳也比10年前稍暗了。

太阳黑子是太阳活动最明显的迹象——它是太阳表面一种炽热气体的巨大漩涡，因为比太阳光球层表面温度要低，所以从地球看上去像一些深暗色的斑点——事实上它们的消失意味着我们进入了一个太阳活动低迷期。

最终会是什么样子？黑子的活动周期为11.2年，但也会经历更长期的变化，顶峰和低估可以持续一个世纪左右。

“一个新的11年周期在一两年前开始了，目前为止它活动极其微弱，”剑桥大学的 Nigel Weiss说。和位于Dübendorf的瑞士联邦水产科技学院的Jose Abreu及其他一些人一起，Weiss最近预测，我们自二战后长期享受的充足太阳能量已经结束，目前能量正在减少，将在2020年左右达到最低点。他们的预测是基于积聚在地壳的罕见同位素的水平，轻微的太阳风使宇宙射线穿透地球的大气层。

还有一种可能，Weiss说，我们可能会与1645－1715年间发生在整个欧洲、北美和格陵兰岛的降温过程非常相似。当时，伦敦的泰晤士河、巴黎的塞纳河，以及格陵兰地区的所有河流均结冰冻结，格陵兰岛上的居民还不得不放弃了部分居民点。我们会不会再经历一次这样的事情？

这些声称主要是由于激烈的太阳活动导致过去一个世纪地球变暖的人可能会主张我们会经历十七世纪欧洲发生的事情，但这种人是少数。大多数科学家认为人类是全球气候变暖的罪魁祸首，Weiss也不例外。他指出，那次冰冻时期后太阳活动活跃起来了冰山依然在欧洲保留。即使这种情况会发生，他说，大概也只能导致地球温度下降0.1摄氏度左右，实在是微不足道。